Pipe schedule dimensions and weights

Pipe is specified by a Nominal Pipe Size and a schedule number, and neither is a dimension. NPS is a label: for NPS 1/2 through 12 it is loosely related to the historic bore, and for NPS 14 and above it happens to equal the outside diameter in inches. What is fixed for a given NPS is the outside diameter, so that the same flanges, fittings, valves and pipe supports fit every schedule. Increasing the schedule thickens the wall inward and reduces the bore.

The schedule number itself derives from the old Barlow relation Sch ≈ 1000·P/S — the ratio of a working pressure to an allowable stress. That heritage means a given schedule is not a constant pressure rating across sizes: NPS 2 Sch 40 has a much higher pressure capacity than NPS 24 Sch 40, because the wall grows far more slowly than the diameter. Always compute the pressure capacity for the actual diameter and wall rather than assuming a schedule carries a rating.

Two standards apply. ASME B36.10M covers welded and seamless wrought steel pipe and gives the plain schedule numbers 10, 20, 30, 40, 80, 160 along with STD, XS and XXS. ASME B36.19M covers stainless steel pipe and gives the S-suffixed schedules 5S, 10S, 40S and 80S, which are generally thinner because stainless has better corrosion resistance and needs less allowance. Note that STD and Sch 40 are identical up to NPS 10 and then diverge — STD is fixed at 9.53 mm from NPS 12 up, while Sch 40 keeps growing — and XS and Sch 80 are identical up to NPS 8, with XS fixed at 12.70 mm above that. XXS is not defined above NPS 12.

For any strength or fitness-for-service calculation, remember that these are nominal walls. Seamless and welded pipe to most specifications carries a 12.5 % mill undertolerance, so a nominal 9.53 mm wall may legitimately be delivered at 8.34 mm. Design calculations and remaining-life assessments should use the minimum wall, not the nominal, unless the actual thickness has been measured.