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Pipe Wall Thickness Chart

Contents

Pipe wall thickness is one of the most important dimensions used when selecting pipe for industrial piping, oil and gas, chemical processing, power generation, water treatment, and mechanical systems. It affects the pipe’s internal diameter, pressure capacity, weight, flow area, mechanical strength, and material cost.

A pipe wall thickness chart shows the standard wall thickness available for each nominal pipe size and pipe schedule. For example, an NPS 2 pipe has the same outside diameter regardless of whether it is Schedule 10, Schedule 40, or Schedule 80. However, its wall thickness and inside diameter change with the selected schedule.

Most metallic pipe dimensions are standardized by ASME B36.10M for welded and seamless wrought steel pipe and ASME B36.19M for stainless steel pipe. Common schedule designations include Schedule 5, 10, 20, 30, 40, 60, 80, 100, 120, 140, and 160. Stainless steel pipes may also use schedules such as 5S, 10S, 40S, and 80S.

This article explains pipe wall thickness terminology, pipe schedules, dimensional standards, and methods for reading a pipe wall thickness chart. It also provides practical guidance for selecting the correct pipe thickness for pressure, temperature, corrosion, and service conditions.

1. What Is a Pipe Wall Thickness Chart?

What Is a Pipe Wall Thickness Chart?

A pipe wall thickness chart is a reference table that lists the standardized wall thickness for different nominal pipe sizes and pipe schedules. Depending on the chart, the dimensions may be shown in inches, millimeters, or both.

A typical pipe wall thickness chart includes:

  • Nominal Pipe Size (NPS)
  • Nominal Diameter (DN)
  • Outside diameter (OD)
  • Pipe schedule
  • Wall thickness
  • Inside diameter (ID)
  • Pipe weight per unit length

The chart allows engineers, designers, fabricators, inspectors, and purchasers to identify the physical dimensions of a pipe without calculating them individually.

For example, an NPS 2 pipe has a standardized outside diameter of 2.375 inches, or 60.3 mm. Its wall thickness depends on the schedule:

Pipe size Outside diameter Schedule Wall thickness Approximate inside diameter
NPS 2 2.375 in Sch 10 0.109 in 2.157 in
NPS 2 2.375 in Sch 40 0.154 in 2.067 in
NPS 2 2.375 in Sch 80 0.218 in 1.939 in
NPS 2 2.375 in Sch 160 0.344 in 1.687 in

This example demonstrates an important pipe-sizing principle: increasing the schedule increases the wall thickness but does not change the outside diameter. As a result, the available inside diameter becomes smaller.

The approximate inside diameter can be calculated using the following WordPress-safe formula:

Inside diameter = Outside diameter − (2 × Wall thickness)

For an NPS 2 Schedule 40 pipe:

Inside diameter = 2.375 − (2 × 0.154)

Inside diameter = 2.067 inches

A pipe wall thickness chart is useful for:

  • Selecting pipe for a required pressure and temperature
  • Determining the actual flow area
  • Estimating pipe weight and material quantity
  • Selecting compatible fittings, flanges, and welding procedures
  • Checking available corrosion allowance
  • Verifying pipe dimensions during inspection
  • Preparing piping specifications and material takeoffs

However, a wall thickness chart alone does not confirm that a particular schedule is safe for an application. The required thickness must be calculated according to the applicable piping code, design pressure, design temperature, material strength, corrosion allowance, manufacturing tolerance, and other design factors.

2. Pipe Wall Thickness Terminology and Dimensions

Understanding the terminology used in a pipe wall thickness chart is essential because nominal pipe size does not directly represent the pipe’s measured diameter.

Nominal Pipe Size (NPS)

Nominal Pipe Size is the North American system used to identify standardized pipe sizes. NPS is a nominal designation rather than an exact physical measurement.

For pipe sizes from NPS 1/8 through NPS 12, the NPS number generally does not equal the actual outside diameter. For NPS 14 and larger, the numerical NPS designation normally matches the outside diameter in inches.

Examples:

Nominal pipe size Actual outside diameter
NPS 1/2 0.840 in
NPS 1 1.315 in
NPS 2 2.375 in
NPS 6 6.625 in
NPS 12 12.750 in
NPS 14 14.000 in

Nominal Diameter (DN)

DN is the metric nominal designation corresponding approximately to NPS. It is expressed as a dimensionless whole number and should not be treated as the exact outside or inside diameter in millimeters.

Common NPS-to-DN equivalents include:

NPS DN
1/2 15
1 25
2 50
4 100
6 150
8 200
12 300

For example, an NPS 2 pipe is commonly designated as DN 50, although its actual outside diameter is 60.3 mm.

Outside Diameter (OD)

Outside diameter is the measured distance across the exterior of the pipe through its centerline. For a given NPS, the outside diameter normally remains constant across the available pipe schedules.

This fixed outside diameter allows pipes of different schedules to connect to standardized fittings, flanges, supports, and other piping components.

For example, NPS 4 Schedule 10, Schedule 40, and Schedule 80 pipes all have an outside diameter of 4.500 inches, or 114.3 mm.

Inside Diameter (ID)

Inside diameter is the distance across the internal opening of the pipe. Unlike outside diameter, it is not fixed for a particular NPS because it changes with wall thickness.

The inside diameter is calculated as:

ID = OD − (2 × t)

Where:

  • ID = inside diameter
  • OD = outside diameter
  • t = nominal wall thickness

A thicker pipe wall produces a smaller inside diameter when the outside diameter remains constant. This affects flow capacity, fluid velocity, pressure drop, and the ability to insert cleaning or inspection equipment.

Nominal Wall Thickness

Nominal wall thickness is the standardized thickness listed for a particular pipe size and schedule. It is called “nominal” because the actual manufactured thickness may vary within the tolerance allowed by the applicable product specification.

Wall thickness may be expressed in:

  • Inches
  • Millimeters
  • Pipe schedule
  • Standard weight designations such as STD, XS, and XXS

The nominal wall thickness should not automatically be treated as the minimum guaranteed wall thickness.

Minimum Wall Thickness

Minimum wall thickness is the lowest acceptable thickness after accounting for manufacturing tolerance. Many pipe specifications permit a negative wall thickness tolerance, commonly 12.5%, although the actual tolerance must be confirmed from the applicable material standard.

When a 12.5% negative tolerance applies:

Minimum manufactured thickness = Nominal wall thickness × 0.875

For example, if the nominal wall thickness is 10.0 mm:

Minimum manufactured thickness = 10.0 × 0.875 = 8.75 mm

This is different from the calculated design thickness. Engineers usually calculate the required minimum thickness first and then select a nominal pipe wall thick enough to remain acceptable after manufacturing tolerance and other allowances are considered.

Pipe Schedule

Pipe schedule is a dimensionless designation used to identify a standardized wall thickness for a particular nominal pipe size.

Common schedules include:

  • Schedule 5
  • Schedule 10
  • Schedule 20
  • Schedule 30
  • Schedule 40
  • Schedule 60
  • Schedule 80
  • Schedule 100
  • Schedule 120
  • Schedule 140
  • Schedule 160

In general, a higher schedule number indicates a thicker wall for the same NPS. However, the schedule number is not the wall thickness and should not be used as a direct measurement.

For example, Schedule 40 does not mean that the pipe wall is 40 mm or 0.040 inches thick.

Stainless Steel Schedule Designations

Stainless steel pipe commonly uses schedule designations ending in the letter “S,” including:

  • Schedule 5S
  • Schedule 10S
  • Schedule 40S
  • Schedule 80S

These dimensions are primarily covered by ASME B36.19M. A schedule with an “S” suffix is not always identical to the corresponding carbon steel schedule in every pipe size. The applicable dimensional table should therefore be checked instead of assuming that Schedule 40 and Schedule 40S are always interchangeable.

STD, XS, and XXS

Older and commonly used wall designations include:

  • STD: Standard Weight
  • XS: Extra Strong
  • XXS: Double Extra Strong

These designations correspond to schedule thicknesses only within certain size ranges.

For many smaller pipe sizes:

  • STD generally corresponds to Schedule 40
  • XS generally corresponds to Schedule 80
  • XXS has its own wall thickness and should not be assumed to equal Schedule 160

At larger sizes, STD and XS may no longer match Schedule 40 and Schedule 80. The dimensional standard must be consulted to confirm the correct thickness.

Wall Thickness Tolerance

Wall thickness tolerance is the permitted difference between the nominal and actual manufactured wall thickness. It depends on the pipe manufacturing method and material specification.

Tolerance is important because the pipe delivered by the manufacturer may be slightly thinner than its nominal chart value. It must therefore be considered when selecting a schedule based on calculated design thickness.

Corrosion Allowance

Corrosion allowance is additional thickness added to compensate for expected metal loss during the pipe’s service life.

A simplified relationship is:

Required nominal thickness = Design thickness + Corrosion allowance + Other allowances

Manufacturing tolerance must then be applied according to the governing design code and material specification.

Corrosion allowance may depend on:

  • Process fluid
  • Material compatibility
  • Operating temperature
  • Expected service life
  • Corrosion rate
  • Erosion conditions
  • Inspection and maintenance strategy

Pipe Wall Thickness and Flow Area

As wall thickness increases, the inside diameter and available flow area decrease.

The internal flow area can be calculated using:

Flow area = π × ID² ÷ 4

Where:

  • Flow area = internal cross-sectional area
  • π = approximately 3.1416
  • ID = inside diameter

For the same flow rate, reducing the inside diameter generally increases fluid velocity and may increase frictional pressure loss. Pipe schedule selection must therefore consider both mechanical strength and hydraulic performance.

3. Pipe Schedule and Wall Thickness Standards

Pipe Schedule and Wall Thickness Standards

Pipe dimensions and wall thicknesses are controlled by recognized dimensional standards. These standards ensure that pipes produced by different manufacturers have compatible outside diameters and standardized nominal wall thicknesses.

The two principal standards are:

  • ASME B36.10M: Welded and seamless wrought steel pipe
  • ASME B36.19M: Stainless steel pipe

Material specifications such as ASTM A53, ASTM A106, ASTM A312, and API 5L define material grades, chemical composition, mechanical properties, manufacturing methods, testing, and tolerances. They should not be confused with the dimensional standards.

ASME B36.10M

ASME B36.10M covers the dimensions of welded and seamless wrought steel pipe used at high and low pressures and temperatures. It is commonly applied to carbon steel and alloy steel piping.

The standard includes:

  • Nominal Pipe Size from NPS 1/8 through NPS 80
  • Standardized outside diameters
  • Schedule wall thicknesses
  • Plain-end pipe weights
  • STD, XS, and XXS wall designations

Common ASME B36.10M schedules include:

  • Schedule 5
  • Schedule 10
  • Schedule 20
  • Schedule 30
  • Schedule 40
  • Schedule 60
  • Schedule 80
  • Schedule 100
  • Schedule 120
  • Schedule 140
  • Schedule 160

Not every schedule is produced for every nominal pipe size. The absence of a thickness in a dimensional table means that the particular size-and-schedule combination is not listed in that table.

ASME B36.19M

ASME B36.19M establishes dimensions for stainless steel pipe. Its most common schedule designations are:

  • Schedule 5S
  • Schedule 10S
  • Schedule 40S
  • Schedule 80S

The letter S identifies a stainless steel pipe schedule. The outside diameters are coordinated with those in ASME B36.10M, allowing stainless steel pipes to connect to standardized flanges and fittings.

Schedule 40S and Schedule 80S correspond to Schedule 40 and Schedule 80 dimensions for many smaller sizes. However, they are not identical across every size range. The correct ASME table should always be checked.

ASTM and API Material Specifications

Dimensional standards define pipe size and wall thickness, while material specifications define how the pipe is manufactured and tested.

Common specifications include:

Specification Typical application
ASTM A53 Welded and seamless carbon steel pipe for mechanical and pressure service
ASTM A106 Seamless carbon steel pipe for high-temperature service
ASTM A333 Carbon and alloy steel pipe for low-temperature service
ASTM A312 Seamless, welded, and heavily cold-worked austenitic stainless steel pipe
ASTM A335 Seamless ferritic alloy steel pipe for high-temperature service
API 5L Seamless and welded pipe for pipeline transportation systems

A purchase description should include both the material specification and the required pipe dimensions.

For example:

ASTM A106 Grade B, NPS 4, Schedule 40, seamless pipe

This description identifies:

  • Material specification: ASTM A106
  • Material grade: Grade B
  • Manufacturing method: Seamless
  • Nominal size: NPS 4
  • Wall designation: Schedule 40

Standard Weight, Extra Strong, and Double Extra Strong

Before schedule numbers became widely used, pipe wall thickness was commonly described using weight classes:

  • STD: Standard Weight
  • XS: Extra Strong
  • XXS: Double Extra Strong

These designations are still found in piping specifications, valve connections, fittings, and older engineering documents.

Their relationship with pipe schedules is size-dependent:

Traditional designation General relationship
STD Same as Schedule 40 through NPS 10; 0.375-inch wall at NPS 12 and larger
XS Same as Schedule 80 through NPS 8; 0.500-inch wall at NPS 10 and larger
XXS Separate thickness series; not generally equivalent to Schedule 160

Therefore, STD should not automatically be interpreted as Schedule 40, and XS should not automatically be interpreted as Schedule 80 for all pipe sizes.

Schedule Number and Pressure Capacity

A schedule number does not provide a universal pressure rating. Two pipes with the same schedule but different nominal sizes do not necessarily have the same pressure capacity.

The allowable pressure depends on factors such as:

  • Outside diameter
  • Actual wall thickness
  • Pipe material
  • Allowable material stress
  • Design temperature
  • Weld joint quality factor
  • Corrosion allowance
  • Manufacturing tolerance
  • Applicable piping code

Higher schedules generally provide thicker walls and greater pressure capacity for the same nominal size and material. However, the final selection must be supported by a code calculation.

Applicable Piping Codes

Pipe dimensional standards should be used together with the applicable design code. Common examples include:

  • ASME B31.1 for power piping
  • ASME B31.3 for process piping
  • ASME B31.4 for liquid transportation pipelines
  • ASME B31.8 for gas transmission and distribution piping
  • ASME Section VIII for pressure vessels and related pressure components

The dimensional chart identifies available pipe thicknesses. The design code determines the minimum thickness required for the operating conditions.

4. Complete Pipe Wall Thickness Chart

Complete Pipe Wall Thickness Chart

The following chart gives the outside diameter and nominal wall thicknesses for commonly used pipe sizes. Dimensions are shown in millimeters for Schedule 5, Schedule 10, Schedule 40, Schedule 80, and Schedule 160.

A dash indicates that the schedule is not commonly listed for that nominal size in the referenced dimensional series.

NPS DN OD (mm) Sch 5 (mm) Sch 10 (mm) Sch 40 (mm) Sch 80 (mm) Sch 160 (mm)
1/8 6 10.3 1.24 1.73 2.41
1/4 8 13.7 1.65 2.24 3.02
3/8 10 17.1 1.65 2.31 3.20
1/2 15 21.3 1.65 2.11 2.77 3.73 4.78
3/4 20 26.7 1.65 2.11 2.87 3.91 5.56
1 25 33.4 1.65 2.77 3.38 4.55 6.35
1 1/4 32 42.2 1.65 2.77 3.56 4.85 6.35
1 1/2 40 48.3 1.65 2.77 3.68 5.08 7.14
2 50 60.3 1.65 2.77 3.91 5.54 8.74
2 1/2 65 73.0 2.11 3.05 5.16 7.01 9.53
3 80 88.9 2.11 3.05 5.49 7.62 11.13
3 1/2 90 101.6 2.11 3.05 5.74 8.08
4 100 114.3 2.11 3.05 6.02 8.56 13.49
5 125 141.3 2.77 3.40 6.55 9.53 15.88
6 150 168.3 2.77 3.40 7.11 10.97 18.26
8 200 219.1 2.77 3.76 8.18 12.70 23.01
10 250 273.0 3.40 4.19 9.27 15.09 28.58
12 300 323.8 3.96 4.57 10.31 17.48 33.32
14 350 355.6 3.96 6.35 11.13 19.05 35.71
16 400 406.4 4.19 6.35 12.70 21.44 40.49
18 450 457.2 4.19 6.35 14.27 23.83 45.24
20 500 508.0 4.78 6.35 15.09 26.19 50.01
22 550 559.0 4.78 6.35 28.58 53.98
24 600 610.0 5.54 6.35 17.48 30.96 59.54

The values in the table are nominal wall thicknesses. Actual thickness may vary within the tolerance permitted by the applicable pipe specification.

How to Use the Chart

To use the chart:

  1. Locate the required NPS or DN.
  2. Confirm the standardized outside diameter.
  3. Move across the row to the required schedule.
  4. Read the corresponding nominal wall thickness.
  5. Calculate the inside diameter if required.
  6. Verify that the selected wall satisfies the applicable design code.

For example, an NPS 6 Schedule 40 pipe has:

  • Outside diameter: 168.3 mm
  • Nominal wall thickness: 7.11 mm

The approximate inside diameter is:

ID = OD − (2 × Wall thickness)

ID = 168.3 − (2 × 7.11)

ID = 154.08 mm

The calculated value is a nominal inside diameter. The actual dimension may differ because of wall thickness and outside-diameter tolerances.

Important Limitations of the Chart

The table should be used as a dimensional reference rather than a pressure-rating table. Before specifying a pipe, verify:

  • Current edition of the applicable dimensional standard
  • Material specification and grade
  • Availability from the manufacturer
  • Negative wall thickness tolerance
  • Design pressure and temperature
  • Corrosion or erosion allowance
  • Threading or machining allowance
  • Required mechanical strength
  • Governing piping code

Some schedule-and-size combinations may be dimensionally standardized but not readily available from local suppliers.

5. Carbon Steel Pipe Wall Thickness Chart

Carbon Steel Pipe Wall Thickness Chart

 

Carbon steel pipe dimensions are generally selected according to ASME B36.10M. Common carbon steel materials include ASTM A53 Grade B, ASTM A106 Grade B, ASTM A333 Grade 6, and API 5L grades.

The following table provides nominal wall thicknesses in inches for common carbon steel pipe schedules.

NPS OD (in) Sch 10 Sch 20 Sch 40 Sch 80 Sch 120 Sch 160
1/8 0.405 0.049 0.068 0.095
1/4 0.540 0.065 0.088 0.119
3/8 0.675 0.065 0.091 0.126
1/2 0.840 0.083 0.109 0.147 0.188
3/4 1.050 0.083 0.113 0.154 0.219
1 1.315 0.109 0.133 0.179 0.250
1 1/4 1.660 0.109 0.140 0.191 0.250
1 1/2 1.900 0.109 0.145 0.200 0.281
2 2.375 0.109 0.154 0.218 0.344
2 1/2 2.875 0.120 0.203 0.276 0.375
3 3.500 0.120 0.216 0.300 0.438
3 1/2 4.000 0.120 0.226 0.318
4 4.500 0.120 0.237 0.337 0.438 0.531
5 5.563 0.134 0.258 0.375 0.500 0.625
6 6.625 0.134 0.280 0.432 0.562 0.719
8 8.625 0.148 0.250 0.322 0.500 0.719 0.906
10 10.750 0.165 0.250 0.365 0.594 0.844 1.125
12 12.750 0.180 0.250 0.406 0.688 1.000 1.312
14 14.000 0.250 0.312 0.438 0.750 1.094 1.406
16 16.000 0.250 0.312 0.500 0.844 1.219 1.594
18 18.000 0.250 0.312 0.562 0.938 1.375 1.781
20 20.000 0.250 0.375 0.594 1.031 1.500 1.969
22 22.000 0.250 0.375 1.125 1.625 2.125
24 24.000 0.250 0.375 0.688 1.219 1.812 2.344

Common Carbon Steel Pipe Schedules

The following schedules are frequently used in carbon steel piping systems:

  • Schedule 10: Light-wall pipe for relatively low-pressure service
  • Schedule 40: Common general-purpose wall thickness
  • Schedule 80: Heavy-wall pipe for higher-pressure or more mechanically demanding service
  • Schedule 120: Very heavy-wall pipe used in selected high-pressure applications
  • Schedule 160: Extremely heavy-wall pipe for severe pressure service

These descriptions are general only. A Schedule 10 pipe may be acceptable in one high-pressure application but unsuitable in another because material strength, size, temperature, code requirements, and corrosion allowance also affect the design.

Carbon Steel Pipe Inside Diameter Example

Consider an NPS 4 Schedule 80 carbon steel pipe:

  • Outside diameter = 4.500 inches
  • Wall thickness = 0.337 inches

The approximate inside diameter is:

ID = 4.500 − (2 × 0.337)

ID = 3.826 inches

For comparison, an NPS 4 Schedule 40 pipe has a wall thickness of 0.237 inches:

ID = 4.500 − (2 × 0.237)

ID = 4.026 inches

Changing from Schedule 40 to Schedule 80 reduces the inside diameter by approximately 0.200 inches. This reduction should be included in flow and pressure-drop calculations.

Selecting Carbon Steel Pipe Thickness

When selecting a carbon steel pipe schedule, consider:

  • Required pressure design thickness
  • Design temperature
  • Allowable stress of the selected grade
  • Corrosion and erosion allowance
  • Mill undertolerance
  • Threading, grooving, or machining allowance
  • External mechanical loads
  • Vacuum or external-pressure conditions
  • Cyclic and fatigue loading
  • Minimum thickness required for handling and fabrication
  • Availability of matching fittings and branch connections

The selected nominal wall thickness must be equal to or greater than the minimum required nominal thickness calculated under the governing piping code.

6. Stainless Steel Pipe Wall Thickness Chart

Stainless Steel Pipe Wall Thickness Chart

Stainless steel pipe dimensions are generally specified according to ASME B36.19M. Common materials covered by related product specifications include ASTM A312 grades TP304/304L and TP316/316L.

Stainless steel pipe schedules normally use the suffix S:

  • Schedule 5S
  • Schedule 10S
  • Schedule 40S
  • Schedule 80S

The letter “S” distinguishes stainless steel schedule designations from the schedules primarily associated with carbon steel pipe under ASME B36.10M.

The following table shows nominal stainless steel pipe wall thicknesses. Values are given in both inches and millimeters.

NPS OD (in) OD (mm) Sch 5S Sch 10S Sch 40S Sch 80S
1/8 0.405 10.3 0.049 in / 1.24 mm 0.068 in / 1.73 mm 0.095 in / 2.41 mm
1/4 0.540 13.7 0.065 in / 1.65 mm 0.088 in / 2.24 mm 0.119 in / 3.02 mm
3/8 0.675 17.1 0.065 in / 1.65 mm 0.091 in / 2.31 mm 0.126 in / 3.20 mm
1/2 0.840 21.3 0.065 in / 1.65 mm 0.083 in / 2.11 mm 0.109 in / 2.77 mm 0.147 in / 3.73 mm
3/4 1.050 26.7 0.065 in / 1.65 mm 0.083 in / 2.11 mm 0.113 in / 2.87 mm 0.154 in / 3.91 mm
1 1.315 33.4 0.065 in / 1.65 mm 0.109 in / 2.77 mm 0.133 in / 3.38 mm 0.179 in / 4.55 mm
1 1/4 1.660 42.2 0.065 in / 1.65 mm 0.109 in / 2.77 mm 0.140 in / 3.56 mm 0.191 in / 4.85 mm
1 1/2 1.900 48.3 0.065 in / 1.65 mm 0.109 in / 2.77 mm 0.145 in / 3.68 mm 0.200 in / 5.08 mm
2 2.375 60.3 0.065 in / 1.65 mm 0.109 in / 2.77 mm 0.154 in / 3.91 mm 0.218 in / 5.54 mm
2 1/2 2.875 73.0 0.083 in / 2.11 mm 0.120 in / 3.05 mm 0.203 in / 5.16 mm 0.276 in / 7.01 mm
3 3.500 88.9 0.083 in / 2.11 mm 0.120 in / 3.05 mm 0.216 in / 5.49 mm 0.300 in / 7.62 mm
3 1/2 4.000 101.6 0.083 in / 2.11 mm 0.120 in / 3.05 mm 0.226 in / 5.74 mm 0.318 in / 8.08 mm
4 4.500 114.3 0.083 in / 2.11 mm 0.120 in / 3.05 mm 0.237 in / 6.02 mm 0.337 in / 8.56 mm
5 5.563 141.3 0.109 in / 2.77 mm 0.134 in / 3.40 mm 0.258 in / 6.55 mm 0.375 in / 9.53 mm
6 6.625 168.3 0.109 in / 2.77 mm 0.134 in / 3.40 mm 0.280 in / 7.11 mm 0.432 in / 10.97 mm
8 8.625 219.1 0.109 in / 2.77 mm 0.148 in / 3.76 mm 0.322 in / 8.18 mm 0.500 in / 12.70 mm
10 10.750 273.0 0.134 in / 3.40 mm 0.165 in / 4.19 mm 0.365 in / 9.27 mm 0.500 in / 12.70 mm
12 12.750 323.8 0.156 in / 3.96 mm 0.180 in / 4.57 mm 0.375 in / 9.53 mm 0.500 in / 12.70 mm
14 14.000 355.6 0.156 in / 3.96 mm 0.188 in / 4.78 mm 0.375 in / 9.53 mm 0.500 in / 12.70 mm
16 16.000 406.4 0.165 in / 4.19 mm 0.188 in / 4.78 mm 0.375 in / 9.53 mm 0.500 in / 12.70 mm
18 18.000 457.2 0.165 in / 4.19 mm 0.188 in / 4.78 mm 0.375 in / 9.53 mm 0.500 in / 12.70 mm
20 20.000 508.0 0.188 in / 4.78 mm 0.218 in / 5.54 mm 0.375 in / 9.53 mm 0.500 in / 12.70 mm
22 22.000 559.0 0.188 in / 4.78 mm 0.218 in / 5.54 mm 0.375 in / 9.53 mm 0.500 in / 12.70 mm
24 24.000 610.0 0.218 in / 5.54 mm 0.250 in / 6.35 mm 0.375 in / 9.53 mm 0.500 in / 12.70 mm

The table contains nominal dimensions. Actual pipe thickness must comply with the tolerance requirements of the applicable material specification.

Schedule 40 vs. Schedule 40S

Schedule 40 and Schedule 40S have identical wall thicknesses for many smaller pipe sizes, but they are not identical throughout the complete size range.

For example, at NPS 12:

  • Schedule 40 under ASME B36.10M: 0.406 inches
  • Schedule 40S under ASME B36.19M: 0.375 inches

Schedule 40S remains at a nominal wall thickness of 0.375 inches for several larger pipe sizes, while Schedule 40 under ASME B36.10M continues to increase.

Therefore, a specification should clearly state whether Schedule 40 or Schedule 40S is required.

Schedule 80 vs. Schedule 80S

Schedule 80 and Schedule 80S are also identical for many smaller sizes. At larger sizes, however, Schedule 80S is limited to a nominal wall thickness of 0.500 inches, while Schedule 80 continues to increase.

For example, at NPS 12:

  • Schedule 80: 0.688 inches
  • Schedule 80S: 0.500 inches

The schedule suffix should never be omitted when it affects the required pipe dimension.

Selecting a Stainless Steel Pipe Schedule

Schedule 10S is widely used for stainless steel process piping because stainless steel provides good corrosion resistance without requiring the heavier walls commonly used for carbon steel corrosion allowance.

However, Schedule 10S is not automatically suitable for every application. Selection should account for:

  • Internal design pressure
  • Design temperature
  • External loads
  • Corrosion allowance
  • Manufacturing tolerance
  • Welding and fabrication requirements
  • Minimum wall thickness for branch connections
  • Threading or mechanical joining
  • Vacuum and external pressure
  • Applicable piping code

Thin-wall stainless steel pipe may also require special attention during welding because excessive heat input can cause distortion, excessive penetration, oxidation, or loss of dimensional control.

7. How to Read and Use a Pipe Wall Thickness Chart

A pipe wall thickness chart should be read in a logical sequence. The nominal pipe size, outside diameter, schedule, and wall thickness must not be confused with one another.

Step 1: Identify the Nominal Pipe Size

Start by finding the required NPS or DN in the first column.

For example:

  • NPS 1 corresponds to DN 25
  • NPS 2 corresponds to DN 50
  • NPS 4 corresponds to DN 100
  • NPS 8 corresponds to DN 200

NPS and DN are nominal designations. They are not exact measured diameters.

Step 2: Confirm the Outside Diameter

Read the actual outside diameter from the chart. For a given NPS, this diameter normally remains constant regardless of the pipe schedule.

For example, all NPS 4 pipes have an outside diameter of:

  • 4.500 inches
  • 114.3 mm

This applies whether the pipe is Schedule 10, Schedule 40, or Schedule 80.

Step 3: Select the Required Schedule

Move horizontally across the selected NPS row until reaching the required schedule column.

For an NPS 4 pipe:

Schedule Wall thickness
Schedule 10 0.120 in / 3.05 mm
Schedule 40 0.237 in / 6.02 mm
Schedule 80 0.337 in / 8.56 mm
Schedule 160 0.531 in / 13.49 mm

A higher schedule generally means a thicker wall and a smaller inside diameter.

Step 4: Calculate the Inside Diameter

If the inside diameter is not shown, calculate it using:

ID = OD − (2 × t)

Where:

  • ID = nominal inside diameter
  • OD = outside diameter
  • t = nominal wall thickness

For an NPS 4 Schedule 40 pipe:

ID = 4.500 − (2 × 0.237)

ID = 4.026 inches

For an NPS 4 Schedule 80 pipe:

ID = 4.500 − (2 × 0.337)

ID = 3.826 inches

The Schedule 80 pipe has the same outside diameter but a smaller internal flow passage.

Step 5: Check the Dimensional Standard

Confirm whether the chart is based on:

  • ASME B36.10M for carbon and alloy steel pipe
  • ASME B36.19M for stainless steel pipe

This is particularly important when comparing Schedule 40 with 40S or Schedule 80 with 80S.

Step 6: Check the Material Specification

The dimensional standard does not define the material grade. Confirm the product specification separately.

Examples include:

  • ASTM A106 Grade B carbon steel
  • ASTM A333 Grade 6 low-temperature carbon steel
  • ASTM A312 TP304L stainless steel
  • ASTM A312 TP316L stainless steel
  • API 5L Grade B line pipe

The material specification determines allowable manufacturing methods, properties, testing requirements, and dimensional tolerances.

Step 7: Account for Manufacturing Tolerance

The nominal wall shown in a chart may be greater than the actual manufactured wall. Many pipe specifications allow a negative wall thickness tolerance, frequently 12.5%.

When a 12.5% negative tolerance applies:

Minimum possible wall = Nominal wall × 0.875

For a nominal wall thickness of 6.02 mm:

Minimum possible wall = 6.02 × 0.875

Minimum possible wall = 5.27 mm

The actual tolerance must be confirmed from the applicable material specification because it can vary by product and manufacturing process.

Step 8: Verify the Pressure Design

Do not select a schedule only because it is commonly used. Calculate the required wall thickness using the applicable piping code and compare the result with the available nominal wall thicknesses.

The selected pipe must satisfy:

Available minimum wall ≥ Required minimum wall

The available minimum wall is determined after applying the permitted negative manufacturing tolerance.

Step 9: Check Flow Performance

A thicker pipe wall reduces the inside diameter and internal flow area. This can increase fluid velocity and pressure drop.

The internal cross-sectional area is:

Flow area = π × ID² ÷ 4

Therefore, the final schedule selection should be included in hydraulic calculations rather than using the nominal pipe size as the actual flow diameter.

Example of Reading the Chart

Suppose a project requires an NPS 6 Schedule 80 stainless steel pipe.

From the chart:

  • NPS: 6
  • DN: 150
  • Outside diameter: 6.625 inches or 168.3 mm
  • Wall thickness: 0.432 inches or 10.97 mm

Calculate the inside diameter:

ID = 168.3 − (2 × 10.97)

ID = 146.36 mm

If a 12.5% negative tolerance applies:

Minimum possible wall = 10.97 × 0.875

Minimum possible wall = 9.60 mm

The designer must confirm that this minimum available thickness remains greater than or equal to the thickness required by the piping code.

8. How to Calculate the Required Pipe Wall Thickness

The required pipe wall thickness depends on internal pressure, outside diameter, material strength, temperature, corrosion allowance, manufacturing tolerance, and the applicable design code.

For process piping designed according to ASME B31.3, a commonly used internal-pressure equation for straight pipe is:

t = (P × D) ÷ [2 × (S × E × W + P × Y)]

Where:

  • t = pressure design thickness
  • P = internal design pressure
  • D = pipe outside diameter
  • S = allowable material stress at design temperature
  • E = longitudinal weld joint quality factor
  • W = weld joint strength reduction factor
  • Y = coefficient determined by material and temperature

All quantities must use a consistent unit system. For example, if pressure and allowable stress are in MPa and diameter is in millimeters, the calculated thickness will be in millimeters.

This equation is subject to the applicability limits and requirements of the governing code edition. Other equations may apply to certain materials, dimensions, loading conditions, or code sections.

Step 1: Establish the Design Conditions

Identify:

  • Design pressure
  • Design temperature
  • Pipe material and grade
  • Nominal pipe size
  • Pipe manufacturing method
  • Fluid corrosiveness
  • Required service life
  • Applicable piping code

Design pressure and temperature should include the most severe coincident condition expected during operation, startup, shutdown, cleaning, or upset conditions.

Step 2: Determine the Allowable Stress

Obtain the allowable stress S from the applicable code table for the selected material and design temperature.

Allowable stress generally decreases as temperature increases. The room-temperature value should not be used when the pipe will operate at an elevated design temperature.

Step 3: Determine the Code Factors

Select the correct values for:

  • E: Weld joint quality factor
  • W: Weld joint strength reduction factor
  • Y: Code coefficient

These values depend on material, construction type, examination, temperature, and the applicable code.

For seamless pipe, E may commonly be 1.0, but the correct value must still be verified. The value of W is often 1.0 below specified temperature limits but may be lower at elevated temperatures.

Step 4: Calculate the Pressure Design Thickness

Consider an illustrative example with the following assumed values:

  • Design pressure, P = 5 MPa
  • Outside diameter, D = 114.3 mm
  • Allowable stress, S = 120 MPa
  • Weld joint quality factor, E = 1.0
  • Weld strength reduction factor, W = 1.0
  • Coefficient, Y = 0.4

Apply the formula:

t = (5 × 114.3) ÷ [2 × (120 × 1.0 × 1.0 + 5 × 0.4)]

t = 571.5 ÷ [2 × (120 + 2)]

t = 571.5 ÷ 244

t = 2.34 mm

The pressure design thickness is 2.34 mm.

Step 5: Add Corrosion and Other Allowances

The pressure design thickness does not normally include corrosion, erosion, threading, grooving, or other mechanical allowances.

The required minimum thickness is:

tm = t + c

Where:

  • tm = required minimum thickness
  • t = pressure design thickness
  • c = total mechanical, corrosion, and erosion allowance

Assume a corrosion allowance of 1.5 mm and no other allowance:

tm = 2.34 + 1.50

tm = 3.84 mm

The pipe must retain at least 3.84 mm of acceptable wall thickness after applicable allowances are considered.

Step 6: Account for Mill Undertolerance

If the pipe specification permits a 12.5% negative wall tolerance, the required nominal thickness is:

Required nominal thickness = tm ÷ (1 − 0.125)

Required nominal thickness = 3.84 ÷ 0.875

Required nominal thickness = 4.39 mm

A pipe with a nominal wall thickness below 4.39 mm would not satisfy this simplified example.

Step 7: Select the Next Available Schedule

For an NPS 4 carbon steel pipe:

  • Schedule 10 wall thickness = 3.05 mm
  • Schedule 40 wall thickness = 6.02 mm
  • Schedule 80 wall thickness = 8.56 mm

Because the calculated required nominal thickness is 4.39 mm:

  • Schedule 10 is insufficient.
  • Schedule 40 satisfies the calculated thickness.
  • Schedule 80 is thicker than required for internal pressure alone.

Based on this calculation, Schedule 40 would be the first listed schedule thick enough to meet the requirement.

Step 8: Verify the Selected Schedule

Check the minimum possible manufactured wall of Schedule 40:

Minimum possible wall = 6.02 × 0.875

Minimum possible wall = 5.27 mm

Compare this with the required minimum thickness:

  • Available minimum wall: 5.27 mm
  • Required minimum wall: 3.84 mm

Therefore, the selected Schedule 40 wall satisfies the simplified internal-pressure calculation.

Additional Design Checks

Internal pressure is only one part of pipe design. The selected thickness may also need to satisfy requirements for:

  • External pressure and vacuum
  • Pipe weight and fluid weight
  • Wind, earthquake, and support loads
  • Thermal expansion
  • Water hammer and pressure surge
  • Cyclic loading and fatigue
  • Vibration
  • Branch connections
  • Local loads from valves and equipment
  • Threaded or grooved joints
  • Mechanical handling
  • Minimum fabrication thickness
  • Corrosion under insulation
  • Erosion and localized corrosion

The final wall thickness should be verified by a qualified piping engineer using the current edition of the applicable design code and the project’s piping specifications.

9. How to Select the Correct Pipe Wall Thickness

Selecting the correct pipe wall thickness requires more than choosing a commonly used schedule. The selected pipe must withstand all expected pressure, temperature, corrosion, fabrication, and mechanical loading conditions throughout its intended service life.

A practical selection process should include the following considerations.

Identify the Applicable Design Code

Begin by identifying the code governing the piping system. Common examples include:

  • ASME B31.1 for power piping
  • ASME B31.3 for process piping
  • ASME B31.4 for liquid transportation pipelines
  • ASME B31.8 for gas transmission and distribution piping
  • ASME Section VIII for pressure-vessel-related applications
  • Local regulations and project-specific engineering standards

The design code determines the required calculation method, allowable stresses, material factors, allowances, and minimum thickness requirements.

Confirm the Pipe Material

Select a material that is compatible with the process fluid, design temperature, operating environment, and required mechanical properties.

Common pipe materials include:

  • Carbon steel
  • Low-temperature carbon steel
  • Austenitic stainless steel
  • Duplex stainless steel
  • Alloy steel
  • Nickel alloys
  • Nonmetallic materials

Different materials have different allowable stresses, corrosion resistance, manufacturing tolerances, and temperature limits. Two pipes with the same NPS and schedule may have different allowable pressure ratings if they are made from different materials.

Determine Design Pressure and Temperature

Use the maximum design pressure and temperature that may occur under the most severe expected coincident operating condition.

Consider:

  • Normal operation
  • Startup and shutdown
  • Equipment isolation
  • Pump shutoff pressure
  • Compressor discharge conditions
  • Pressure surges
  • Thermal expansion of trapped liquid
  • Steam-out or cleaning operations
  • Regeneration cycles
  • Process upset conditions

Operating pressure alone should not be used if the design pressure is higher.

Calculate the Pressure Design Thickness

Calculate the pressure design thickness using the applicable code formula. The calculation should use:

  • Pipe outside diameter
  • Design pressure
  • Allowable stress at design temperature
  • Weld joint quality factor
  • Weld strength reduction factor
  • Code coefficient
  • Applicable material and construction factors

The result represents the thickness required to resist internal pressure before additional allowances are included.

Add Corrosion and Erosion Allowance

Add sufficient thickness for the expected metal loss during service.

The total allowance may include:

  • General corrosion allowance
  • Erosion allowance
  • Threading allowance
  • Grooving allowance
  • Machining allowance
  • Other mechanical allowances

A simplified relationship is:

Required minimum wall = Pressure design thickness + Total allowances

Corrosion allowance should be based on material compatibility, expected corrosion rate, service life, process conditions, and inspection strategy.

Stainless steel pipe is sometimes specified without a general corrosion allowance when the material is confirmed to be resistant to the process fluid. However, localized corrosion mechanisms such as pitting, crevice corrosion, and chloride stress-corrosion cracking must still be evaluated.

Account for Manufacturing Tolerance

The nominal thickness listed in a pipe chart is not necessarily the minimum manufactured thickness. The selected nominal wall must remain adequate after the permitted negative tolerance is applied.

The general selection relationship is:

Selected nominal wall × (1 − Mill tolerance) ≥ Required minimum wall

Alternatively:

Required nominal wall = Required minimum wall ÷ (1 − Mill tolerance)

For a 12.5% negative tolerance:

Required nominal wall = Required minimum wall ÷ 0.875

Always verify the actual tolerance in the applicable material specification.

Select the Next Available Standard Schedule

After calculating the required nominal thickness, use the pipe wall thickness chart to select the next available schedule with an equal or greater wall thickness.

For example, assume the calculated required nominal wall is 5.0 mm for an NPS 4 carbon steel pipe:

Schedule Nominal wall Selection result
Schedule 10 3.05 mm Insufficient
Schedule 40 6.02 mm Acceptable
Schedule 80 8.56 mm Acceptable but heavier

Schedule 40 would normally be the first suitable schedule based on wall thickness alone.

The final decision may still require a heavier schedule because of mechanical loads, standardization, corrosion risk, or project requirements.

Check External Pressure and Vacuum

A pipe that can withstand high internal pressure may still collapse under external pressure or vacuum.

External-pressure resistance depends on:

  • Outside diameter
  • Wall thickness
  • Material properties
  • Pipe ovality
  • Unsupported length
  • Design temperature
  • Stiffening and restraint
  • External hydrostatic pressure

Vacuum lines, jacketed pipes, subsea pipelines, and buried piping may require separate buckling or external-pressure calculations.

Evaluate Mechanical Loads

The pipe wall must also withstand loads unrelated to internal pressure, including:

  • Pipe self-weight
  • Weight of the contained fluid
  • Insulation and cladding
  • Valves and inline equipment
  • Support reactions
  • Wind and seismic loads
  • Thermal expansion
  • Nozzle loads
  • Vibration
  • Occasional impact
  • Transportation and installation loads

Thin-wall pipe may satisfy the pressure calculation but lack sufficient rigidity for fabrication, handling, or support spacing.

Consider Branch Connections

Branch openings remove material from the run pipe and create localized stresses. A branch reinforcement calculation may be required by the piping code.

The selected pipe wall affects:

  • Available reinforcement area
  • Branch fitting selection
  • Weldolet or sockolet size
  • Reinforcement pad requirements
  • Weld dimensions
  • Local stress concentration

A schedule adequate for straight pipe may be insufficient at an unreinforced branch connection.

Consider the Joining Method

The pipe wall must be compatible with the intended joining and fabrication method.

Butt Welding

Thin-wall pipe requires careful fit-up and heat control. Excessive heat input can cause burn-through, distortion, or excessive internal penetration.

Threading

Threading removes part of the pipe wall. Threaded carbon steel pipe is therefore commonly specified with a wall thick enough to retain adequate material beneath the thread root.

Grooved Connections

Roll grooving and cut grooving have different minimum wall requirements. The groove depth and joining-system manufacturer’s limitations must be checked.

Socket Welding

The pipe wall, socket dimensions, and pressure class of the fitting must be compatible. Socket-welded construction may also have service restrictions under the applicable project specification.

Check Fitting and Valve Compatibility

The selected pipe schedule should be coordinated with:

  • Butt-weld fitting schedule
  • Branch fittings
  • Valve end dimensions
  • Flange bore
  • Welding end preparation
  • Gasket and bolting arrangement
  • Mechanical couplings

A mismatch between pipe wall thickness and fitting bore can create an internal step, increase turbulence, complicate welding, or require internal tapering.

Evaluate Hydraulic Performance

Increasing wall thickness reduces the inside diameter. For the same flow rate, this may result in:

  • Higher fluid velocity
  • Greater frictional pressure drop
  • Increased pump or compressor power
  • Higher noise
  • Greater erosion risk
  • Reduced system capacity

The actual inside diameter associated with the selected schedule should be used in flow calculations.

Consider Weight and Cost

A heavier schedule increases:

  • Pipe material cost
  • Transportation weight
  • Support loads
  • Welding time
  • Cutting and beveling time
  • Installation labor
  • Lifting requirements

Selecting a substantially heavier schedule than required can increase project cost without providing a meaningful engineering benefit.

However, standardizing several line classes around one commonly available schedule may sometimes reduce procurement complexity and overall project cost.

Check Commercial Availability

Not every standardized schedule is readily available in every material, size, or location. Before finalizing the specification, confirm:

  • Manufacturer availability
  • Minimum order quantity
  • Seamless or welded construction
  • Available pipe lengths
  • Required material grade
  • Delivery time
  • Matching fittings and flanges
  • Testing and certification requirements

A theoretically optimal schedule may not be practical if it requires a special production run or creates a long delivery delay.

Review the Complete Piping Specification

The final pipe designation should clearly state:

  • Material specification
  • Material grade
  • Nominal pipe size
  • Pipe schedule or nominal wall thickness
  • Seamless or welded construction
  • End preparation
  • Heat treatment, if applicable
  • Inspection and testing requirements
  • Surface finish or cleaning requirements
  • Certification requirements

An example specification is:

ASTM A312 TP316L, NPS 4, Schedule 10S, seamless, plain end

For critical services, the piping material specification should also define supplementary testing, nondestructive examination, impact testing, hardness limits, corrosion testing, and documentation requirements.

Pipe Wall Thickness Selection Checklist

Before approving the selected schedule, verify that:

  • The correct design code has been used.
  • Design pressure and temperature are confirmed.
  • The pipe material and grade are suitable.
  • Allowable stress is taken at design temperature.
  • Pressure design thickness has been calculated.
  • Corrosion, erosion, and mechanical allowances are included.
  • Manufacturing undertolerance has been applied.
  • The selected schedule is commercially available.
  • External pressure and vacuum have been evaluated.
  • Mechanical and occasional loads are acceptable.
  • Branch reinforcement has been checked.
  • The joining method is compatible with the wall thickness.
  • Fittings, valves, and flanges are properly matched.
  • The actual inside diameter has been used for flow calculations.
  • Project and client specifications have been satisfied.

Conclusion

A pipe wall thickness chart provides a convenient way to compare standardized wall thicknesses for different nominal pipe sizes and schedules. It helps engineers and fabricators identify the outside diameter, nominal wall thickness, approximate inside diameter, and dimensional relationship between schedules such as Schedule 10, Schedule 40, Schedule 80, and Schedule 160.

ASME B36.10M is generally used for carbon and alloy steel pipe, while ASME B36.19M provides the commonly used 5S, 10S, 40S, and 80S dimensions for stainless steel pipe. Schedule designations identify standardized dimensions, but they do not provide a universal pressure rating.

The correct pipe wall thickness must be determined from the applicable design code using the design pressure, design temperature, outside diameter, allowable material stress, weld factors, corrosion allowance, and manufacturing tolerance. External pressure, mechanical loads, fabrication requirements, flow performance, and commercial availability must also be considered.

A pipe wall thickness chart should therefore be used as a dimensional selection tool after the required thickness has been calculated—not as a substitute for engineering design.

NPS to DN Conversion Chart

DN Pipe Size Chart: DN to NPS Conversion and Dimensions

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