DN Pipe Size Chart: DN to NPS Conversion and Dimensions

7. How to Read and Use a DN Pipe Size Chart

A DN pipe size chart connects the nominal pipe designation with the physical dimensions needed for design, purchasing, fabrication, installation, and inspection.

To use the chart correctly, identify the DN size and then determine:

• Equivalent NPS
• Actual outside diameter
• Pipe schedule
• Wall thickness
• Approximate inside diameter
• Applicable dimensional standard

Step 1: Identify the DN Size

Start with the DN designation shown in the piping document or marked on the component.

The DN size may be found on:

• Piping and instrumentation diagrams
• Piping specifications
• Equipment datasheets
• Isometric drawings
• General arrangement drawings
• Bills of materials
• Valve and flange markings
• Equipment nozzles

Assume that a piping specification requires a DN 80 steel pipe.

Step 2: Find the Equivalent NPS

Use the DN-to-NPS conversion chart to find the corresponding inch-based nominal size.

For this example:

DN 80 = NPS 3

This conversion is helpful when purchasing pipes or components manufactured according to NPS-based standards.

Step 3: Find the Actual Outside Diameter

Locate the actual outside diameter in the chart.

For standard DN 80 steel pipe:

OD = 88.9 mm

The actual OD is used to select or design:

• Pipe supports
• Clamps
• Welding fittings
• Mechanical couplings
• Flanges
• Insulation
• Equipment openings
• Installation clearances

Do not use 80 mm as the actual OD of DN 80 steel pipe.

Step 4: Identify the Pipe Schedule

The DN size and outside diameter do not reveal the pipe wall thickness. The schedule must be obtained from the piping specification, pipe marking, bill of materials, or design documentation.

Assume the pipe is specified as:

DN 80 Schedule 40

The corresponding nominal wall thickness is:

Wall thickness = 5.49 mm

Step 5: Calculate the Approximate Inside Diameter

Use the following WordPress-safe formula:

ID = OD − (2 × t)

Where:

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

For DN 80 Schedule 40:

ID = 88.9 − (2 × 5.49)

ID = 88.9 − 10.98

ID = 77.92 mm

Therefore, the approximate inside diameter is 77.92 mm.

DN 80 Schedule 40 Example

Step 6: Confirm the Pipe Material

A DN chart may provide standard steel pipe dimensions, but similarly named pipe sizes made from other materials may follow different outside-diameter systems.

Confirm whether the pipe is made from:

• Carbon steel
• Stainless steel
• Alloy steel
• PVC
• CPVC
• HDPE
• Copper
• Ductile iron
• Fibreglass-reinforced plastic

Do not assume that every DN 100 pipe has an OD of 114.3 mm. That value is associated with the standard NPS steel pipe series, while other pipe systems may use different dimensions.

Step 7: Check the Applicable Standard

The dimensional standard determines the recognized outside diameters, wall-thickness series, schedules, and tolerances.

Before using the chart, confirm:

• Product type
• Pipe material
• Governing dimensional standard
• Available schedule
• Wall-thickness tolerance
• Outside-diameter tolerance
• Unit system

This is especially important when combining pipes, fittings, flanges, and valves from different standards.

Step 8: Verify the Complete Pipe Specification

A complete pipe selection requires more than DN and schedule. Depending on the application, the specification may need to include:

• DN or NPS
• Pipe schedule or wall thickness
• Material specification
• Material grade
• Seamless or welded construction
• End preparation
• Corrosion allowance
• Design pressure
• Design temperature
• Inspection and testing requirements
• Applicable design code

An example of a more complete description is:

DN 100, Schedule 40, ASTM A106 Grade B, seamless, bevelled ends

Avoid Confusing DN, OD, and ID

A frequent mistake is treating the DN value as a measured pipe diameter.

For example, DN 100 steel pipe normally has:

• DN designation = DN 100
• Equivalent nominal size = NPS 4
• Actual OD = 114.3 mm
• ID = dependent on wall thickness

Use each value for its intended purpose:

8. How to Measure and Identify DN Pipe Size

The most reliable way to identify the DN size of a standard pipe is to measure its outside diameter and compare the result with a DN pipe size chart.

Inside diameter alone should not normally be used because it changes with pipe schedule, corrosion, lining thickness, and manufacturing tolerances.

Tools for Measuring Pipe Size

Common measuring tools include:

• Vernier caliper
• Digital caliper
• Outside micrometer
• Diameter tape
• Flexible measuring tape
• Ultrasonic thickness gauge
• Pipe wall-thickness gauge

A caliper is suitable for small and accessible pipes. A diameter tape is useful for larger installed pipes. An ultrasonic thickness gauge may be required when the pipe end is unavailable.

Method 1: Measure the Outside Diameter Directly

If the full pipe diameter is accessible, use a caliper or outside micrometer.

Follow these steps:

1. Remove loose rust, dirt, insulation residue, and thick paint.
2. Position the measuring tool perpendicular to the pipe axis.
3. Measure across the centre of the pipe.
4. Repeat the measurement at several angular positions.
5. Compare the measured OD with a DN chart.

Example:

If the measured outside diameter is approximately 60.3 mm:

OD 60.3 mm = DN 50 = NPS 2

Other common examples include:

Method 2: Measure the Pipe Circumference

For an installed pipe where direct OD measurement is difficult, wrap a flexible tape around the pipe and measure its outside circumference.

Use this WordPress-safe formula:

OD = Circumference ÷ 3.1416

Abbreviated form:

OD = C ÷ 3.1416

Where:

• OD = outside diameter
• C = measured outside circumference
• 3.1416 = approximate value of pi

For example, suppose the measured circumference is 359.1 mm.

OD = 359.1 ÷ 3.1416

OD = 114.3 mm

An OD of approximately 114.3 mm corresponds to:

DN 100 = NPS 4

Method 3: Calculate Circumference From a Known OD

If the outside diameter is known, its approximate circumference can be calculated using:

Circumference = Outside diameter × 3.1416

Abbreviated form:

C = OD × 3.1416

For DN 50 steel pipe:

C = 60.3 × 3.1416

C = 189.44 mm

Therefore, a DN 50 steel pipe has an outside circumference of approximately 189.4 mm, excluding coatings and measurement tolerances.

Method 4: Use a Diameter Tape

A diameter tape converts circumference directly into diameter. When wrapped around the pipe, it displays the approximate OD without requiring a manual calculation.

For an accurate result:

• Keep the tape flat against the pipe.
• Position it perpendicular to the pipe axis.
• Avoid measuring over weld reinforcement.
• Remove loose scale and thick coatings.
• Pull the tape firmly without stretching it.
• Repeat the measurement at different positions.

Identify the Pipe Schedule

Measuring the outside diameter identifies the likely DN or NPS size, but it does not identify the pipe schedule.

Pipes with the same nominal size normally share the same OD. Their schedules are differentiated by wall thickness.

For example, DN 50 pipes may have:

If the pipe end is accessible, measure the wall thickness directly with a caliper. If the pipe is installed, an ultrasonic thickness gauge may be required.

Calculate Wall Thickness From OD and ID

When both the outside and inside diameters are known, wall thickness can be estimated using:

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

Abbreviated form:

t = (OD − ID) ÷ 2

For example, suppose a pipe has:

• OD = 60.3 mm
• ID = 52.48 mm

Calculation:

t = (60.3 − 52.48) ÷ 2

t = 7.82 ÷ 2

t = 3.91 mm

A DN 50 pipe with a nominal wall thickness of 3.91 mm is normally Schedule 40.

Check the Pipe Marking

New pipes are usually marked with identifying information. Depending on the product standard and manufacturer, the marking may include:

• Manufacturer’s name or symbol
• NPS or nominal size
• Schedule or wall thickness
• Material specification
• Material grade
• Seamless or welded construction
• Heat number
• Manufacturing method
• Inspection mark

For example:

4 SCH 40 ASTM A106 GR B SMLS

This marking generally means:

• NPS 4
• DN 100 equivalent
• Schedule 40
• ASTM A106 Grade B
• Seamless construction

Consider Coating Thickness

Paint, galvanizing, wrapping, fireproofing, insulation residue, and other coatings can increase the measured outside diameter.

For accurate identification:

• Measure a clean, representative section.
• Remove loose scale and corrosion products.
• Avoid measuring over weld seams.
• Deduct known coating thickness when necessary.
• Take measurements at several locations.

If a coating of thickness c covers the entire pipe, the approximate bare pipe OD may be calculated as:

Bare pipe OD = Measured coated OD − (2 × Coating thickness)

Abbreviated form:

Bare OD = Coated OD − (2 × c)

This calculation is only useful when the coating thickness is known and reasonably uniform.

Account for Corrosion and Ovality

An in-service pipe may no longer match its original nominal dimensions exactly because of:

• External corrosion
• Internal corrosion
• Erosion
• Mechanical deformation
• Manufacturing ovality
• Scale or deposit buildup

Measure at several locations and directions. Do not determine the original schedule solely from one remaining-wall measurement on a used pipe.

Do Not Identify DN From ID Alone

Inside diameter is an unreliable standalone indicator of nominal pipe size because it varies with:

• Pipe schedule
• Wall-thickness tolerance
• Internal lining
• Corrosion
• Erosion
• Deposits
• Product material and standard

The recommended identification sequence is:

1. Measure the outside diameter.
2. Match the OD to a DN or NPS chart.
3. Measure the wall thickness.
4. Compare the wall thickness with a schedule chart.
5. Check pipe markings and documentation.
6. Verify the applicable material and product standard.

9. How to Select the Correct DN Pipe Size

Selecting the correct DN pipe size requires more than matching the connection size of a valve, pump, or piece of equipment. The pipe must provide adequate flow while meeting pressure, temperature, velocity, mechanical, material, and installation requirements.

The selection process should consider:

• Required flow rate
• Allowable fluid velocity
• Acceptable pressure drop
• Fluid properties
• Pipe schedule
• Design pressure
• Design temperature
• Material compatibility
• Corrosion allowance
• Connection standard
• Future operating requirements

Step 1: Determine the Required Flow Rate

The required flow rate is one of the main inputs for pipe sizing.

Depending on the system, flow may be expressed in:

• Cubic metres per hour
• Litres per minute
• Litres per second
• Cubic feet per minute
• Gallons per minute
• Kilograms per hour

The design flow should include credible maximum operating conditions rather than only normal flow.

Step 2: Select an Acceptable Fluid Velocity

Once the flow rate is known, choose an appropriate design velocity.

Excessive velocity may cause:

• High pressure drop
• Noise
• Vibration
• Erosion
• Water hammer
• Increased energy consumption

Very low velocity may cause:

• Solids deposition
• Poor transport of suspended material
• Oversized piping
• Higher installation cost
• Longer system response time

Acceptable velocity depends on the fluid, service, pipe material, pressure, and applicable design practice.

Step 3: Estimate the Required Inside Diameter

The relationship between flow rate, velocity, and flow area is:

Flow rate = Flow area × Fluid velocity

Abbreviated form:

Q = A × v

Where:

• Q = volumetric flow rate
• A = internal flow area
• v = average fluid velocity

Rearranging the formula:

Flow area = Flow rate ÷ Fluid velocity

Abbreviated form:

A = Q ÷ v

For a circular pipe:

Flow area = 3.1416 × Inside diameter × Inside diameter ÷ 4

Abbreviated form:

A = 3.1416 × ID × ID ÷ 4

The required inside diameter can therefore be calculated using:

ID = Square root of [(4 × Q) ÷ (3.1416 × v)]

All units must be consistent. If flow rate is expressed in cubic metres per second and velocity in metres per second, the calculated ID will be in metres.

Pipe Sizing Example

Assume that a water line requires:

• Flow rate = 0.01 cubic metre per second
• Selected velocity = 2 metres per second

First, calculate the required flow area:

A = Q ÷ v

A = 0.01 ÷ 2

A = 0.005 square metre

Next, calculate the required inside diameter:

ID = Square root of [(4 × 0.01) ÷ (3.1416 × 2)]

ID = Square root of (0.006366)

ID = 0.0798 metre

Convert the result to millimetres:

0.0798 metre = 79.8 mm

The pipe therefore requires an inside diameter of approximately 79.8 mm at the selected velocity.

A standard DN size and schedule must then be selected with an actual ID suitable for the calculated requirement. The next larger standard size may be necessary after considering pressure drop and wall thickness.

Step 4: Check the Actual Inside Diameter

DN is only a nominal designation. Flow calculations must use the actual inside diameter associated with the selected pipe size and schedule.

For example, two DN 100 pipes may have different IDs:

• DN 100 Schedule 40: approximate ID of 102.26 mm
• DN 100 Schedule 80: approximate ID of 97.18 mm

The thicker Schedule 80 wall reduces the flow area and increases pressure drop compared with Schedule 40.

Step 5: Calculate the Pressure Drop

After selecting a preliminary DN size, calculate the pressure drop through:

• Straight pipe
• Elbows
• Tees
• Reducers
• Valves
• Strainers
• Flowmeters
• Flexible hoses
• Equipment connections

Pressure drop depends on:

• Pipe length
• Inside diameter
• Flow rate
• Fluid density
• Fluid viscosity
• Surface roughness
• Fitting quantity
• Valve type and position
• Flow regime

If the pressure drop is too high, a larger DN size may be required.

Step 6: Determine the Required Wall Thickness

Pipe size and pipe schedule are related but separate selections.

The required wall thickness depends on:

• Design pressure
• Design temperature
• Material allowable stress
• Pipe outside diameter
• Manufacturing tolerance
• Corrosion allowance
• Erosion allowance
• Joint quality factor
• Applicable piping code

The selected schedule must provide at least the minimum required wall thickness after all applicable allowances are considered.

A preliminary selection based only on flow may provide sufficient internal diameter but insufficient pressure resistance.

Step 7: Confirm Material Compatibility

The pipe material must be suitable for the process fluid and operating environment.

Check compatibility with:

• Process fluid
• Cleaning chemicals
• Atmospheric exposure
• Chlorides
• Acids and alkalis
• Temperature
• External corrosion
• Galvanic effects
• Required cleanliness level

Common pipe materials include:

• Carbon steel
• Stainless steel
• Alloy steel
• Copper
• PVC
• CPVC
• HDPE
• Ductile iron
• Fibreglass-reinforced plastic

Different materials may use different dimensional systems even when they have the same DN designation.

Step 8: Verify Connections and Components

Every component in the piping system must match the selected nominal size and applicable connection standard.

Verify:

• Valve DN or NPS
• Flange standard
• PN rating or ASME class
• Flange facing
• Bolt pattern
• Fitting schedule or bore
• Equipment nozzle size
• Gasket dimensions
• Thread standard
• Welding end dimensions

A DN 100 valve cannot be selected based on size alone. Its pressure rating, face-to-face dimensions, connection standard, material, and bore must also meet the piping specification.

Step 9: Consider Installation Requirements

The selected pipe size must fit within the available installation space.

Consider:

• Routing clearances
• Pipe support spacing
• Insulation thickness
• Valve accessibility
• Welding clearance
• Thermal expansion
• Drainability
• Venting requirements
• Maintenance access
• Equipment removal space

A larger DN size may reduce pressure drop but increase cost, weight, support requirements, and installation space.

Step 10: Consider Future Capacity

Where justified, the pipe may include reasonable capacity for:

• Future equipment
• Process expansion
• Increased production
• Temporary peak demand
• Normal fouling
• Gradual internal roughness increase

However, unnecessary oversizing may increase:

• Material cost
• Installation cost
• Fluid inventory
• Heat loss
• Residence time
• Drainage difficulty

Future allowance should be based on realistic project requirements rather than an arbitrary increase in pipe size.

Final DN Pipe Selection Checklist

Before finalizing the DN pipe size, confirm:

• Required design flow
• Acceptable velocity
• Calculated inside diameter
• Selected DN and NPS
• Actual ID for the selected schedule
• Total system pressure drop
• Required wall thickness
• Design pressure and temperature
• Material compatibility
• Corrosion allowance
• Dimensional standard
• Connection compatibility
• Installation space
• Applicable piping code

The correct DN size is the smallest practical standard pipe size that safely provides the required flow, acceptable pressure drop, suitable velocity, adequate mechanical strength, and compatibility with the complete piping system.