Choosing Pipe Schedule

Quick pick (TL;DR)

Compute B31.3 pressure thickness, add corrosion allowance and mill tolerance, then round UP to a standard schedule.
Small-bore CS (≤ 2″) → Sch 80 minimum for robustness.
Larger lines → Sch 40 / STD as the default.
The project material class usually dictates the schedule anyway.

What schedule means

Same OD, different wall — pressure thickness + allowance, rounded up to a standard schedule.

Worked calculation

6″ A106-B line

Given P = 2.0 MPa , D = 168.3 mm (6″) S = 137.9 MPa , E = 1.0 , W = 1.0 , Y = 0.4 corrosion allowance c = 1.5 mm , mill tol = 12.5% Step 1 — Pressure thickness t = (P·D) / [2·(S·E·W + P·Y)] t = (2.0 × 168.3) / [2 × (137.9 + 0.8)] t = 336.6 / 277.4 = 1.21 mm Step 2 — Add allowance t_min = 1.21 + 1.5 = 2.71 mm Step 3 — Mill tolerance order = 2.71 / (1 − 0.125) = 3.10 mm Step 4 — Select schedule Sch 40 (6″) = 7.11 mm ≥ 3.10 → ✓ ample conclusion: pressure does not govern; pick Sch 40

Pipe wall-thickness calculator (ASME B31.3)

Pressure thickness → schedule

t = P·D/(2·(S·E + P·Y)) + CA + mill tolerance → lightest schedule that passes.

Nominal size

Design pressure (barg)

Allowable S (MPa)

Joint efficiency E

Y coefficient

Corrosion allow. (mm)

Mill tolerance (%)

Internal-pressure screening (B31.3 §304.1.2). Not for external pressure / bending / thick-wall.

Reference — schedule wall thickness (mm)

NPS	OD (mm)	Sch 40	Sch 80	Sch 160
2"	60.3	3.91	5.54	8.74
3"	88.9	5.49	7.62	11.13
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
10"	273.0	9.27	15.09	28.58
12"	323.8	10.31	17.48	33.32

Decision flow

1. t = P·D/(2(S·E+P·Y))

2. + CA, ÷ (1 − mill tol)

Pick scheduleLightest Sch ≥ ordered t

OftenSch 40 ample; mechanical min governs

High pressureSch 80/160/XXS

At low pressure the schedule is set by mechanical minimums, not pressure.

Schedules at a glance

Schedule	Typical use
Sch 10 / 10S	Low-pressure utility / stainless.
Sch 40 / STD	Default for most lines.
Sch 80 / XS	Higher pressure, small bore, threaded.
Sch 160 / XXS	High-pressure / severe service.

How to choose

Compute the B31.3 pressure thickness.
Add corrosion allowance.
Add mill tolerance, round up.
Apply a minimum schedule for small bore.
Reconcile with the material class.

Common mistakes

Pressure-only sizing, too thin.
Forgetting tolerance/allowance.
Mixing schedules at fittings.

Frequently asked questions

What should I know about Choosing Pipe Schedule?

Compute B31.3 pressure thickness, add corrosion allowance, add mill tolerance, then round UP to a standard schedule. Small-bore CS (≤ 2″) → Sch 80 minimum for robustness. Larger lines → Sch 40 / STD default. The project material class usually dictates the schedule anyway.

Which standards or codes apply?

ASME B31.3 (thickness); ASME B36.10M (steel) / B36.19M (stainless) dimensions & schedules.

Standards — ASME B31.3; B36.10M / B36.19M.