Nominal Pipe Size

Nominal Pipe Size (NPS) is a North American set of standard sizes for pipes used for high or low pressures and temperatures.^[1] Pipe size is specified with two non-dimensional numbers: a nominal pipe size (NPS) for diameter based on inches, and a schedule (Sched. or Sch.) for wall thickness. NPS is often incorrectly called National Pipe Size, due to confusion with national pipe thread (NPT). The European designation equivalent to NPS is DN (diamètre nominal/nominal diameter/Durchmesser nach Norm), in which sizes are measured in millimetres.^[2] The term NB (nominal bore) is also frequently used interchangeably with NPS.

1 History
2 Application
3 NPS tables for selected sizes
4 See also
5 Notes
6 References
- 6.1 Bibliography
7 External links

History

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. The committee surveyed the industry and created a system of schedule numbers that designated wall thicknesses based on smaller steps between sizes,^[3] although IPS and NPS numbers remain equivalent.

The original intent was that each schedule would relate to a given pressure rating, however the numbers deviated so far from wall thicknesses in common use that this original intent could not be accomplished.^[3]^{[note 1]} 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.^[3]

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.^[4]

Application

Based on the NPS and schedule of a pipe, 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 14 inches and a wall thickness of 0.437 inches. However the NPS and OD values are not always equal, which can create confusion.

For NPS ⅛ to 12 inches, the NPS and OD values are different. For example, the OD of an NPS 12 pipe is actually 12.75 inches. To find the actual OD for each NPS value, refer to the tables below. (Note that for tubing, the size is always the actual OD.)
For NPS 14 inches and up, the NPS and OD values are equal. In other words, an NPS 14 pipe is actually 14 inches OD.

The reason for the discrepancy for NPS ⅛ 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.^{[note 1]}

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 1/8 to NPS 10, inclusive. XS is identical to SCH 80S, and 80S is identical to 80 for NPS 1/8 to NPS 8, inclusive. XXS wall is thicker than schedule 160 from NPS 1/8" to NPS 6" inclusive, and schedule 160 is thicker than XXS wall for NPS 8" and larger.

The "S" designation, for example "NPS Sch 10S", most often indicates stainless steel pipes. However some stainless steel pipes are available in steel designations, so strictly speaking the "S" designation only differentiates B36.19M pipe from B36.10M pipe.^[1]

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

NPS tables for selected sizes

NPS ⅛ to NPS 3½

NPS^[5]	DN ^[2]	OD [in (mm)]	Wall thickness [in (mm)]
NPS^[5]	DN ^[2]	OD [in (mm)]	SCH 5	SCH 10s/10	SCH 30	SCH 40s/40 /STD	SCH 80s/80 /XS	SCH 120	SCH 160	XXS
⅛	6	0.405 (10.29)	0.035 (0.889)	0.049 (1.245)	0.057 (1.448)	0.068 (1.727)	0.095 (2.413)	—	—	—
¼	8	0.540 (13.72)	0.049 (1.245)	0.065 (1.651)	0.073 (1.854)	0.088 (2.235)	0.119 (3.023)	—	—	—
⅜	10	0.675 (17.15)	0.049 (1.245)	0.065 (1.651)	0.073 (1.854)	0.091 (2.311)	0.126 (3.200)	—	—	—
½	15	0.840 (21.34)	0.065 (1.651)	0.083 (2.108)	0.095 (2.413)	0.109 (2.769)	0.147 (3.734)	—	0.188 (4.775)	0.294 (7.468)
¾	20	1.050 (26.67)	0.065 (1.651)	0.083 (2.108)	0.095 (2.413)	0.113 (2.870)	0.154 (3.912)	—	0.219 (5.563)	0.308 (7.823)
1	25	1.315 (33.40)	0.065 (1.651)	0.109 (2.769)	0.114 (2.896)	0.133 (3.378)	0.179 (4.547)	—	0.250 (6.350)	0.358 (9.093)
1¼	32	1.660 (42.16)	0.065 (1.651)	0.109 (2.769)	0.117 (2.972)	0.140 (3.556)	0.191 (4.851)	—	0.250 (6.350)	0.382 (9.703)
1½	40	1.900 (48.26)	0.065 (1.651)	0.109 (2.769)	0.125 (3.175)	0.145 (3.683)	0.200 (5.080)	—	0.281 (7.137)	0.400 (10.160)
2	50	2.375 (60.33)	0.065 (1.651)	0.109 (2.769)	0.125 (3.175)	0.154 (3.912)	0.218 (5.537)	0.250 (6.350)	0.343 (8.712)	0.436 (11.074)
2½	65	2.875 (73.03)	0.083 (2.108)	0.120 (3.048)	0.188 (4.775)	0.203 (5.156)	0.276 (7.010)	0.300 (7.620)	0.375 (9.525)	0.552 (14.021)
3	80	3.500 (88.90)	0.083 (2.108)	0.120 (3.048)	0.188 (4.775)	0.216 (5.486)	0.300 (7.620)	0.350 (8.890)	0.438 (11.125)	0.600 (15.240)
3½	90	4.000 (101.60)	0.083 (2.108)	0.120 (3.048)	0.188 (4.775)	0.226 (5.740)	0.318 (8.077)	—	—	0.636 (16.154)

NPS 4 to NPS 9

NPS^[5]	DN ^[2]	OD [in (mm)]	Wall thickness [in (mm)]
NPS^[5]	DN ^[2]	OD [in (mm)]	SCH 5	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^[5]
4	100	4.500 (114.30)	0.083 (2.108)	0.120 (3.048)	—	0.188 (4.775)	0.237 (6.020)	0.281 (7.137)	0.337 (8.560)	0.437 (11.100)	—	—	0.531 (13.487)	0.674 (17.120)	4½	115	5.000 (127.00)	—	—	—	—	0.247 (6.274)	—	0.355 (9.017)	—	—	—	—	0.710 (18.034)
5	125	5.563 (141.30)	0.109 (2.769)	0.134 (3.404)	—	—	0.258 (6.553)	—	0.375 (9.525)	—	0.500 (12.700)	—	0.625 (15.875)	0.750 (19.050)
6	150	6.625 (168.28)	0.109 (2.769)	0.134 (3.404)	—	—	0.280 (7.112)	—	0.432 (10.973)	—	0.562 (14.275)	—	0.718 (18.237)	0.864 (21.946)
7^[5]	—	7.625 (193.68)	—	—	—	—	0.301 (7.645)	—	0.500 (12.700)	—	—	—	—	0.875 (22.225)
8	200	8.625 (219.08)	0.109 (2.769)	0.148 (3.759)	0.250 (6.350)	0.277 (7.036)	0.322 (8.179)	0.406 (10.312)	0.500 (12.700)	0.593 (15.062)	0.718 (18.237)	0.812 (20.625)	0.906 (23.012)	0.875 (22.225)
9^[5]	—	9.625 (244.48)	—	—	—	—	0.342 (8.687)	—	0.500 (12.700)	—	—	—	—	—

NPS 10 to NPS 24

NPS^[5]	DN ^[2]	OD [in (mm)]	Wall thickness [in (mm)]
NPS^[5]	DN ^[2]	OD [in (mm)]	SCH 5s	SCH 5	SCH richs	SCH 10	SCH 20	SCH 30	SCH 40s/STD
10	250	10.75 (273.05)	0.134 (3.404)	0.134 (3.404)	0.165 (4.191)	0.165 (4.191)	0.250 (6.350)	0.307 (7.798)	0.365 (9.271)
12	300	12.75 (323.85)	0.156 (3.962)	0.165 (4.191)	0.180 (4.572)	0.180 (4.572)	0.250 (6.350)	0.330 (8.382)	0.375 (9.525)
14	350	14.00 (355.60)	0.156 (3.962)	0.156 (3.962)	0.188 (4.775)	0.250 (6.350)	0.312 (7.925)	0.375 (9.525)	0.375 (9.525)
16	400	16.00 (406.40)	0.165 (4.191)	0.165 (4.191)	0.188 (4.775)	0.250 (6.350)	0.312 (7.925)	0.375 (9.525)	0.375 (9.525)
18	450	18.00 (457.20)	0.165 (4.191)	0.165 (4.191)	0.188 (4.775)	0.250 (6.350)	0.312 (7.925)	0.437 (11.100)	0.375 (9.525)
20	500	20.00 (508.00)	0.188 (4.775)	0.188 (4.775)	0.218 (5.537)	0.250 (6.350)	0.375 (9.525)	0.500 (12.700)	0.375 (9.525)
24	600	24.00 (609.60)	0.218 (5.537)	0.218 (5.537)	0.250 (6.350)	0.250 (6.350)	0.375 (9.525)	0.562 (14.275)	0.375 (9.525)

NPS^[5]	Wall thickness [in (mm)]
NPS^[5]	SCH 40	SCH 60	SCH 80s/XS	SCH 80	SCH 100	SCH 120	SCH 140	SCH 160
10	0.365 (9.271)	0.500 (12.700)	0.500 (12.700)	0.593 (15.062)	0.718 (18.237)	0.843 (21.412)	1.000 (25.400)	1.125 (28.575)
12	0.406 (10.312)	0.562 (14.275)	0.500 (12.700)	0.687 (17.450)	0.843 (21.412)	1.000 (25.400)	1.125 (28.575)	1.312 (33.325)
14	0.437 (11.100)	0.593 (15.062)	0.500 (12.700)	0.750 (19.050)	0.937 (23.800)	1.093 (27.762)	1.250 (31.750)	1.406 (35.712)
16	0.500 (12.700)	0.656 (16.662)	0.500 (12.700)	0.843 (21.412)	1.031 (26.187)	1.218 (30.937)	1.437 (36.500)	1.593 (40.462)
18	0.562 (14.275)	0.750 (19.050)	0.500 (12.700)	0.937 (23.800)	1.156 (29.362)	1.375 (34.925)	1.562 (39.675)	1.781 (45.237)
20	0.593 (15.062)	0.812 (20.625)	0.500 (12.700)	1.031 (26.187)	1.280 (32.512)	1.500 (38.100)	1.750 (44.450)	1.968 (49.987)
24	0.687 (17.450)	0.968 (24.587)	0.500 (12.700)	1.218 (30.937)	1.531 (38.887)	1.812 (46.025)	2.062 (52.375)	2.343 (59.512)

NPS 26 to NPS 36

NPS^[5]	DN ^[2]	OD [in (mm)]	Wall thickness [in (mm)]
NPS^[5]	DN ^[2]	OD [in (mm)]	SCH 5s	SCH 10s	SCH 10	SCH 20	SCH 30	SCH 40s/STD	SCH 40	SCH 80s/XS
26	650	26.000 (660.400)	—	—	0.312 (7.925)	0.500 (12.700)	0.375 (9.525)	—	0.500 (12.700)
28	700	28.000 (711.200)	—	—	0.312 (7.925)	0.500 (12.700)	0.625 (15.875)	0.375 (9.525)	—	—
30	750	30.000 (762.000)	0.250 (6.350)	0.312 (7.925)	0.312 (7.925)	0.500 (12.700)	0.625 (15.875)	0.375 (9.525)	—	0.500 (12.700)
32	800	32.000 (812.800)	—	—	0.312 (7.925)	0.500 (12.700)	0.625 (15.875)	0.375 (9.525)	0.688 (17.475)	0.500 (12.700)
34	850	34.000 (863.600)	—	—	0.312 (7.925)	0.500 (12.700)	0.625 (15.875)	0.375 (9.525)	0.688 (17.475)
36	900	36.000 (914.400)	—	—	0.312 (7.925)	—	0.625 (15.875)	0.375 (9.525)	0.750 (19.050)	0.500 (12.700)

Additional sizes (NPS)

NPS	DN ^[2]	OD [in (mm)]	Wall thickness [in (mm)]
22	550	22.000 (558.800)	?
40	1000	40.000 (1,016.000)	?
42	1050	42.000 (1,066.800)	42.000
44	1100	44.000 (1,117.600)	?
48	1200	48.000 (1,219.200)	48.000
52	1300	52.000 (1,320.800)	?
56	1400	56.000 (1,422.400)	?
60	1500	60.000 (1,524.000)	?
64	1600	64.000 (1,625.600)	?
68	1700	68.000 (1,727.200)	?
72	1800	72.000 (1,828.800)	?
76	1900	76.000 (1,930.400)	?
80	2000	80.000 (2,032.000)	?
88	2200	88.000 (2,235.200)	?

Notes

^ ^a ^b This is contrary to the McGraw Hill Piping Handbook, which says that the schedule number can be converted to pressure by dividing the schedule by 1000 and multiplying by the allowable stress of the material. [Nayyar and Mohinder, 2000.]

References

^ ^a ^b ASME B36.10M-2004, p. 1
^ ^a ^b ^c ^d ^e ^f NPS - "Nominal Pipe Size" and DN - "Diamètre Nominale", EngineeringToolbox.com
^ ^a ^b ^c ASME B36.10M-2004, p. iv
^ ASME Standard 36.19M-2004, p. iv
^ ^a ^b ^c ^d ^e ^f ^g ^h Table of Pipe Schedules from EngineersEdge.com.

Bibliography

Nayyar, P.E., Mohinder L. (2000). "A1". In Mohinder L. Nayyar, P.E.. Piping Handbook (7th ed.). New York: McGraw-Hill. ISBN 0-07-047106-1.
ASME B36.10M - Welded and Seamless Wrought Steel Pipe.
ANSI/ASME Standard 36.19M - Stainless Steel Pipe.

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.
[1] - Online Pipe Schedule Calculator