Nominal Pipe Size (NPS) Chart

Nominal Pipe Size (NPS) Chart: Dimensions, DN, NB, OD & Pipe Schedule Guide

Choosing the correct pipe size is essential for pipeline design, fluid transportation, pressure calculations, and selecting compatible fittings. However, many engineers, buyers, and contractors are often confused by terms such as Nominal Pipe Size (NPS), Nominal Bore (NB), and Diameter Nominal (DN) because these designations do not represent the actual pipe dimensions.

For example, a 2-inch NPS pipe does not have an outside diameter of exactly 2 inches. Instead, it has a standardized outside diameter of 2.375 inches (60.3 mm), while its inside diameter changes depending on the pipe schedule (wall thickness).

What Is Nominal Pipe Size (NPS)?

Nominal Pipe Size (NPS) is the standard sizing system used primarily in North America to identify steel pipe sizes. Rather than representing the pipe's exact inside or outside diameter, NPS is a nominal (reference) designation that corresponds to a standardized outside diameter (OD).
Unlike tubing, where the stated size usually matches the outside diameter, steel pipes use a nominal sizing system because different wall thicknesses can share the same outside diameter.

Why Is It Called "Nominal"?

The word nominal means in name only or approximately designated rather than representing an exact measurement.
Historically, early steel pipes were identified by their approximate inside diameter because fluid capacity depended primarily on the bore. As manufacturing technology evolved and different wall thicknesses became available, the outside diameter was standardized while the inside diameter varied.
As a result:
Pipe Size = Standard Outside Diameter + Variable Wall Thickness
Therefore:
Outside Diameter (OD) remains fixed
Wall Thickness depends on Schedule
Inside Diameter (ID) changes accordingly
NPS remains the same
This system simplifies manufacturing and ensures compatibility across fittings, valves, supports, and piping components.

Understanding Pipe Dimensions

Before selecting a pipe, it is important to understand the four dimensions commonly used in piping systems.

Term	Meaning	Changes with Schedule?	Used For
NPS	Nominal Pipe Size	No	Pipe designation
OD	Outside Diameter	No	Fittings, flanges, supports
Wall Thickness	Pipe wall thickness	Yes	Pressure rating
ID	Inside Diameter	Yes	Flow capacity

The relationship between these dimensions can be expressed as:
Inside Diameter = Outside Diameter − (2 × Wall Thickness)
This simple formula explains why two pipes with the same NPS can have different internal flow capacities.

NPS, NB and DN – What's the Difference?

One of the most common questions is whether NPS, NB, and DN refer to the same pipe size.
Although these terms are closely related, they originate from different standards and regions.

Term	Full Name	Unit	Primary Region
NPS	Nominal Pipe Size	Inch	North America
NB	Nominal Bore	Inch	UK, Australia, Asia
DN	Diameter Nominal	Millimetres	Europe & ISO countries

Nominal Pipe Size (NPS)
NPS is defined by ASME standards and is widely used in the United States, Canada, and industries following ASTM or API specifications.
Examples include:
API 5L Line Pipe
ASTM A53 Pipe
ASTM A106 Pipe
ASME Pressure Piping

Diameter Nominal (DN)
DN is the international metric designation defined by ISO standards.
Unlike NPS, DN uses metric numbers such as:
DN15
DN20
DN50
DN100
However, DN values are still nominal rather than actual measured diameters.
For instance:
DN50 corresponds approximately to NPS 2
DN100 corresponds approximately to NPS 4
The actual outside diameter is determined by the corresponding NPS standard.

NPS to DN Conversion Chart

The following chart provides the most commonly used conversions between North American and metric pipe size designations.

NPS (inch)	DN (mm)	NB	Outside Diameter (mm)
1/8	DN6	6 NB	10.3
1/4	DN8	8 NB	13.7
3/8	DN10	10 NB	17.1
1/2	DN15	15 NB	21.3
3/4	DN20	20 NB	26.7
1	DN25	25 NB	33.4
1¼	DN32	32 NB	42.2
1½	DN40	40 NB	48.3
2	DN50	50 NB	60.3
2½	DN65	65 NB	73
3	DN80	80 NB	88.9
4	DN100	100 NB	114.3
6	DN150	150 NB	168.3
8	DN200	200 NB	219.1
10	DN250	250 NB	273.1
12	DN300	300 NB	323.9

Note: DN values are nominal metric designations and should not be interpreted as actual outside or inside diameters. Always refer to ASME B36.10M or ASME B36.19M dimensions when specifying pipe sizes.

Nominal Pipe Size Chart (Standard Dimensions)

The following chart lists the standard outside diameters for the most commonly used Nominal Pipe Sizes.

NPS	DN	Outside Diameter (in)	Outside Diameter (mm)
1/8	6	0.405	10.3
1/4	8	0.54	13.7
3/8	10	0.675	17.1
1/2	15	0.84	21.3
3/4	20	1.05	26.7
1	25	1.315	33.4
1¼	32	1.66	42.2
1½	40	1.9	48.3
2	50	2.375	60.3
2½	65	2.875	73
3	80	3.5	88.9
4	100	4.5	114.3
5	125	5.563	141.3
6	150	6.625	168.3
8	200	8.625	219.1
10	250	10.75	273.1
12	300	12.75	323.9

Important: For NPS sizes up to 12 inches, the outside diameter is larger than the nominal size. From NPS 14 (DN350) and above, the nominal size and outside diameter are numerically the same in inches, a historical convention retained in modern piping standards.

How Pipe Schedule Affects Nominal Pipe Size ？

One of the biggest misconceptions about Nominal Pipe Size (NPS) is that all pipes with the same NPS have the same inside diameter. In reality, while the outside diameter (OD) remains constant, the inside diameter (ID) changes according to the pipe schedule.
A pipe schedule refers to the wall thickness of a pipe. Higher schedule numbers indicate thicker walls, resulting in a smaller inside diameter and a higher pressure rating.
For example, a 2-inch NPS pipe has a fixed outside diameter of 2.375 inches (60.3 mm) regardless of the schedule. However, the inside diameter decreases as the wall becomes thicker.

NPS	Schedule	Wall Thickness (in)	Inside Diameter (in)	Outside Diameter (in)
2	Sch 10	0.109	2.157	2.375
2	Sch 40	0.154	2.067	2.375
2	Sch 80	0.218	1.939	2.375
2	Sch 160	0.344	1.687	2.375

NPS does not determine the wall thickness.
Pipe Schedule determines the pressure capability.
Outside diameter remains unchanged for the same NPS.
Inside diameter decreases as wall thickness increases.

This standardized system ensures that valves, flanges, fittings, clamps, and supports remain compatible across different pressure classes.

Nominal Pipe Size vs Actual Pipe Size

Many first-time buyers expect a 2-inch pipe to measure exactly 2 inches across its outside diameter. However, this is one of the most common misunderstandings in the piping industry.

The following table compares nominal pipe sizes with their actual outside diameters.

Nominal Pipe Size (NPS)	Actual Outside Diameter
1/2"	0.840" (21.3 mm)
3/4"	1.050" (26.7 mm)
1"	1.315" (33.4 mm)
2"	2.375" (60.3 mm)
3"	3.500" (88.9 mm)
4"	4.500" (114.3 mm)
6"	6.625" (168.3 mm)
8"	8.625" (219.1 mm)
10"	10.750" (273.1 mm)
12"	12.750" (323.9 mm)

Why Are They Different?

Historically, pipe sizes were based on the approximate inside diameter because flow capacity was the primary concern. As manufacturing evolved and multiple wall thicknesses became available, the outside diameter was standardized to maintain compatibility with fittings and accessories.
This is why the nominal size no longer matches the actual measured diameter for most pipe sizes.

Note: From NPS 14 (DN350) and above, the nominal pipe size and outside diameter are numerically the same in inches.

Pipe Schedule Chart (Common Wall Thickness)

The following chart lists the most commonly used pipe schedules and their typical applications.

Pipe Schedule	Relative Wall Thickness	Typical Applications
Sch 5	Very Thin	Low-pressure stainless steel systems
Sch 10	Thin	Water supply, food processing, HVAC
Sch 20	Light	Drainage and low-pressure piping
Sch 40 (STD)	Standard	General industrial and construction piping
Sch 80 (XS)	Heavy	High-pressure oil & gas, steam, chemical plants
Sch 120	Extra Heavy	High-pressure industrial systems
Sch 160	Very Heavy	Refineries, power plants, offshore projects
XXS	Double Extra Strong	Extreme pressure applications