Pipe Schedules Explained: NPS, Wall Thickness and What the Numbers Mean

Schedule 40, Schedule 80, XS, XXS — the schedule system fixes a pipe's wall thickness for each nominal size. Here is how it works and how to think about pressure.

The basic idea

A pipe size has two independent parts: the nominal size (NPS in inches, DN in millimetres) and the schedule, which selects the wall thickness. For any given NPS, the outside diameter is fixed by ASME B36.10M (carbon steel) or B36.19M (stainless); changing the schedule changes only the wall — and therefore the inside diameter and the weight.

Where schedule numbers came from

The schedule number was originally set as roughly 1000 × P/S, where P is the internal pressure and S the allowable stress of the material, both in psi. That direct meaning has faded — modern schedules are simply a standardized ladder of wall thicknesses — but it explains why a higher schedule always means a thicker, stronger, heavier pipe at the same NPS.

The common designations

Designation	Meaning
Sch 5, 10, 20, 30	Thin walls, low pressure and weight-driven service
Sch 40 / STD	The general-purpose wall; "STD" (standard) equals Sch 40 up to NPS 10
Sch 80 / XS	"XS" (extra strong) equals Sch 80 up to NPS 8
Sch 120, 160	Heavy walls for high pressure or severe service
XXS	Double extra strong — thicker than Sch 160 in small sizes, not a fixed schedule above NPS 8

Stainless schedules carry an S suffix (5S, 10S, 40S, 80S) under ASME B36.19M; the walls match the carbon-steel equivalents in small sizes but diverge in larger ones.

A worked example

NPS 6 pipe has an OD of 168.3 mm in every schedule:

NPS 6	Wall (mm)	Weight (kg/m)
Sch 40	7.11	28.26
Sch 80	10.97	42.56
Sch 160	18.26	about 66

Same flanges, same fittings OD — but the bore shrinks from 154.1 mm to 131.0 mm between Sch 40 and Sch 160, which matters for flow and for matching fittings.

Thinking about pressure

There is no universal pressure rating for "Sch 40". Pressure capacity follows the Barlow relationship — it rises with wall thickness and material strength and falls as diameter grows — and design codes (ASME B31.1, B31.3, EN 13480) then apply temperature derating, corrosion allowance, weld joint factors and safety margins. A Sch 40 pipe in a strong alloy at room temperature and the same pipe in plain carbon steel at 400 °C are entirely different pressure components. Always rate the system — pipe, material, temperature and code together — never the schedule alone.

Specifying tips

State NPS (or DN), schedule, material standard and grade together; any one alone is incomplete.
For stainless, write the S suffix — Sch 40 and Sch 40S walls differ at NPS 12 and above.
Remember STD and XS are legacy labels that only coincide with Sch 40/80 in part of the size range.