Pipe schedules explained
A steel pipe is specified by two numbers: the nominal pipe size (NPS), which fixes the outside diameter, and the schedule, which fixes the wall thickness. Both come from ASME B36.10M for carbon steel and B36.19M for stainless steel.
Nominal pipe size is not the outside diameter
Up to NPS 12 the nominal size is a rough bore size, not a measurement: a 4 inch pipe has a 4.5 in outside diameter. From NPS 14 the outside diameter equals the nominal size. The outside diameter stays the same in every schedule so the same fittings and flanges fit.
Schedule numbers
Schedules run 5, 10, 20, 30, 40, 60, 80, 100, 120, 140 and 160. A higher number means a thicker wall for the same NPS, but the same schedule number does not mean the same wall thickness across sizes: the wall grows with the diameter so that each schedule gives roughly the same pressure rating.
| NPS 4" | Wall (in) | ID (in) | lb/ft |
|---|---|---|---|
| Sch 10 | 0.120 | 4.260 | 5.62 |
| Sch 30 | 0.188 | 4.124 | 8.67 |
| STD | 0.237 | 4.026 | 10.80 |
| Sch 40 | 0.237 | 4.026 | 10.80 |
| Sch 80 | 0.337 | 3.826 | 15.00 |
| Sch 120 | 0.437 | 3.626 | 18.98 |
| Sch 160 | 0.531 | 3.438 | 22.53 |
STD, XS and XXS
Before schedule numbers, pipe was sold as standard (STD), extra strong (XS) and double extra strong (XXS). These names are still used:
- STD has the same wall as Sch 40 up to NPS 10. From NPS 12 it stays at 0.375 in.
- XS has the same wall as Sch 80 up to NPS 8. From NPS 10 it stays at 0.500 in.
- XXS is about twice the XS wall in small sizes and has no schedule equivalent.
Stainless steel schedules (5S, 10S, 40S, 80S)
ASME B36.19M adds an S suffix for stainless pipe. 40S and 80S match Sch 40 and 80 up to NPS 10, but differ for larger sizes. Stainless pipe is about 1 % heavier than carbon steel pipe of the same size because of its higher density.
How schedule affects weight
Weight per foot = 10.69 × (OD − t) × t, so weight rises almost in proportion to the wall. Every size has a page with all its schedules, e.g. 4 inch pipe, and the pipe schedule chart lists them all.
Frequently asked questions
What is the difference between Schedule 40 and Schedule 80 pipe?
Same outside diameter, thicker wall. 4 inch Sch 40 has a 0.237 in wall and weighs 10.80 lb/ft; Sch 80 has a 0.337 in wall and weighs 15.00 lb/ft. The thicker wall takes more pressure and leaves a smaller bore.
Is STD the same as Schedule 40?
Up to NPS 10 they have the same wall. From NPS 12, STD stays at 0.375 in while Sch 40 keeps getting thicker; for 12 inch pipe Sch 40 is 0.406 in.
Does the pipe schedule change the outside diameter?
No. For a given nominal pipe size the outside diameter is fixed so that fittings and flanges fit every schedule. The schedule only changes the wall thickness and therefore the inside diameter and weight.