Pipe Schedule Explained: NPS, Outside Diameter and Wall Thickness of SCH40 Seamless Pipe

Steel pipe is ordered with a short code such as "2 inch SCH40 seamless". This guide explains what the schedule number means, why "2 inch" is not the outside diameter, how wall thickness changes with schedule, and lists the SCH40 seamless pipe sizes in the Ahan Asa catalog with their mass per metre and per length, so engineers and importers can check a specification before ordering.

What a pipe schedule is

A schedule (SCH) is a designation for the nominal wall thickness of steel pipe. As Kloeckner Metals puts it, schedule numbers are non-dimensional: the 40 in SCH40 is neither millimetres nor inches but the name of a thickness class. The tables in Wikipedia's "Nominal Pipe Size" article list schedules such as 10, 20, 30, 40, 60, 80, 100, 120, 140 and 160, alongside the older names STD (standard weight), XS (extra-strong) and XXS (double extra-strong). According to the same source, STD is identical to 40S, and 40S is identical to SCH40 for NPS 1/8 to NPS 10.

The dimensions are set by ASME B36.10M, whose scope is the standardisation of dimensions of welded and seamless wrought steel pipe for high or low temperatures and pressures.

Nominal size, outside diameter and DN

A pipe is named first by its nominal size: NPS in the North American system and DN in the European and international system (for example DN 50). Wikipedia notes a one-to-one correspondence between the two.

The nominal size is not the outside diameter. Under the scope of ASME B36.10M, pipe of NPS 12 (DN 300) and smaller has an outside diameter numerically larger than its size; a 4 inch pipe measures 114.3 mm OD, not 101.6 mm. According to Wikipedia, from NPS 14 upward size and OD are equal. The ASME B36.10M scope also points out that for tube the OD always equals the size number, so pipe and tube cannot be read the same way.

Fixed outside diameter, changing wall

For a given nominal size the outside diameter stays fixed and the wall gets thicker as the schedule rises, so the bore gets smaller. Wikipedia's tables give a 2 inch pipe a 3.91 mm wall in SCH40 and a 5.54 mm wall in SCH80, with the same 60.3 mm OD. Kloeckner Metals adds that SCH40 has a thinner wall than SCH80 and that SCH80 handles higher pressures.

The SCH40 wall is not the same across sizes: 2.77 mm at 1/2 inch and 8.18 mm at 8 inch. Which schedule a line needs depends on its pressure, temperature and design code, and is the piping designer's decision.

SCH40 seamless pipe in the Ahan Asa catalog

The catalog lists seamless pipe to ASTM A106 Grade B in 12 sizes, all SCH40, dimensioned to ASME B36.10M, in 6 m lengths. OD, wall and mass come from the catalog; DN from the Wikipedia table. Values are rounded.

All sizes are in the pipe category. Nominal mass is for estimating; deliveries are invoiced on actual weight.

Estimating mass per metre

With the outside diameter D and wall t in millimetres, the approximate mass of steel pipe (density about 7850 kg/m³) is 0.02466 × (D − t) × t kg/m. For 4 inch SCH40 this gives about 16.1 kg/m against the catalog's 16.03; the small gap comes from rounding and the density assumed. Use the catalog figure for orders and the formula as a quick check. The same per-metre logic for rebar and beams is in our rebar and IPE weight tables.

What ASTM A106 adds

The schedule fixes dimensions only; the material standard governs chemistry and testing. According to its scope, ASTM A106/A106M covers seamless carbon steel pipe for high-temperature service from NPS 1/8 to NPS 48 (DN 6 to DN 1200), with nominal wall thickness as given in ASME B36.10M, and the pipe is suitable for bending, flanging and similar forming, and for welding.

Before you order

• State size and schedule together, e.g. "3 inch SCH40"; a size alone does not define the wall.
• Check the OD against the drawing or fittings so pipe is not confused with tube.
• Specify the material standard and grade (in our catalog ASTM A106 Grade B) and check the mill test certificate against it.
• Give the number of lengths; catalog pipe comes in 6 m lengths. For site storage, see storing steel on site.

Summary

A pipe schedule is a unitless wall-thickness class. Within a nominal size the OD is fixed and the wall thickens as the schedule rises; up to 12 inch the OD is larger than the size number. Ahan Asa's seamless pipe covers 12 sizes from 1/2 to 8 inch, all SCH40 to ASTM A106 Grade B in 6 m lengths. To request a quotation, contact us.