Pipe wall thickness calculator (ASME B31.3)
Required wall thickness of straight pipe under internal pressure per ASME B31.3 equation (3a), with corrosion allowance and the 12.5% mill under-tolerance — then the lightest schedule that meets it.
Example S values at ambient: A106 Gr B ≈ 137.9 MPa (20 ksi); A312 TP316L ≈ 115 MPa (16.7 ksi). S falls with temperature — take it from ASME B31.3 Table A-1 for your material and design temperature. Y = 0.4 for ferritic and austenitic steels below 482 °C. This tool is a check, not a substitute for a qualified engineer's calculation.
Formula
t = P·D / (2·(S·E·W + P·Y)), with D = outside diameter and W = 1 below creep range. Minimum required thickness tm = t + c (corrosion, erosion and thread allowances). Because seamless and welded pipe may be up to 12.5% under nominal, the nominal wall must be at least tm ÷ (1 − 0.125).
Frequently asked questions
What is Y in the B31.3 wall thickness formula?
A coefficient that depends on material and temperature; 0.4 for ferritic and austenitic steels below 482 °C (900 °F).
Why divide by 0.875?
Pipe specifications allow wall thickness up to 12.5% below nominal, so the ordered nominal wall must be tm / 0.875 to guarantee the minimum.
What allowable stress should I use for A106 Grade B?
137.9 MPa (20 ksi) up to about 38 °C per ASME B31.3 Table A-1; lower at higher temperatures.
Last updated 2026-11-02