Pipe Schedules Explained: NPS, OD, STD, XS and Sch 40/80

NPS and outside diameter

Pipe sizes were originally named after the approximate inside diameter of early standard pipe. When walls changed, the names stayed. Today:

• NPS 1/8 to NPS 12: the NPS is only a name. NPS 2 has an outside diameter of 60.3 mm (2.375 in); NPS 6 has 168.3 mm (6.625 in).
• NPS 14 and above: the outside diameter in inches equals the NPS. NPS 16 is 16 in (406.4 mm).

Europe and many specifications also use DN (diameter nominal) in millimetres: DN 50 corresponds to NPS 2, DN 150 to NPS 6.

Schedules

For each NPS the outside diameter is fixed so that fittings and flanges match. The wall thickness changes by schedule:

• numbered schedules: 5, 10, 20, 30, 40, 60, 80, 100, 120, 140, 160,
• weight classes: STD (standard), XS (extra strong) and XXS (double extra strong).

Up to NPS 10, Sch 40 equals STD; from NPS 12 upward they differ (STD stays at 9.53 mm while Sch 40 keeps growing). Up to NPS 8, Sch 80 equals XS.

A few common sizes (carbon steel, ASME B36.10M)

NPS	OD (mm)	Sch 40 (mm)	Sch 80 (mm)	Sch 160 (mm)
2	60.3	3.91	5.54	8.74
4	114.3	6.02	8.56	13.49
6	168.3	7.11	10.97	18.26
8	219.1	8.18	12.70	23.01
12	323.9	10.31	17.48	33.32

For all sizes and schedules, use the pipe schedule lookup on this site.

Stainless steel schedules

ASME B36.19M covers stainless steel pipe with schedules marked “S”: 5S, 10S, 40S and 80S. Thin-walled 10S pipe is common because stainless rarely needs corrosion allowance. Up to NPS 10, 40S equals Sch 40 and 80S equals Sch 80; at NPS 12, 40S and 80S follow STD and XS.

Choosing the schedule

The schedule is chosen from the pressure design wall thickness plus allowances and mill tolerance, rounded up to the next standard schedule. Try the wall thickness calculator for B31.3 straight pipe.

What to take back to your desk

When you write a size on a datasheet, always give both NPS and schedule (or wall thickness in mm). “6 inch pipe” on its own could be five different products.