NPS Pipe Size Chart: Nominal Sizes & Dimensions

Choosing the correct pipe size is easier when you understand NPS (Nominal Pipe Size), outside diameter, inside diameter, wall thickness, and pipe schedule. One of the most common mistakes is assuming that the NPS number equals the pipe’s actual outside or inside diameter. In many sizes, it does not.

This NPS pipe size chart provides a practical reference for standard nominal sizes, DN equivalents, outside diameters, and common Schedule 40 dimensions. It also explains how NPS relates to pipe schedules, how to measure an existing pipe, and how to select the right size for a piping application.

The dimensional information is based primarily on the standard pipe dimensions associated with ASME B36.10M for welded and seamless wrought steel pipe. ASME notes that NPS 12 and smaller pipe sizes have outside diameters numerically larger than their nominal designations.

What Is NPS in Pipe Size?

NPS stands for Nominal Pipe Size. It is a standardized, dimensionless designation used primarily in North America to identify pipe size.

For example:

• NPS 1/2 means nominal 1/2-inch pipe.
• NPS 2 means nominal 2-inch pipe.
• NPS 6 means nominal 6-inch pipe.
• NPS 12 means nominal 12-inch pipe.
• NPS 24 means nominal 24-inch pipe.

The important point is that NPS is a nominal designation, not a direct measurement of the pipe’s actual diameter.

A nominal 2-inch steel pipe, for example, has an actual outside diameter of approximately 2.375 inches (60.3 mm), not 2 inches. Likewise, a nominal 4-inch pipe has an outside diameter of 4.500 inches (114.3 mm).

This system allows pipes, fittings, flanges, and other components to be standardized so that compatible components can be selected without relying on the actual inside diameter.

Why Is the NPS Number Different From the Actual Diameter?

Historically, pipe sizing was associated with approximate internal dimensions and standard wall thicknesses. As manufacturing practices developed, the outside diameter became standardized for many pipe sizes while different wall thicknesses could be produced.

Consequently, the same NPS can have different inside diameters depending on its schedule or wall thickness.

For example, an NPS 2 pipe can have:

• The same nominal size
• The same standard outside diameter
• Different wall thicknesses
• Different inside diameters
• Different weights and pressure capabilities

That distinction is essential when selecting pipe for an engineering or installation project.

NPS Pipe Size Chart: Standard Sizes and Outside Diameters

The following chart provides common NPS designations, their approximate DN equivalents, and standard outside diameters. The outside diameter is especially important when selecting fittings, clamps, flanges, and other components.

The relationship between nominal size and actual OD changes notably around NPS 14. For NPS 12 and smaller, the actual outside diameter is generally larger than the nominal designation. Beginning with NPS 14, the nominal size and outside diameter are numerically the same in inches.

NPS vs DN: What Is the Difference?

NPS and DN identify similar pipe-size concepts, but they come from different sizing systems.

NPS is the North American nominal pipe size system and is normally expressed in inches, although the designation itself is nominal rather than a direct dimensional measurement.

DN, or Diamètre Nominal, is the nominal size designation commonly used in international metric-oriented standards and documentation.

For example:

The DN number should not be treated as an exact metric conversion of NPS. DN is itself a nominal designation.

For example, NPS 2 corresponds to DN 50, but DN 50 does not mean the pipe has an exact 50 mm inside or outside diameter.

NPS Pipe Dimensions: OD, ID, and Wall Thickness

Three dimensions are especially important when working with pipe:

• OD — Outside Diameter
• ID — Inside Diameter
• WT — Wall Thickness

These dimensions are related, but they do not mean the same thing.

The outside diameter is generally controlled by the nominal pipe size and applicable dimensional standard. The wall thickness is determined by the selected schedule or other specified wall designation. The inside diameter is then affected by the wall thickness.

A simplified relationship is:

Inside Diameter = Outside Diameter − 2 × Wall Thickness

For example, an NPS 2 Schedule 40 pipe has an outside diameter of approximately 2.375 inches and a wall thickness of approximately 0.154 inch.

Therefore:

ID ≈ 2.375 − (2 × 0.154) = 2.067 inches

This demonstrates why an NPS 2 pipe does not have a 2-inch internal bore.

Why Inside Diameter Matters

Inside diameter can be critical when calculating:

• Fluid flow
• Pressure drop
• Flow velocity
• Internal volume
• Hydraulic capacity
• Gas capacity
• Cleaning requirements
• Pigging or inspection access

For that reason, simply choosing an NPS based on the desired internal diameter may produce the wrong result. The pipe schedule and actual wall thickness must also be considered.

Schedule and NPS: How Pipe Wall Thickness Works

A pipe’s NPS tells you the nominal size, while the schedule identifies a standardized wall-thickness designation for many common steel pipe dimensions.

Common schedule designations include:

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

Other traditional designations include STD (Standard Weight), XS (Extra Strong), and XXS (Double Extra Strong).

For a given NPS, changing the schedule generally changes the wall thickness while the standard outside diameter remains the same. As wall thickness increases, the internal diameter decreases.

Schedule 40 NPS Pipe Dimensions

Schedule 40 is one of the most commonly referenced pipe schedules. The following chart provides useful dimensions for NPS sizes from 1/8 through 24.

The values above are standard reference dimensions for Schedule 40 steel pipe and are useful for general sizing and comparison. Schedule tables should always be checked against the applicable standard and material specification for a critical engineering application.

Schedule 40 vs Schedule 80 Pipe Sizes

A common question is whether Schedule 80 pipe has a larger outside diameter than Schedule 40 pipe.

For the same NPS, the standard outside diameter is generally the same. The major difference is wall thickness.

For example, NPS 2 has an OD of approximately 2.375 inches for both Schedule 40 and Schedule 80.

The exact available schedule and wall thickness can vary by NPS and material standard. The important principle is that a higher schedule generally means a thicker wall and smaller internal bore for the same nominal pipe size.

Schedule selection should not be based solely on the idea that “higher is better.” Wall thickness must be appropriate for pressure, temperature, corrosion allowance, mechanical loads, joining requirements, and the applicable design code.

Also Read:

• Bolt Size Chart: Standard Metric & SAE Bolt Sizes

How to Measure NPS Pipe Size

If the pipe designation is unknown, measuring the pipe can help identify its approximate NPS. However, measurement alone may not be enough because nominal pipe dimensions are standardized and can differ significantly from intuitive measurements.

Step 1: Measure the Outside Diameter

Use a caliper for small pipe or a suitable tape and measurement method for larger pipe.

Measure across the widest part of the pipe from one outside edge to the opposite outside edge.

For example, if the outside diameter is close to 2.375 inches, the pipe may be NPS 2.

Step 2: Measure Wall Thickness if Possible

If you can access an open end, measure the wall thickness directly with a suitable gauge or precision measuring instrument.

Alternatively, measure the inside diameter and compare it with the outside diameter.

Step 3: Compare the Measurement With an NPS Chart

Do not simply round the measured OD to the nearest inch.

For example:

• Approximately 1.315 inches OD → NPS 1
• Approximately 2.375 inches OD → NPS 2
• Approximately 4.500 inches OD → NPS 4
• Approximately 6.625 inches OD → NPS 6
• Approximately 8.625 inches OD → NPS 8

Comparing the measured dimension with a recognized dimensional table is much more reliable than assuming the nominal size from the measurement alone.

Also Read:

• Screw Size Chart: Standard Metric & SAE Screw Sizes

How to Choose the Correct NPS Pipe Size

Selecting pipe size involves more than matching the connection diameter. The correct NPS depends on the system’s functional and mechanical requirements.

Consider these factors:

1. Required Flow Rate

For liquid or gas systems, the required flow rate is a major factor.

A pipe that is too small can create excessive velocity and pressure loss. A pipe that is unnecessarily large may increase material, installation, support, and insulation costs.

2. Required Inside Diameter

The internal bore determines the available flow area.

Because different schedules have different wall thicknesses, two pipes with the same NPS may have different inside diameters.

3. Pressure and Temperature

The pipe must be suitable for the system’s operating pressure and temperature.

Wall thickness, material grade, allowable stress, corrosion allowance, joining method, and applicable piping code can all affect the final selection.

4. Material

NPS is a dimensional designation, not a material specification.

Pipe may be manufactured from materials such as:

• Carbon steel
• Stainless steel
• Alloy steel
• Cast iron
• Copper
• Plastic and other engineered materials

The applicable dimensional standard can differ by material. For example, ASME B36.10 covers welded and seamless wrought steel pipe, while ASME B36.19 covers welded and seamless wrought stainless steel pipe. ASME specifically notes differences between some B36.19M stainless schedules and B36.10M dimensions.

5. Connection and Fittings

The selected pipe must be compatible with the system’s fittings, valves, flanges, threads, couplings, or welding requirements.

The outside diameter is particularly important for components that fit around the pipe.

Also Read:

• Threaded Rod Size Chart: Standard Sizes & Dimensions

NPS Pipe Size vs Tube Size

Pipe and tube should not be treated as interchangeable sizing systems.

For standard pipe, NPS is a nominal designation and the outside diameter may not equal the nominal size. Tube dimensions, on the other hand, are commonly specified by actual outside diameter and wall thickness.

ASME explains that for NPS 12 and smaller, pipe outside diameters are numerically larger than the corresponding nominal sizes, whereas tube outside diameters are numerically identical to the size designation.

This distinction matters when selecting:

• Fittings
• Heat exchangers
• Structural components
• Instrument tubing
• Process piping
• Compression fittings

A component designed around a specific tube OD should not automatically be assumed to fit an NPS pipe with a similar nominal number.

Common NPS Pipe Size Mistakes

Understanding the following mistakes can prevent incorrect pipe selection.

Mistake 1: Assuming NPS Equals OD

An NPS 2 pipe does not have a 2-inch outside diameter. Its standard OD is approximately 2.375 inches.

Mistake 2: Assuming NPS Equals ID

The internal diameter changes with wall thickness. Therefore, NPS alone does not tell you the actual bore.

Mistake 3: Ignoring Pipe Schedule

Two NPS 2 pipes can have different wall thicknesses and internal diameters because they have different schedules.

Mistake 4: Treating DN as an Exact Conversion

DN 50 and NPS 2 are corresponding nominal designations, but DN 50 does not mean exactly 50 mm OD or ID.

Mistake 5: Mixing Pipe and Tube Dimensions

A tube’s actual OD should not be confused with an NPS pipe designation.

Mistake 6: Choosing Schedule From Pressure Alone

Pipe schedule should be selected using the applicable engineering calculations, material properties, design code, temperature, corrosion allowance, and service conditions—not simply by choosing the thickest available pipe.

NPS, Schedule, and Inside Diameter: A Simple Example

Suppose a piping system requires an NPS 4 steel pipe.

The standard outside diameter is approximately 4.500 inches.

If Schedule 40 is selected, the wall thickness is approximately 0.237 inch, producing an inside diameter of approximately 4.026 inches.

If a heavier schedule is selected, the outside diameter remains based on the NPS standard, but the wall becomes thicker and the available internal diameter becomes smaller.

This example shows why a pipe specification normally needs more information than simply “4-inch pipe.”

A practical specification might identify:

NPS 4 + material specification + schedule/wall thickness + applicable standard

Depending on the project, additional information may include length, end preparation, manufacturing method, grade, corrosion allowance, testing requirements, and applicable piping code.

What Standard Covers NPS Pipe Dimensions?

For welded and seamless wrought steel pipe, ASME B36.10M is a key dimensional standard. The current ASME listing identifies B36.10-2022 as the standard for welded and seamless wrought steel pipe.

For stainless steel pipe, ASME B36.19M is the relevant dimensional standard. The stainless steel standard uses an “S” suffix in schedule designations to distinguish B36.19M pipe from B36.10M pipe.

Other standards may apply depending on:

• Pipe material
• Product form
• Pressure service
• Manufacturing method
• Industry
• Country or project specification

For critical applications, the manufacturer’s certified dimensional data and the applicable standard should take precedence over a general online chart.

Frequently Asked Questions About NPS Pipe Sizes

Is NPS the same as pipe diameter?

No. NPS is a nominal size designation. The actual outside diameter is standardized separately and may differ substantially from the NPS number, especially for smaller sizes.

What does NPS 2 mean?

NPS 2 identifies a nominal 2-inch pipe size. A typical steel NPS 2 pipe has an outside diameter of approximately 2.375 inches (60.3 mm).

What is the difference between NPS and DN?

NPS is the North American nominal pipe size designation, while DN is a nominal metric-oriented designation used widely in international standards and specifications. For example, NPS 2 corresponds approximately to DN 50.

Does a higher pipe schedule mean a larger pipe?

Not in outside diameter. For the same NPS, a higher schedule generally means a thicker pipe wall and smaller inside diameter.

What is the OD of NPS 4 pipe?

The standard outside diameter of NPS 4 pipe is approximately 4.500 inches (114.3 mm).

What is the OD of NPS 6 pipe?

NPS 6 has a standard outside diameter of approximately 6.625 inches (168.3 mm).

What is the OD of NPS 8 pipe?

NPS 8 has a standard outside diameter of approximately 8.625 inches (219.1 mm).

Is NPS 1 actually 1 inch in diameter?

No. A standard NPS 1 steel pipe has an outside diameter of approximately 1.315 inches (33.4 mm).

How do I identify an unknown pipe?

Start by measuring the outside diameter and comparing it with an NPS chart. If possible, determine the wall thickness or inside diameter as well. Then confirm the material and applicable pipe standard before ordering replacement components.

Conclusion

An NPS pipe size chart is useful because it connects nominal pipe designations with the actual dimensions needed for design, fabrication, installation, and component selection. The most important concept to remember is that NPS is nominal: it does not necessarily equal the pipe’s outside diameter or inside diameter.

For accurate pipe selection, consider the complete dimensional specification, including NPS, outside diameter, wall thickness, inside diameter, schedule, material, and applicable standard. For steel pipe, ASME B36.10M provides an important reference for standardized dimensions, while stainless steel pipe dimensions are covered by ASME B36.19M.

When replacing an existing pipe, do not rely on the nominal number alone. Measure the pipe where practical, identify the schedule and material, and verify the final dimensions against the applicable manufacturer documentation or engineering standard.