Master Guide to Pipe Sizes, Schedules & Wall Thickness

What it is

Steel pipe is sized by Nominal Pipe Size (NPS). NPS is a name, not a measurement. For NPS 1/8 through 12 the actual outside diameter (OD) is larger than the nominal number. From NPS 14 up, the OD equals the NPS in inches. Within each NPS the OD stays the same and the schedule sets the wall thickness, so a heavier schedule has a smaller bore. Carbon steel dimensions are in ASME B36.10M. Stainless pipe (schedules 5S, 10S, 40S, 80S) is in ASME B36.19M.

How to identify

• Measure the OD with a caliper or pipe tape and match it to the table to find the NPS.
• Measure the wall at a cut end or with an ultrasonic gauge, and compare it to the schedule columns.
• Read the mill stencil, which shows the specification (e.g. ASTM A53 Gr B, A106 Gr B), the NPS, the schedule or wall, and the heat number.
• STD, XS and XXS are weight-class designations. They match Sch 40 and Sch 80 for many small sizes, but not for every size.

Nominal dimensions (inches)

These are nominal values. Mill tolerance on wall thickness is set by the product specification (for example ASTM A53 or A106), so a measured wall can be thinner than nominal and still conform.

Key points

• The ID depends on the schedule. Approximate ID = OD − 2 × wall.
• PVC and CPVC Schedule 40 and 80 pipe use the same ODs as steel pipe of the same NPS. Copper tube does not.
• For NPS 10 and 12, STD wall is not the same as Sch 40. Confirm which one the spec calls for.
• Pressure capacity depends on material, wall, temperature, joint type, corrosion allowance and the governing code (ASME B31.1, B31.3, B31.9 or the plumbing code). There is no single number per schedule.

Common mistakes

• Measuring the ID and calling it the size.
• Assuming a heavier schedule always fits the same fittings. Socket-weld and butt-weld fittings are matched to a specific schedule or wall.
• Treating "Sch 40 stainless" and "40S" as the same in every size.

Governing standards: ASME B36.10M (welded and seamless wrought steel pipe); ASME B36.19M (stainless steel pipe); ASTM A53; ASTM A106; ASTM A530 (general requirements and tolerances).

Reference only. Verify against the current governing code or standard, the manufacturer's data and the project documents before use. Apps for Distributors disclaims liability arising from reliance on this reference.