Rule of thumb · applies

Schedule is not a wall thickness – Sch 40 means nothing without the NPS

Schedule is a wall-thickness series – not the wall thickness itself.

A rule of thumb replaces neither the design calculation nor the applicable standard.

Category: robust rule
Standards referenced: ASME B36.10M, ASME B36.19M, EN 10220

Rule of thumb

"Sch 40" without the NPS is not a dimension.

Where does the rule come from?

The schedule number comes from a rough formula of the 1930s: schedule ≈ 1000 · p / S, with the working pressure p and the allowable stress S in psi. A pipe meant to carry 40 psi at an allowable stress of 1000 psi was "schedule 40" – and because that yields a different wall thickness at every nominal size, each schedule has its own millimetre value per NPS. Today the formula is an explanation, not a sizing rule: the wall thicknesses are fixed in the table of the standard and have been adjusted to manufacturing practice several times.

What is technically correct?

Schedule (Sch 5, 10, 20, 30, 40, 60, 80, 100, 120, 140, 160) is an index that points to a fixed wall thickness per NPS. Sch 40 means 3.91 mm at NPS 2 (60.3 mm), 6.02 mm at NPS 4 (114.3 mm), 10.31 mm at NPS 12 (323.9 mm). Sch 80 at the same NPS 2 is 5.54 mm, at NPS 4 8.56 mm. In addition there are the older weight classes STD (standard), XS (extra strong) and XXS: STD coincides with Sch 40 up to NPS 10, XS with Sch 80 up to NPS 8 – from NPS 12 they diverge (NPS 12: STD 9.53 mm, Sch 40 10.31 mm). For stainless steel (B36.19M) the thin series Sch 5S, 10S, 40S, 80S are added, whose values partly differ from the "non-S" ones.

A wall thickness is therefore only defined by NPS and schedule – and ordering an inch pipe needs both plus the material (ASTM A106 Gr. B, A312 TP316L …).

Where does the rule end?

Sch 40 is not "medium" and Sch 80 is not "double": from NPS 2 to NPS 12 the Sch 40 wall grows from 3.9 to 10.3 mm, and the ratio Sch 80 to Sch 40 lies between 1.4 and 1.7 depending on NPS.
STD = Sch 40 and XS = Sch 80 only hold up to NPS 10 and NPS 8 respectively – applying the equation at NPS 12 and above orders the wrong wall.
Conversion into the EN world is a question of dimensions, not of approval: 60.3 × 3.6 (EN 10220) is the dimensional neighbour of NPS 2 Sch 40 (3.91 mm), but material, tolerances and testing follow other standards.
Fittings to ASME B16.9 and flanges to B16.5 are ordered with the schedule of the pipe so that wall thicknesses match at the weld – again: NPS and schedule, never one alone.

What does the standard say?

ASME B36.10M specifies, for welded and seamless steel pipe, the outside diameters per NPS and the wall thicknesses per schedule; ASME B36.19M does the same for stainless-steel pipe with the S series. The schedule numbers are table columns there, not quantities to calculate with. EN 10220 knows no schedule: per outside diameter it lists a stepped series of wall thicknesses that can be combined freely (at 60.3 mm for example 2.9 · 3.2 · 3.6 · 4.0 · 4.5 · 5.0 mm and onwards).

DIN/EN versus ASME/ASTM

Two systems, the same separation of diameter and wall, but different handling: EN states the wall thickness directly in mm and allows combinations; ASME states an index and fixes the combination. The outside diameters of the inch series are practically identical to the EN series, the wall thicknesses are not – NPS 2 Sch 40 (3.91 mm) hits no EN step (3.6 or 4.0 mm). When welding EN pipe to an ASME fitting, the wall thickness at the joint has to be checked, not the DN.

Worked example

One customer orders "2 inch pipe schedule 40", another "pipe 60.3 × 3.6 EN 10216-2 P235GH" (core range). Both mean a pipe with 60.3 mm outside diameter; the first has a 3.91 mm wall to B36.10M, the second 3.6 mm. For a flange DN 50 PN 16 both fit; for a 2" Sch 40 fitting to ASME B16.9 only the first fits without an offset at the weld. At NPS 12 the difference between STD (9.53 mm) and Sch 40 (10.31 mm) is regularly overlooked in bill-of-materials checks – exactly the case the rule is made for.

Checked today

Traffic light: applies (class A). By the standard, schedule is an index; the wall thickness follows only from NPS and schedule. The historical formula 1000·p/S is origin, not rule.

Frequently asked questions

How thick is schedule 40?

It depends on the nominal size: NPS 2 → 3.91 mm, NPS 4 → 6.02 mm, NPS 12 → 10.31 mm (ASME B36.10M).

Is STD the same as Sch 40?

Only up to NPS 10. From NPS 12 STD is thinner (9.53 mm) than Sch 40 (10.31 mm).

Which EN pipe corresponds to 2" Sch 40?

Dimensionally 60.3 mm outside with 3.6 or 4.0 mm wall as neighbours of 3.91 mm – none of them is equivalent in approval terms; material and standard have to be settled separately.

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