NPS, Nominal Pipe Size

Nominal Pipe Size () is a North American set of standard sizes for pipes used for high or low pressures and temperatures. The name is based on the earlier “Iron Pipe Size” (IPS) system.

Stainless Steel Pipe dimensions determined by ASME B36.19 covering the outside diameter and the Schedule wall thickness. Note that stainless wall thicknesses to ASME B36.19 all have an “S” suffix. Sizes without an “S” suffix are to ASME B36.10 which is intended for carbon steel pipes.

The International Standards Organization (ISO) also employs a system with a dimensionless designator.
Diameter nominal (DN) is used in the metric unit system. It indicates standard pipe size when followed by the specific size designation number without a millimeter symbol. For example, DN 80 is the equivalent designation of NPS 3. Below a table with equivalents for NPS and DN pipe sizes.

NPS ½ ¾ 1 2 3 4
Note: For NPS ≥ 4, the related DN = 25 multiplied by the NPS number.

That IPS system was established to designate the pipe size. The size represented the approximate inside diameter of the pipe in inches. An IPS 6″ pipe is one whose inside diameter is approximately 6 inches. Users started to call the pipe as 2inch, 4inch, 6inch pipe and so on. To begin, each pipe size was produced to have one thickness, which later was termed as standard (STD) or standard weight (STD.WT.). The outside diameter of the pipe was standardized.
As the industrial requirements handling higher pressure fluids, pipes were manufactured with thicker walls, which has become known as an extra strong (XS) or extra heavy (XH). The higher pressure requirements increased further, with thicker wall pipes. Accordingly, pipes were made with double extra strong (XXS) or double extra heavy (XXH) walls, while the standardized outside diameters are unchanged. Note that on this website only terms XS and XXS are used.

Actual outside diameters

  • NPS 1 actual O.D. = 1.5/16″ (33.4 mm)
  • NPS 2 actual O.D. = 2.3/8″ (60.3 mm)
  • NPS 3 actual O.D. = 3½” (99.9 mm)
  • NPS 4 actual O.D. = 4.1/2″ (114.3 mm)
  • NPS 12 actual O.D. = 12.3/4″ (323.9 mm)
  • NPS 14 actual O.D. = 14″ (355.6 mm)

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 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.

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