Pipe schedules: Sch 40, STD, XS, 40S explained

A schedule number sets the wall thickness for a given size. Sch 40 at 2" is 3.91 mm while Sch 40 at 8" is 8.18 mm, so a schedule is not one fixed thickness.

STD (standard), XS (extra strong) and XXS (double extra strong) are older weight classes. STD equals Sch 40 up to NPS 10 and stays 9.53 mm above it. XS equals Sch 80 up to NPS 8 and stays 12.70 mm above it.

Schedules with an "S" suffix (5S, 10S, 40S, 80S) come from ASME B36.19M for stainless steel. 40S and 80S match STD and XS, but differ from Sch 40 and 80 at some larger sizes.

Mill tolerance on seamless pipe wall is typically −12.5 %, which is why design codes add it back when sizing the wall.