Is NPS the same as the actual pipe diameter?

No. NPS is a designation; the actual outside diameter is fixed by the standard and only matches the NPS number approximately at larger sizes.

What is the difference between SCH 40 and SCH 80?

Same outside diameter, thicker wall and smaller bore for SCH 80, giving higher weight per metre.

Dimensional standard

ASME B36.10M / B36.19M

Welded and Seamless Wrought Steel Pipe Dimensions

ASME B36.10M gives standardised outside diameters and wall thickness schedules for carbon and alloy steel pipe; B36.19M covers stainless steel pipe with the S-suffix schedules.

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Scope

What it covers

Nominal pipe sizes with fixed outside diameters across all schedules.
Schedules such as 10, 20, 30, 40, 80, 120, 160, STD, XS and XXS.
Stainless schedules 5S, 10S, 40S and 80S under B36.19M.

Practical notes

Points to specify

For a given NPS the outside diameter stays constant; increasing the schedule reduces the bore, not the OD.
Weight per metre rises with schedule because wall thickness increases.
Use the pipe dimensions chart and weight calculator on this site to check OD, wall thickness and kg/m.

Products quoted against ASME B36.10M / B36.19M

MS ERW Pipes

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FAQ

Common questions about ASME B36.10M / B36.19M

Is NPS the same as the actual pipe diameter?: No. NPS is a designation; the actual outside diameter is fixed by the standard and only matches the NPS number approximately at larger sizes.
What is the difference between SCH 40 and SCH 80?: Same outside diameter, thicker wall and smaller bore for SCH 80, giving higher weight per metre.

More references

ASME B16.5

Pipe Flanges and Flanged Fittings (NPS 1/2 through NPS 24)

ASME B16.9

Factory-Made Wrought Buttwelding Fittings

ASME B16.11

Forged Fittings, Socket-Welding and Threaded

General guidance only. Work from the current published edition of the standard and your project specification; sizes, grades, classes and documentation are confirmed per enquiry.