Nominal Pipe Size Explained

Nominal Pipe Size (NPS) is a North American set of standard sizes for pipes used for high or low pressures and temperatures. "Nominal" refers to pipe in non-specific terms and identifies the diameter of the hole with a non-dimensional number (for example – 2-inch nominal steel pipe" consists of many varieties of steel pipe with the only criterion being a 2.375inches outside diameter). Specific pipe is identified by pipe diameter and another non-dimensional number for wall thickness referred to as the Schedule (Sched. or Sch., for example – "2-inch diameter pipe, Schedule 40"). NPS is often incorrectly called National Pipe Size, due to confusion with the American standard for pipe threads, "national pipe straight", which also abbreviates as "NPS". The European and international designation equivalent to NPS is DN (diamètre nominal/nominal diameter/Nennweite), in which sizes are measured in millimetres, see ISO 6708.^[1] The term NB (nominal bore) is also frequently used interchangeably with DN.

In March 1927 the American Standards Association authorized a committee to standardize the dimensions of wrought steel and wrought iron pipe and tubing. At that time only a small selection of wall thicknesses were in use: standard weight (STD), extra-strong (XS), and double extra-strong (XXS), based on the iron pipe size (IPS) system of the day. However these three sizes did not fit all applications. Also, in 1939, it was hoped that the designations of STD, XS, and XXS would be phased out by schedule numbers, however those original terms are still in common use today (although sometimes referred to as standard, extra-heavy (XH), and double extra-heavy (XXH), respectively). Since the original schedules were created, there have been many revisions and additions to the tables of pipe sizes based on industry use and on standards from API, ASTM, and others.

Stainless steel pipes, which were coming into more common use in the mid 20th century, permitted the use of thinner pipe walls with much less risk of failure due to corrosion. By 1949 thinner schedules 5S and 10S, which were based on the pressure requirements modified to the nearest BWG number, had been created, and other "S" sizes followed later. Due to their thin walls, the smaller "S" sizes can not be threaded together according to ASME code, but must be fusion welded, brazed, roll grooved, or joined with press fittings.

Application

Based on the NPS and schedule of a pipe,^[2] the pipe outside diameter (OD) and wall thickness can be obtained from reference tables such as those below, which are based on ASME standards B36.10M and B36.19M. For example, NPS 14 Sch 40 has an OD of and a wall thickness of . However, the NPS and OD values are not always equal, which can create confusion.

• For NPS to 12, the NPS and OD values are different. For example, the OD of an NPS 12 pipe is actually . To find the actual OD for each NPS value, refer to the tables below. (Note that for tubing, the size indicates actual dimensions, not nominal.)
• For NPS 14 and up, the NPS and OD values are equal. In other words, an NPS 14 pipe is actually OD.

The reason for the discrepancy for NPS to 12 inches is that these NPS values were originally set to give the same inside diameter (ID) based on wall thicknesses standard at the time. However, as the set of available wall thicknesses evolved, the ID changed and NPS became only indirectly related to ID and OD.

For a given NPS, the OD stays fixed and the wall thickness increases with schedule. For a given schedule, the OD increases with NPS while the wall thickness stays constant or increases. Using equations and rules in ASME B31.3 Process Piping, it can be shown that pressure rating decreases with increasing NPS and constant schedule.

Some specifications use pipe schedules called standard wall (STD), extra strong (XS), and double extra strong (XXS), although these actually belong to an older system called iron pipe size (IPS). The IPS number is the same as the NPS number. STD is identical to SCH 40S, and 40S is identical to 40 for NPS to NPS 10, inclusive. XS is identical to SCH 80S, and 80S is identical to 80 for NPS to NPS 8, inclusive. XXS wall is thicker than schedule 160 from NPS  in to NPS 6 in inclusive, and schedule 160 is thicker than XXS wall for NPS 8 in and larger.

Blockage or ball test

When a pipe is welded or bent the most common method to inspect blockages, misalignment, ovality, and weld bead dimensional conformity is to pass a round ball through the pipe coil or circuit. If the inner pipe dimension is to be measured then the weld bead should be subtracted, if welding is applicable. Typically, the clearance tolerance for the ball must not exceed . Allowable ovality of any pipe is measured on the inside dimension of the pipe, normally 5% to 10% ovality can be accepted. If no other test is conducted to verify ovality, or blockages, this test must be seen as a standard requirement. A flow test can not be used in lieu of a blockage or ball test. See pipe dimensional table, Specification ASME B36.10M or B36.19M for pipe dimensions per schedule.

Stainless steel pipe is most often available in standard weight sizes (noted by the S designation; for example, NPS Sch 10S). However stainless steel pipe can also be available in other schedules.

Both polyvinyl chloride pipe (PVC) and chlorinated polyvinyl chloride pipe (CPVC) are made in NPS sizes.

NPS tables for selected sizes

NPS to NPS

DN does not exactly correspond to a size in millimeters, because ISO 6708 defines it as being a dimensionless specification only indirectly related to a diameter. The ISO 6708 sizes provide a metric name for existing inch sizes, resulting in a 1:1 correlation between NPS and DN sizes. ISO 6708 does not include values for "DN 6" or "DN 8", however ASME B36.10M list the "DN 6" and "DN 8" . Also, the European Standard EN 12 516-1 (Industrial valves - Shell design strength - Part 1: Tabulation method for steel valve shells) specifies the dimensions "DN 6" and "DN 8", respectively their equivalents NPS "and NPS ".

NPS^[3]	DN (dimensionless)	OD [in (mm)]	Wall thickness [in (mm)]
			Sch. 5s	Sch. 10s/10	Sch. 30	Sch. 40s/40 /STD	Sch. 80s/80 /XS	Sch. 120	Sch. 160	XXS
	6	0.4052NaN2^[4]	0.0353NaN3	0.0493NaN3	0.0573NaN3	0.0683NaN3	0.0953NaN3	—	—	—
	8	0.542NaN2	0.0493NaN3	0.0653NaN3	0.0733NaN3	0.0883NaN3	0.1193NaN3	—	—	—
	10	0.6752NaN2	0.0493NaN3	0.0653NaN3	0.0733NaN3	0.0913NaN3	0.1263NaN3	—	—	—
	15	0.842NaN2	0.0653NaN3	0.0833NaN3	0.0953NaN3	0.1093NaN3	0.1473NaN3	—	0.1883NaN3	0.2943NaN3
	20	1.052NaN2	0.0653NaN3	0.0833NaN3	0.0953NaN3	0.1133NaN3	0.1543NaN3	—	0.2193NaN3	0.3083NaN3
1	25	1.3152NaN2	0.0653NaN3	0.1093NaN3	0.1143NaN3	0.1333NaN3	0.1793NaN3	—	0.253NaN3	0.3583NaN3
	32	1.662NaN2	0.0653NaN3	0.1093NaN3	0.1173NaN3	0.143NaN3	0.1913NaN3	—	0.253NaN3	0.3823NaN3
	40	1.92NaN2	0.0653NaN3	0.1093NaN3	0.1253NaN3	0.1453NaN3	0.23NaN3	—	0.2813NaN3	0.43NaN3
2	50	2.3752NaN2	0.0653NaN3	0.1093NaN3	0.1253NaN3	0.1543NaN3	0.2183NaN3	—	0.3443NaN3	0.4363NaN3
	65	2.8752NaN2	0.0833NaN3	0.123NaN3	0.1883NaN3	0.2033NaN3	0.2763NaN3	0.33NaN3	0.3753NaN3	0.5523NaN3
3	80	3.52NaN2	0.0833NaN3	0.123NaN3	0.1883NaN3	0.2163NaN3	0.33NaN3	0.353NaN3	0.4383NaN3	0.63NaN3
	90	42NaN2	0.0833NaN3	0.123NaN3	0.1883NaN3	0.2263NaN3	0.3183NaN3	—	—	0.6363NaN3

Tolerance: The tolerance on pipe OD is + (0.0156) inch (NaNinches), − (0.0312) inch (NaNinches).

As per ASME B36.10M -2018 Pipe wall thickness are rounded to nearest 0.01mm, while converting wall thickness from inch to millimetre.

NPS 4 to NPS 9

NPS	DN	OD [in (mm)]	Wall thickness [in (mm)]
			Sch. 5s	Sch. 10s/10	Sch. 20	Sch. 30	Sch. 40s/40 /STD	Sch. 60	Sch. 80s/80 /XS	Sch. 100	Sch. 120	Sch. 140	Sch. 160	XXS
4	100	4.52NaN2	0.0833NaN3	0.123NaN3	—	0.1883NaN3	0.2373NaN3	—	0.3373NaN3	—	0.4373NaN3	—	0.5313NaN3	0.6743NaN3
	115	52NaN2	—	—	—	—	0.2473NaN3	—	0.3553NaN3	—	—	—	—	0.713NaN3
5	125	5.5632NaN2	0.1093NaN3	0.1343NaN3	—	—	0.2583NaN3	—	0.3753NaN3	—	0.53NaN3	—	0.6253NaN3	0.753NaN3
6	150	6.6252NaN2	0.1093NaN3	0.1343NaN3	—	—	0.283NaN3	—	0.4323NaN3	—	0.5623NaN3	—	0.7193NaN3	0.8643NaN3
7	—	7.6252NaN2	—	—	—	—	0.3013NaN3	—	0.53NaN3	—	—	—	—	0.8753NaN3
8	200	8.6252NaN2	0.1093NaN3	0.1483NaN3	0.253NaN3	0.2773NaN3	0.3223NaN3	0.4063NaN3	0.53NaN3	0.5933NaN3	0.7193NaN3	0.8123NaN3	0.9063NaN3	0.8753NaN3
9	—	9.6252NaN2	—	—	—	—	0.3423NaN3	—	0.53NaN3	—	—	—	—	—

NPS 10 to NPS 24

NPS	DN	OD [in (mm)]	Wall thickness [in (mm)]
			Sch. 5s	Sch. 10	Sch. 10s	Sch. 20	Sch. 30	Sch. Std./40S
10	250	10.752NaN2	0.1343NaN3	0.1653NaN3	0.165 (4.191)	0.253NaN3	0.3073NaN3	0.3653NaN3
12	300	12.752NaN2	0.1563NaN3	0.183NaN3	0.180 (4.572)	0.253NaN3	0.333NaN3	0.3753NaN3
14	350	142NaN2	0.1563NaN3	0.253NaN3	0.188 (4.775)	0.3123NaN3	0.3753NaN3	0.3753NaN3
16	400	162NaN2	0.1653NaN3	0.253NaN3	0.188 (4.775)	0.3123NaN3	0.3753NaN3	0.3753NaN3
18	450	182NaN2	0.1653NaN3	0.253NaN3	0.188 (4.775)	0.3123NaN3	0.4373NaN3	0.3753NaN3
20	500	202NaN2	0.1883NaN3	0.253NaN3	0.218 (5.537)	0.3753NaN3	0.53NaN3	0.3753NaN3
22	550	222NaN2	0.1883NaN3	0.253NaN3	0.218 (5.537)	0.3753NaN3	0.53NaN3	0.3753NaN3
24	600	242NaN2	0.2183NaN3	0.253NaN3	0.250 (6.350)	0.3753NaN3	0.5623NaN3	0.3753NaN3

NPS	DN	Wall thickness [in (mm)]
		Sch. 40	Sch. 60	Sch. 80s/XS	Sch. 80	Sch. 100	Sch. 120	Sch. 140	Sch. 160
10	250	0.3653NaN3	0.53NaN3	0.53NaN3	0.5943NaN3	0.7183NaN3	0.8433NaN3	13NaN3	1.1253NaN3
12	300	0.4063NaN3	0.5623NaN3	0.53NaN3	0.6873NaN3	0.8433NaN3	13NaN3	1.1253NaN3	1.3123NaN3
14	350	0.4373NaN3	0.5933NaN3	0.53NaN3	0.753NaN3	0.9373NaN3	1.0933NaN3	1.253NaN3	1.4063NaN3
16	400	0.53NaN3	0.6563NaN3	0.53NaN3	0.8433NaN3	1.0313NaN3	1.2183NaN3	1.4373NaN3	1.5943NaN3
18	450	0.5623NaN3	0.753NaN3	0.53NaN3	0.9373NaN3	1.1563NaN3	1.3753NaN3	1.5623NaN3	1.7813NaN3
20	500	0.5933NaN3	0.8123NaN3	0.53NaN3	1.0313NaN3	1.283NaN3	1.53NaN3	1.753NaN3	1.9683NaN3
22	550	—	0.8753NaN3	0.53NaN3	1.1253NaN3	1.3753NaN3	1.6253NaN3	1.8753NaN3	2.1253NaN3
24	600	0.6873NaN3	0.9683NaN3	0.53NaN3	1.2183NaN3	1.5313NaN3	1.8123NaN3	2.0623NaN3	2.3433NaN3

NPS 26 to NPS 36

NPS	DN	OD [in (mm)]	Wall thickness [in (mm)]
			Sch. 5S	Sch. 10S	Sch. 10	Sch. 20/XS	Sch. 30	Sch. 40s/Std.	Sch. 40
26	650	263NaN3	—	—	0.3123NaN3	0.53NaN3	—	0.3753NaN3	—
28	700	283NaN3	—	?	0.3123NaN3	0.53NaN3	0.6253NaN3	0.3753NaN3	—
30	750	303NaN3	0.253NaN3	0.3123NaN3	0.3123NaN3	0.53NaN3	0.6253NaN3	0.3753NaN3	—
32	800	323NaN3	—	—	0.3123NaN3	0.53NaN3	0.6253NaN3	0.3753NaN3	0.6883NaN3
34	850	343NaN3	—	—	0.3123NaN3	0.53NaN3	0.6253NaN3	0.3753NaN3	0.6883NaN3
36	900	363NaN3	—	—	0.3123NaN3	0.53NaN3	0.625 (15.875)	0.3753NaN3	0.75 (19.05)

Additional sizes (NPS)

NPS
	DN	OD [in (mm)]	Wall thickness [in (mm)]
			Std. Wt.	Sch. XS	Sch. XXS
40	1000	403NaN3	0.3753NaN3	0.53NaN3	13NaN3
42	1050	423NaN3	0.3753NaN3	0.53NaN3	13NaN3
44	1100	443NaN3	0.3753NaN3	0.53NaN3	13NaN3
46	1150	463NaN3	0.3753NaN3	0.53NaN3	13NaN3
48	1200	483NaN3	0.3753NaN3	0.53NaN3	13NaN3
52	1300	523NaN3	0.3753NaN3	0.53NaN3	13NaN3
56	1400	563NaN3	0.3753NaN3	0.53NaN3	13NaN3
60	1500	603NaN3	0.3753NaN3	0.53NaN3	13NaN3
64	1600	643NaN3	0.3753NaN3	0.53NaN3	13NaN3
68	1700	683NaN3	0.3753NaN3	0.53NaN3	13NaN3
72	1800	723NaN3	0.3753NaN3	0.53NaN3	13NaN3
76	1900	763NaN3
80	2000	803NaN3
88	2200	883NaN3
92	2300	923NaN3
96	2400	963NaN3
100	2500	1003NaN3
104	2600	1043NaN3
108	2700	1083NaN3	≥−

See also

• British standard pipe thread sizes
• Copper tubing sizes
• Pipe thread sizes
• National pipe thread sizes
• Pipe (fluid conveyance)
• Pipe sizes
• Standard dimension ratio

Bibliography

• • Book: Nayyar, Mohinder L. . 2000 . Piping Handbook . A1 . Mohinder L. . Nayyar . 7th . McGraw-Hill . New York . 0-07-047106-1 .
• Book: The American Society of Mechanical Engineers . B36.10M Welded and Seamless Wrought Steel Pipe . 2015 . 2015-10-02 . 9780791870273 . .
• Book: The American Society of Mechanical Engineers . B36.19M Stainless Steel Pipe . 2004 . 2015-10-02 . 0791829170. .

External links

• Notes on Pipe—PVC Pipe weights and max PSI
• Quick calculator to determine standard pipe dimensions For Carbon Steel and Stainless Steel pipes as per ANSI. (Requires Membership)

Notes and References

1. http://www.engineeringtoolbox.com/nps-nominal-pipe-sizes-d_45.html NPS - "Nominal Pipe Size" and DN - "Diamètre Nominal"
2. Web site: Pipe Schedule Chart. HYSP Steel Pipe.
3. http://www.engineersedge.com/pipe_schedules.htm Table of Pipe Schedules
4. News: A Complete Guide to Pipe Sizes and Pipe Schedule – Free Pocket Chart. HardHat Engineer. 2017-11-07. en-GB.