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. Pipe size is specified with two non-dimensional numbers: a nominal pipe size (NPS) based on inches, and a schedule (Sched. or Sch.). NPS is often incorrectly called National Pipe Size, due to confusion with national pipe thread (NPT)

National pipe thread

National Pipe Thread Tapered Thread is a U.S. standard for tapered threads used to join pipes and fittings. ANSI/ASME standard B1.20.1 covers threads of 60-degree form with flat crests and roots in sizes from ¹/₁₆ inch to 24 inch Nominal Pipe Size [NPS]...

. For other pipe size standards, see pipe (material) - sizes. The European designation equivalent to NPS is DN (diamètre nominal/nominal diameter), in which sizes are measured in millimetres.

Nominal Pipe Size (NPS) is a North American set of standard sizes for pipes used for high or low pressures and temperatures. Pipe size is specified with two non-dimensional numbers: a nominal pipe size (NPS) based on inches, and a schedule (Sched. or Sch.). NPS is often incorrectly called National Pipe Size, due to confusion with national pipe thread (NPT)

National pipe thread

National Pipe Thread Tapered Thread is a U.S. standard for tapered threads used to join pipes and fittings. ANSI/ASME standard B1.20.1 covers threads of 60-degree form with flat crests and roots in sizes from ¹/₁₆ inch to 24 inch Nominal Pipe Size [NPS]...

. For other pipe size standards, see pipe (material) - sizes. The European designation equivalent to NPS is DN (diamètre nominal/nominal diameter), in which sizes are measured in millimetres. The term NB (nominal bore) is also frequently used interchangeably with NPS.

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)

Iron pipe size

Iron Pipe Size refers to an old pipe sizing system still in use by some industries, including major PVC pipe manufacturers, as well as some legacy drawings and equipment....

 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, 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. 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

American Petroleum Institute

The American Petroleum Institute, commonly referred to as API, is the main U.S trade association for the oil and natural gas industry, representing about 400 corporations involved in production, refinement, distribution, and many other aspects of the petroleum industry...

, ASTM

ASTM International

ASTM International , originally known as the American Society for Testing and Materials, is an international standards organization that develops and publishes voluntary consensus technical standards for a wide range of materials, products, systems, and services...

, 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

Stubs Iron Wire Gauge

The Stubs Iron Wire Gauge system is used in medicine to measure the diameter of hypodermic needles, catheters, and suture wires...

 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.

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
  Tubing (material)
  Tubing is a pipe or hollow cylinder for the conveyance of fluids . The terms "pipe" and "tubing" are almost interchangeable, although minor distinctions exist — generally, "tubing" implies tighter engineering requirements than "pipe". Both "pipe" and "tube" imply a level of rigidity and...
  
  , 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.

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

Iron pipe size

Iron Pipe Size refers to an old pipe sizing system still in use by some industries, including major PVC pipe manufacturers, as well as some legacy drawings and equipment....

 (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. Different definitions exist for XXS, but it is generally thicker than schedule 160.

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.

Both polyvinyl chloride

Polyvinyl chloride

Polyvinyl chloride, commonly abbreviated PVC, is the third most widely used thermoplastic polymer after polyethylene and polypropylene. In terms of revenue generated, it is one of the most valuable products of the chemical industry. Around the world, over 50% of PVC manufactured is used in...

 pipe (PVC) and chlorinated polyvinyl chloride

Chlorinated polyvinyl chloride

Chlorinated polyvinyl chloride is a thermoplastic produced by chlorination of polyvinyl chloride resin. Uses include hot and cold water pipe, and industrial liquid handling.- Production process :...

 pipe (CPVC) are made in NPS sizes.

NPS ⅛ to NPS 3½

NPS	DN mm	OD inches (millimeters)	Wall Thickness inches (millimeters)
			SCH 5	SCH 10	SCH 30	SCH 40	SCH 80	SCH 120	XXS
⅛	6							---	---
¼	8							---	---
⅜	10							---	---
½	15				---			---
¾	20				---			---
1	25				---			---
1¼	32							---
1½	40							---
2	50
2½	65
3	80
3½	90							---

NPS 4 to NPS 9

NPS	DN mm	OD inches (millimeters)	Wall Thickness inches (millimeters)
			SCH 5	SCH 10	SCH 20	SCH 30	SCH 40/STD	SCH 60	SCH 80	SCH 100	SCH 120	SCH 140	SCH 160
4	100				---					---		---
4½	115		---	---	---	---		---		---	---	---	---
5	125				---	---		---		---		---
6	150				---	---		---		---		---
8	200

NPS 10 to NPS 24

NPS	DN mm	OD inches (millimeters)	Wall Thickness inches (millimeters)
			SCH 5s	SCH 5	SCH 10s	SCH 10	SCH 20	SCH 30
10	250
12	300
14	350
16	400
18	450
20	500
24	600

NPS	Wall Thickness inches (millimeters)
	SCH 40s	SCH 40	SCH 60	SCH 80s	SCH 80	SCH 100	SCH 120	SCH 140	SCH 160
10
12
14
16
18
20
24

Note: --- = size not designated.

See also

• British standard pipe sizes
• British standard pipe thread sizes
• Pipe sizes