NPS to DN Conversion Chart

Contents

1 1. What Are NPS and DN Pipe Sizes?
2 2. NPS vs. DN: Key Differences
3 3. Complete NPS to DN Conversion Chart
- 3.1 Important Notes About the Chart
- 3.2 DN Values Are Nominal Designations
4 4. How to Convert NPS to DN
5 5. NPS, DN, and Actual Pipe Diameter
6 6. NPS and DN Pipe Schedule Comparison
7 7. Common NPS and DN Sizes for Pipes and Fittings
8 8. Applicable NPS and DN Standards
9 9. How to Select the Correct NPS or DN Size
10 Conclusion

Pipe sizes are commonly identified using two different designation systems: NPS (Nominal Pipe Size) and DN (Diameter Nominal). NPS is widely used in the United States and other countries that follow inch-based piping standards, while DN is the metric designation used in international and European standards.

Although NPS and DN are often treated as equivalent sizes, neither value directly represents the exact inside diameter or outside diameter of a pipe. They are nominal designations used to standardize the selection and compatibility of pipes, fittings, flanges, valves, and other piping components.

For example, an NPS 2 pipe is generally equivalent to DN 50, but its actual outside diameter is 60.3 mm rather than 50 mm or 2 inches. Understanding this distinction is important when selecting components, preparing piping specifications, interpreting engineering drawings, or converting between ASME and ISO standards.

This guide explains the relationship between NPS and DN, their main differences, and how to use an NPS to DN conversion chart correctly.

1. What Are NPS and DN Pipe Sizes?

What Are NPS and DN Pipe Sizes

NPS and DN are standardized nominal designations used to describe the approximate size of pipes and matching piping components. These systems allow engineers, manufacturers, and installers to specify compatible products without referring to every actual pipe dimension.

What Is NPS?

NPS stands for Nominal Pipe Size. It is a North American pipe-sizing system expressed using dimensionless numbers that are commonly associated with inches.

Typical NPS designations include:

NPS 1/2
NPS 1
NPS 2
NPS 4
NPS 8
NPS 12

For pipe sizes from NPS 1/8 through NPS 12, the NPS designation does not equal the pipe’s actual outside diameter. For example:

NPS 1 pipe has an outside diameter of 1.315 inches.
NPS 2 pipe has an outside diameter of 2.375 inches.
NPS 6 pipe has an outside diameter of 6.625 inches.

For NPS 14 and larger, the NPS number generally matches the actual outside diameter in inches. Therefore, an NPS 14 pipe has an outside diameter of 14 inches, while an NPS 24 pipe has an outside diameter of 24 inches.

The wall thickness is specified separately using a pipe schedule, such as Schedule 10, Schedule 40, Schedule 80, or Schedule 160. Pipes with the same NPS and different schedules have the same outside diameter but different wall thicknesses and inside diameters.

What Is DN?

DN stands for Diameter Nominal, also known as Diamètre Nominal. It is the international metric designation used to identify nominal pipe and component sizes.

Common DN designations include:

DN 15
DN 25
DN 50
DN 100
DN 200
DN 300

DN values are written as whole numbers without the millimetre symbol. For example, the correct designation is DN 50, not DN 50 mm.

Like NPS, DN does not normally represent an exact physical pipe diameter. A DN 50 pipe, for example, generally has an outside diameter of 60.3 mm—not 50 mm. DN is therefore a standardized size designation rather than a direct dimensional measurement.

DN sizes are commonly used with piping systems and components manufactured according to ISO, EN, DIN, and other metric-based standards.

Relationship Between NPS and DN

Each common NPS size has a corresponding DN designation. Some typical equivalents are shown below:

NPS	Corresponding DN	Actual Outside Diameter
1/2	DN 15	21.3 mm
3/4	DN 20	26.7 mm
1	DN 25	33.4 mm
1 1/2	DN 40	48.3 mm
2	DN 50	60.3 mm
3	DN 80	88.9 mm
4	DN 100	114.3 mm
6	DN 150	168.3 mm
8	DN 200	219.1 mm
12	DN 300	323.9 mm

These are standardized equivalents rather than exact mathematical conversions. Multiplying NPS by 25.4 will not always produce the correct DN designation.

2. NPS vs. DN: Key Differences

NPS and DN serve the same basic purpose: they identify standardized pipe sizes and ensure that compatible pipes, fittings, flanges, and valves can be selected. However, they differ in terminology, numbering, standards, and common geographic use.

Feature	NPS	DN
Full term	Nominal Pipe Size	Diameter Nominal
Measurement convention	Inch-based	Metric-based
Typical designation	NPS 2	DN 50
Commonly used in	United States and North America	Europe and international projects
Typical standards	ASME and ASTM	ISO, EN, and DIN
Unit shown after size	No	No
Exact physical diameter	No	No
Wall thickness designation	Pipe schedule	Schedule or standardized wall series

Inch-Based and Metric-Based Designations

NPS uses numbers associated with the inch system, while DN uses rounded metric numbers. For example:

NPS 1/2 corresponds to DN 15.
NPS 1 corresponds to DN 25.
NPS 2 corresponds to DN 50.
NPS 4 corresponds to DN 100.
NPS 8 corresponds to DN 200.

However, DN is not calculated by directly multiplying every NPS value by 25.4. The DN values are standardized, rounded designations intended to provide approximate equivalents.

For instance, converting NPS 2 mathematically gives:

$2 \times 25.4 = 50.8 mm$

The corresponding designation is standardized as DN 50, not DN 50.8.

Nominal Size vs. Actual Diameter

The most important similarity between the systems is that both identify nominal sizes, not necessarily actual pipe diameters.

Consider an NPS 2 or DN 50 pipe:

Nominal designations: NPS 2 and DN 50
Actual outside diameter: 2.375 inches or 60.3 mm
Inside diameter: depends on the pipe schedule

Therefore, NPS 2 does not mean that the pipe has an exact outside diameter of 2 inches, and DN 50 does not mean that it has an exact outside diameter of 50 mm.

Standards and Regional Use

NPS is primarily associated with American piping standards, including:

ASME B36.10M for welded and seamless wrought steel pipe
ASME B36.19M for stainless steel pipe
ASME B16 standards for fittings, flanges, and valves
ASTM material and product specifications

DN is commonly used in international and European standards, including:

ISO piping and component standards
EN European standards
DIN German standards
Metric valve and flange specifications

The selected designation usually depends on the governing project standard. A project based on ASME requirements may use NPS, while a project based on EN or ISO standards may specify DN.

Compatibility of NPS and DN Components

Corresponding NPS and DN designations may describe components intended for the same general pipe size, but the designation alone does not guarantee complete compatibility.

Before connecting components, verify:

Actual pipe outside diameter
Wall thickness or pipe schedule
Flange standard and pressure class
Bolt-hole number and arrangement
Facing type
Fitting dimensions
Thread type
Material specification
Pressure and temperature rating

For example, an ASME NPS 4 Class 150 flange and a European DN 100 PN 16 flange refer to a similar nominal bore, but their bolt patterns, dimensions, and pressure ratings may differ. They should not be considered interchangeable without checking the applicable dimensional standards.

3. Complete NPS to DN Conversion Chart

Complete NPS to DN Conversion Chart

The following chart shows the standardized equivalents between Nominal Pipe Size (NPS) and Diameter Nominal (DN). The actual outside diameter is included because neither NPS nor DN necessarily represents a measured pipe diameter.

NPS	DN	Outside Diameter (in.)	Outside Diameter (mm)
1/8	DN 6	0.405	10.3
1/4	DN 8	0.540	13.7
3/8	DN 10	0.675	17.1
1/2	DN 15	0.840	21.3
3/4	DN 20	1.050	26.7
1	DN 25	1.315	33.4
1 1/4	DN 32	1.660	42.2
1 1/2	DN 40	1.900	48.3
2	DN 50	2.375	60.3
2 1/2	DN 65	2.875	73.0
3	DN 80	3.500	88.9
3 1/2	DN 90	4.000	101.6
4	DN 100	4.500	114.3
5	DN 125	5.563	141.3
6	DN 150	6.625	168.3
8	DN 200	8.625	219.1
10	DN 250	10.750	273.0
12	DN 300	12.750	323.9
14	DN 350	14.000	355.6
16	DN 400	16.000	406.4
18	DN 450	18.000	457.2
20	DN 500	20.000	508.0
22	DN 550	22.000	558.8
24	DN 600	24.000	609.6
26	DN 650	26.000	660.4
28	DN 700	28.000	711.2
30	DN 750	30.000	762.0
32	DN 800	32.000	812.8
34	DN 850	34.000	863.6
36	DN 900	36.000	914.4
38	DN 950	38.000	965.2
40	DN 1000	40.000	1,016.0
42	DN 1050	42.000	1,066.8
44	DN 1100	44.000	1,117.6
46	DN 1150	46.000	1,168.4
48	DN 1200	48.000	1,219.2

Important Notes About the Chart

The conversion chart should be interpreted as a list of standardized size equivalents rather than a table of exact mathematical conversions.

For pipe sizes from NPS 1/8 through NPS 12, the NPS number differs from the actual outside diameter. For example, NPS 4 corresponds to DN 100, but the pipe has an actual outside diameter of 4.500 inches or 114.3 mm.

Beginning at NPS 14, the NPS designation generally equals the pipe outside diameter in inches. An NPS 14 pipe therefore has an outside diameter of 14 inches, and an NPS 24 pipe has an outside diameter of 24 inches.

The chart only identifies the nominal size and outside diameter. It does not specify:

Pipe wall thickness
Inside diameter
Pipe schedule
Pressure rating
Material grade
Flange pressure class
Fitting dimensions

These properties must be checked separately using the applicable pipe or component standard.

DN Values Are Nominal Designations

DN values should not be followed by a unit because they are nominal size designations. The correct notation is:

DN 15
DN 50
DN 100
DN 300

Writing “DN 50 mm” can be misleading because a DN 50 pipe does not have an actual diameter of exactly 50 mm. Its standard outside diameter is normally 60.3 mm.

4. How to Convert NPS to DN

NPS-to-DN conversion is based on standardized equivalent sizes. For most common pipe sizes, a quick approximate conversion can be made by multiplying the NPS value by 25. However, the final result must be matched to a recognized DN designation.

The approximate formula is:

$D N \approx NPS \times 25$

This formula is useful for remembering many common sizes, but it should not replace an official conversion chart.

Example 1: Convert NPS 2 to DN

Using the approximate formula:

$D N \approx 2 \times 25 = 50$

Therefore:

$NPS 2 = DN 50$

The actual outside diameter of this pipe is 60.3 mm.

Example 2: Convert NPS 4 to DN

$D N \approx 4 \times 25 = 100$

Therefore:

$NPS 4 = DN 100$

The actual outside diameter is 114.3 mm.

Example 3: Convert NPS 8 to DN

$D N \approx 8 \times 25 = 200$

Therefore:

$NPS 8 = DN 200$

The actual outside diameter is 219.1 mm.

Converting Fractional NPS Sizes

The multiplication method becomes less reliable for fractional and certain intermediate NPS sizes. The calculated value must be rounded to the standardized DN designation.

For example, for NPS 1/2:

$0.5 \times 25 = 12.5$

However, the standardized equivalent is:

$NPS 1/2 = DN 15$

Similarly:

NPS	Approximate Calculation	Standard DN
1/8	0.125 × 25 = 3.125	DN 6
1/4	0.25 × 25 = 6.25	DN 8
3/8	0.375 × 25 = 9.375	DN 10
1/2	0.5 × 25 = 12.5	DN 15
3/4	0.75 × 25 = 18.75	DN 20
1 1/4	1.25 × 25 = 31.25	DN 32
1 1/2	1.5 × 25 = 37.5	DN 40
2 1/2	2.5 × 25 = 62.5	DN 65
3 1/2	3.5 × 25 = 87.5	DN 90

Therefore, the approximate formula should only be used as a memory aid. A standardized conversion chart is the most reliable method.

Converting DN Back to NPS

For many common sizes, DN can be approximately converted to NPS by dividing the DN value by 25:

$NPS \approx 25 D N$

For example:

$NPS \approx 25100 = 4$

Therefore:

$DN 100 = NPS 4$

This method works well for common whole-number sizes but may not give the correct standardized result for smaller or intermediate sizes. For example:

$25 D N 15 = 0.6$

However, DN 15 corresponds to NPS 1/2, not NPS 0.6.

Recommended Conversion Procedure

To convert NPS to DN correctly:

Identify the NPS designation shown on the drawing, specification, pipe, valve, or fitting.
Find the corresponding DN value in a standardized conversion chart.
Verify the pipe’s actual outside diameter.
Check the wall thickness or pipe schedule separately.
Confirm that the associated components comply with compatible dimensional standards.

Converting the nominal size alone does not confirm that two piping components are interchangeable. Flanges, valves, and fittings must also be checked for pressure rating, connection dimensions, materials, facing type, and applicable standards.

5. NPS, DN, and Actual Pipe Diameter

NPS and DN are nominal size designations, while the outside diameter and inside diameter are actual physical pipe measurements. These values should not be treated as interchangeable.

A pipe has three important diameter-related dimensions:

Nominal size: The standardized NPS or DN designation.
Outside diameter: The measured distance across the external pipe surface.
Inside diameter: The measured internal opening through which fluid flows.

For example, an NPS 4 pipe corresponds to DN 100, but its actual outside diameter is 4.500 inches or 114.3 mm.

Nominal Size	Actual Outside Diameter
NPS 1/2 / DN 15	21.3 mm
NPS 1 / DN 25	33.4 mm
NPS 2 / DN 50	60.3 mm
NPS 3 / DN 80	88.9 mm
NPS 4 / DN 100	114.3 mm
NPS 6 / DN 150	168.3 mm
NPS 8 / DN 200	219.1 mm
NPS 10 / DN 250	273.0 mm
NPS 12 / DN 300	323.9 mm

Outside Diameter

The outside diameter, commonly abbreviated as OD, is the distance across the outside of the pipe. It is a critical dimension because it determines compatibility with fittings, flanges, supports, clamps, and joining equipment.

For a given NPS or DN, the outside diameter normally remains constant regardless of the pipe schedule. Therefore, NPS 4 Schedule 10, Schedule 40, and Schedule 80 pipes all have the same outside diameter of 114.3 mm.

What changes between schedules is the wall thickness and, consequently, the inside diameter.

Inside Diameter

The inside diameter, or ID, depends on both the outside diameter and wall thickness:

$I D = O D - 2 t$

Where:

$I D$ = inside diameter
$O D$ = outside diameter
$t$ = pipe wall thickness

For an NPS 4 pipe with an outside diameter of 4.500 inches:

Schedule 40 wall thickness is approximately 0.237 inches.
Schedule 80 wall thickness is approximately 0.337 inches.

The corresponding inside diameters are:

$I D_{S c h 40} = 4.500 - (2 \times 0.237) = 4.026 in.$ $I D_{S c h 80} = 4.500 - (2 \times 0.337) = 3.826 in.$

Although both pipes are NPS 4 or DN 100, the Schedule 80 pipe has a smaller internal flow area.

Why Actual Diameter Matters

Actual pipe dimensions must be verified when performing:

Flow and pressure-drop calculations
Pipe support and clamp selection
Welding preparation
Mechanical connection design
Insulation quantity calculations
Corrosion allowance evaluation
Pipe weight calculations
Equipment nozzle matching

NPS and DN are sufficient for general size identification, but detailed engineering calculations require actual dimensions from the applicable standard.

6. NPS and DN Pipe Schedule Comparison

A pipe schedule is a standardized designation for wall thickness. It is used together with NPS or DN to provide a more complete pipe specification.

Typical pipe schedules include:

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

The letter S is commonly used for stainless steel pipe dimensions specified under ASME B36.19M. Carbon and alloy steel pipe dimensions are generally covered by ASME B36.10M.

Effect of Pipe Schedule on Dimensions

For a given nominal size, increasing the pipe schedule generally increases wall thickness. Because the outside diameter remains constant, a thicker wall produces a smaller inside diameter.

The following comparison uses NPS 2, equivalent to DN 50:

Nominal Size	Schedule	Outside Diameter	Wall Thickness	Approximate Inside Diameter
NPS 2 / DN 50	Sch 10	60.3 mm	2.77 mm	54.76 mm
NPS 2 / DN 50	Sch 40	60.3 mm	3.91 mm	52.48 mm
NPS 2 / DN 50	Sch 80	60.3 mm	5.54 mm	49.22 mm
NPS 2 / DN 50	Sch 160	60.3 mm	8.74 mm	42.82 mm

The inside diameter is calculated as:

$I D = O D - 2 t$

For NPS 2 Schedule 80:

$I D = 60.3 - (2 \times 5.54) = 49.22 mm$

This demonstrates why DN 50 should not be interpreted as an exact 50 mm inside or outside diameter. Depending on the schedule, the actual inside diameter may be greater than, close to, or less than 50 mm.

NPS and DN Do Not Define Wall Thickness

The designations NPS 2 and DN 50 identify only the nominal pipe size. A complete specification must include the wall-thickness designation.

Examples include:

NPS 2 Schedule 40
NPS 4 Schedule 80
DN 50 Schedule 40
DN 100 Schedule 10S

A complete engineering specification may also include the material and applicable standard, such as:

ASTM A106 Grade B, seamless pipe, NPS 4, Schedule 40, ASME B36.10M

For stainless steel:

ASTM A312 TP316L, seamless pipe, NPS 2, Schedule 10S, ASME B36.19M

Schedule Numbers Are Not Direct Thickness Measurements

A schedule number does not directly equal the wall thickness. For example, Schedule 40 does not mean that the pipe wall is 40 mm or 0.040 inches thick.

The actual wall thickness depends on the nominal pipe size. An NPS 2 Schedule 40 pipe and an NPS 8 Schedule 40 pipe have different wall thicknesses even though both use the same schedule designation.

DN and Metric Wall-Thickness Systems

DN may be used with several wall-thickness designation systems, depending on the governing standard. These may include:

ASME schedule numbers
ISO pipe series
EN wall-thickness series
Standard dimension ratio, or SDR
Specified wall thickness in millimetres

Therefore, DN 100 alone does not provide enough information to determine the pipe’s inside diameter or pressure capability. The governing dimensional standard and wall-thickness designation must also be stated.

7. Common NPS and DN Sizes for Pipes and Fittings

Pipes, valves, flanges, and fittings are manufactured in standardized nominal sizes to allow components within the same piping system to be connected. Each common NPS designation has a corresponding DN designation.

Common Small-Bore Pipe Sizes

Small-bore piping is frequently used for instrument connections, drains, vents, utility services, sampling systems, and process branch lines.

NPS	DN	Common Applications
1/8	DN 6	Instrument and pressure connections
1/4	DN 8	Sample lines and small utility connections
3/8	DN 10	Instrument and small process lines
1/2	DN 15	Instrument air, drains, vents, and utilities
3/4	DN 20	Utility and small process lines
1	DN 25	Water, air, steam, and process branches
1 1/4	DN 32	Utility and process piping
1 1/2	DN 40	Process branches and equipment connections
2	DN 50	Process, drain, and utility piping

Small-bore pipe should not be confused with tubing. Pipe is normally specified by NPS or DN and schedule, while tubing is typically specified by its actual outside diameter and wall thickness.

For example:

NPS 1/2 pipe has an outside diameter of 21.3 mm.
A 1/2-inch tube has an outside diameter of exactly 12.7 mm.

These components are not dimensionally interchangeable.

Common Process-Piping Sizes

Medium pipe sizes are widely used in industrial process systems, cooling-water networks, compressed-air systems, chemical services, and equipment connections.

NPS	DN	Outside Diameter
2 1/2	DN 65	73.0 mm
3	DN 80	88.9 mm
4	DN 100	114.3 mm
5	DN 125	141.3 mm
6	DN 150	168.3 mm
8	DN 200	219.1 mm
10	DN 250	273.0 mm
12	DN 300	323.9 mm

NPS 5 and some intermediate sizes are standardized but less commonly used than NPS 4, NPS 6, or NPS 8. Designers may prefer more readily available sizes when permitted by the required flow capacity and project specifications.

Common Large-Bore Pipe Sizes

Large-bore piping is commonly used for cooling water, firewater, seawater, drainage, transmission, and high-capacity process services.

NPS	DN	Outside Diameter
14	DN 350	355.6 mm
16	DN 400	406.4 mm
18	DN 450	457.2 mm
20	DN 500	508.0 mm
24	DN 600	609.6 mm
30	DN 750	762.0 mm
36	DN 900	914.4 mm
40	DN 1000	1,016.0 mm
48	DN 1200	1,219.2 mm

For NPS 14 and larger, the NPS number generally matches the actual pipe outside diameter in inches. However, the corresponding DN designation remains nominal.

Matching Pipes and Fittings

A fitting marked NPS 4 is normally intended for an NPS 4 pipe. Similarly, a DN 100 fitting is intended for a DN 100 piping system. However, the connection type and governing standard must also match.

Common pipe fittings include:

Elbows
Tees
Reducers
Caps
Couplings
Unions
Flanges
Branch fittings

For welded fittings, verify the matching outside diameter and wall thickness. For threaded fittings, check the thread standard and pressure class. For flanged fittings, confirm the flange dimensions, pressure designation, facing, and bolt pattern.

Flange and Valve Size Compatibility

NPS and DN indicate the nominal bore of a flange or valve but do not fully define its connection dimensions.

For example:

NPS 4 Class 150 is an ASME flange designation.
DN 100 PN 16 is commonly associated with an EN flange designation.

Although both represent approximately the same nominal size, their dimensions and drilling arrangements may differ. Before connecting pipes, valves, or fittings from different standard systems, verify all applicable component dimensions rather than relying only on the NPS-to-DN conversion.

8. Applicable NPS and DN Standards

NPS and DN designations are used across several piping standards. The applicable standard determines actual pipe dimensions, wall thicknesses, tolerances, flange geometry, fitting dimensions, and component pressure ratings.

ASME B36.10M

ASME B36.10M specifies dimensions for welded and seamless wrought steel pipe. It is commonly applied to carbon steel and alloy steel piping.

The standard includes:

Nominal pipe sizes
Standard outside diameters
Schedule numbers
Nominal wall thicknesses
Plain-end pipe weights
Permitted dimensional variations

Common schedules covered by the standard include Schedule 10, 20, 30, 40, 60, 80, 100, 120, 140, and 160. However, not every schedule is available for every nominal size.

ASME B36.19M

ASME B36.19M specifies dimensions for stainless steel pipe. It uses the same general NPS system but includes stainless steel schedule designations such as:

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

The suffix “S” identifies schedule designations associated with stainless steel pipe dimensions. Some wall thicknesses may coincide with those in ASME B36.10M, but the applicable standard should still be stated in the pipe specification.

ASME B16 Piping Component Standards

The ASME B16 series provides dimensional and rating requirements for many piping components used with NPS pipe. Important examples include:

ASME B16.5: Pipe flanges and flanged fittings
ASME B16.9: Factory-made wrought buttwelding fittings
ASME B16.10: Face-to-face and end-to-end dimensions of valves
ASME B16.11: Forged socket-welding and threaded fittings
ASME B16.34: Valves with flanged, threaded, and welding ends
ASME B16.47: Large-diameter steel flanges

These standards use nominal pipe size together with other designations such as schedule, pressure class, material group, and connection type.

ISO 6708

ISO 6708 defines DN as an alphanumeric designation of size for components used in piping systems. The designation consists of the letters DN followed by a dimensionless whole number.

Examples include:

DN 15
DN 50
DN 100
DN 300

DN is indirectly related to the component bore or connection size, but it is not an exact measured diameter. The DN number should not be followed by “mm.”

ISO 4200

ISO 4200 provides dimensions and masses per unit length for plain-end welded and seamless steel tubes. It includes standardized outside diameters and preferred wall thicknesses for steel piping.

ISO 4200 can be used when selecting metric pipe dimensions, but the project specification must still define the required material, manufacturing method, tolerance, and design standard.

EN Pipe and Component Standards

European piping systems commonly use DN designations together with EN standards. Examples include:

EN 10220: Dimensions and masses per unit length of steel tubes
EN 10216: Seamless steel tubes for pressure purposes
EN 10217: Welded steel tubes for pressure purposes
EN 1092-1: Circular steel flanges
EN 10253: Buttwelding pipe fittings
EN 12516: Industrial valve shell design strength

A component specified according to an EN standard may have the same DN designation as an ASME component converted from NPS. However, identical nominal size does not guarantee identical connection dimensions.

Material Standards vs. Dimensional Standards

Material standards and dimensional standards perform different functions.

A material or product specification may define:

Chemical composition
Mechanical properties
Manufacturing method
Heat treatment
Testing requirements
Certification requirements

A dimensional standard defines properties such as:

Outside diameter
Wall thickness
End-to-end dimensions
Flange dimensions
Permitted tolerances

For example, a carbon steel pipe may be specified as:

ASTM A106 Grade B, NPS 4, Schedule 40, seamless, dimensions according to ASME B36.10M

Here, ASTM A106 defines the pipe material and product requirements, while ASME B36.10M defines its dimensional characteristics.

Avoid Mixing Standards Without Verification

A direct NPS-to-DN conversion only identifies a corresponding nominal size. It does not confirm that components manufactured under different standards can be connected.

Before mixing ASME, ISO, or EN components, verify:

Outside diameter
Wall thickness
End preparation
Flange outside diameter
Bolt-circle diameter
Number and diameter of bolt holes
Flange facing
Pressure-temperature rating
Material compatibility
Gasket dimensions

This verification is particularly important for flanged piping systems because an ASME Class flange and an EN PN flange may have different drilling patterns even when their nominal sizes appear equivalent.

9. How to Select the Correct NPS or DN Size

Selecting the correct nominal pipe size involves more than converting between NPS and DN. The pipe must provide the required flow capacity while satisfying pressure, temperature, mechanical, material, and connection requirements.

Determine the Governing Piping Standard

Begin by identifying the standards required by the project, customer, plant, or local regulation.

Use NPS when the system is primarily based on:

ASME piping codes
ASTM pipe specifications
ASME B16 components
Inch-based equipment connections

Use DN when the system is primarily based on:

ISO standards
EN standards
DIN standards
Metric equipment and component specifications

A project should use one consistent designation system whenever practical. If both NPS and DN are shown, the drawings and specifications should clearly state which system governs.

Calculate the Required Flow Diameter

The nominal pipe size should provide sufficient internal flow area for the required flow rate. The preliminary inside diameter can be estimated from the continuity equation:

$Q = v A$

For a circular pipe:

$A = 4 π D ^{2}$

Therefore:

$D = π v 4 Q$

Where:

$Q$ = volumetric flow rate
$v$ = selected fluid velocity
$A$ = internal flow area
$D$ = required inside diameter

After calculating the required diameter, select the next suitable standard NPS or DN size. The final selection should be verified using the actual inside diameter for the chosen schedule.

Check Pressure Drop

A pipe with a smaller inside diameter produces higher fluid velocity and generally greater pressure loss. A larger pipe reduces pressure drop but increases material, installation, insulation, and support costs.

Pressure-drop calculations should consider:

Pipe length
Actual inside diameter
Fluid density and viscosity
Flow rate
Pipe roughness
Valves and fittings
Elevation change
Operating temperature
Two-phase or compressible-flow effects

The nominal pipe size should not be used directly as the internal diameter in hydraulic calculations.

Select the Required Wall Thickness

Once the nominal size has been selected, determine the required wall thickness based on:

Design pressure
Design temperature
Pipe material
Allowable stress
Corrosion allowance
Mechanical allowance
Manufacturing tolerance
Applicable piping code
External loads
Vacuum or external pressure

The calculated minimum wall thickness is then matched to a suitable pipe schedule or specified metric wall thickness.

A higher schedule provides a thicker wall, but it also reduces the pipe’s inside diameter. Flow calculations may therefore need to be repeated after the schedule is selected.

Match the Equipment Connection Size

Pumps, vessels, heat exchangers, control valves, instruments, and other equipment normally have predefined connection sizes and standards.

When connecting piping to equipment, verify:

Nozzle NPS or DN
Flange standard
Pressure class or PN rating
Flange facing
Thread type
Welding-end dimensions
Allowable nozzle loads
Required reducers or expanders

A DN 100 equipment nozzle should not automatically be assumed to match every NPS 4 flange. The applicable flange standards must be compared.

Verify Component Availability

Some nominal sizes are less readily available than adjacent sizes. Before finalizing a design, confirm the local availability of:

Pipe
Elbows and tees
Reducers
Flanges
Valves
Gaskets
Bolting
Supports
Welding consumables

For example, NPS 5 or DN 125 is standardized but may be less commonly stocked than NPS 4 or NPS 6. Selecting a commonly available size can reduce cost and delivery time when the design requirements allow it.

Check Pressure Rating and Compatibility

NPS and DN do not indicate a component’s pressure rating. Pressure capacity is defined separately through pipe wall thickness, material properties, component design, and rating systems such as:

ASME pressure classes: Class 150, 300, 600, 900, 1500, and 2500
European PN ratings: PN 6, 10, 16, 25, 40, and others

ASME Class and PN numbers are not direct equivalents. Their permissible pressure depends on material and temperature, so they should not be converted using a simple numerical formula.

Final Selection Checklist

Before approving an NPS or DN size, confirm:

The required flow rate and design velocity
Acceptable pressure drop
Actual pipe inside diameter
Design pressure and temperature
Required wall thickness or schedule
Corrosion and erosion allowance
Pipe material
Applicable pipe standard
Fitting and valve availability
Equipment nozzle compatibility
Flange standard and rating
Joining method
Maintenance and inspection requirements

The selected nominal size should satisfy the complete piping design rather than only matching an NPS-to-DN conversion chart.

Conclusion

NPS and DN are two standardized systems used to identify nominal pipe and piping-component sizes. NPS is associated primarily with inch-based ASME piping systems, while DN is widely used in ISO, EN, and other metric-based systems.

The two designations can be matched using a standardized conversion chart. For example, NPS 2 corresponds to DN 50, NPS 4 corresponds to DN 100, and NPS 8 corresponds to DN 200. However, these values are nominal designations and do not represent exact measured diameters.

A correct conversion must therefore distinguish among:

Nominal pipe size
Actual outside diameter
Actual inside diameter
Wall thickness or schedule
Component dimensional standard
Pressure rating

An NPS to DN conversion confirms only the corresponding nominal size. It does not automatically establish that pipes, flanges, fittings, or valves made to different standards are interchangeable. Actual dimensions, pressure ratings, connection details, and governing standards must always be verified before components are selected or connected.

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