What Is Pipe Schedule?

Pipe schedule is a dimensionless number that designates the wall thickness of a pipe for a given NPS (Nominal Pipe Size). Higher schedule numbers mean thicker walls. The schedule system is defined in ASME B36.10 for carbon and alloy steel pipe and ASME B36.19 for stainless steel pipe.

A pipe’s schedule does not directly indicate a wall thickness in millimeters or inches. Instead, the schedule number, combined with the NPS, maps to a specific wall thickness from the ASME B36.10/B36.19 tables.

Common Schedule Numbers

Schedule	Category	Typical Use
SCH 5S, 10S	Thin wall (stainless)	Low-pressure stainless applications
SCH 10	Light wall	Low-pressure carbon steel, utilities
SCH 20	Light-medium	Moderate-pressure services
STD (SCH 40)	Standard	General-purpose process piping
SCH 40	Standard	Most common carbon steel schedule
XS (SCH 80)	Extra strong	Higher-pressure services
SCH 80	Extra strong	High-pressure process piping
SCH 120	Heavy wall	High-pressure, high-temperature
SCH 160	Very heavy wall	Very high pressure
XXS	Double extra strong	Maximum wall thickness

Example Wall Thicknesses (NPS 6)

For NPS 6 (OD = 168.3 mm), the wall thicknesses across schedules are:

Schedule	Wall (mm)	Wall (in)	ID (mm)	Weight (kg/m)
SCH 5S	2.77	0.109	162.8	11.31
SCH 10	3.40	0.134	161.5	13.84
SCH 40 (STD)	7.11	0.280	154.1	28.26
SCH 80 (XS)	10.97	0.432	146.4	42.56
SCH 120	14.27	0.562	139.8	54.20
SCH 160	18.26	0.719	131.8	67.56
XXS	21.95	0.864	124.4	79.22

Notice that SCH 40 and STD (Standard) are the same for NPS 1/8 to NPS 10. Above NPS 10, STD remains at 9.53 mm (3/8 in) while SCH 40 increases.

How Schedule Is Determined

The original formula for schedule number was:

Schedule = (1000 x P) / S

Where P = internal design pressure (psi) and S = allowable stress (psi). This formula is no longer used for pipe selection; instead, piping engineers calculate the required wall thickness using ASME B31.3 or B31.1 formulas and then select the nearest standard schedule that meets or exceeds the calculated thickness.

Pro Tip

STD, XS, and XXS are fixed wall thicknesses that do not change with NPS (above NPS 10). Schedule numbers (SCH 10, 20, 40, etc.) do change with NPS. Confirm the exact wall thickness from the ASME B36.10 tables rather than assuming SCH 40 = STD for all sizes.

For stainless steel pipe, ASME B36.19 defines schedules 5S, 10S, 40S, and 80S. The “S” suffix distinguishes stainless schedules, which have different wall thicknesses than the corresponding carbon steel schedules for some NPS values.

Read the full guide to pipe types