Pipe Wall Thickness Calculator
Minimum required pipe wall thickness for internal pressure per ASME B31.3 para 304.1.2, including corrosion allowance and mill tolerance.
When to use this calculator
Use during pipe class selection and pressure design verification — when you have a design pressure and temperature and need to prove a schedule is adequate, or work out which schedule to specify. It answers the three thicknesses a spec needs: the pressure design thickness t, the minimum required tm = t + c, and the nominal thickness to order once mill under-tolerance is allowed for.
Required inputs
- Design pressure P
- Outside diameter D — quick-select from NPS ½"–24" (OD per ASME B36.10M)
- Allowable stress S at design temperature (B31.3 Table A-1)
- Weld joint quality factor E and weld strength reduction factor W
- Temperature coefficient Y (0–0.7, default 0.4)
- Corrosion allowance c and mill tolerance % (default 12.5%)
Expected outputs
- Pressure design thickness t — pressure only
- Minimum required thickness tm = t + c
- Nominal order thickness after mill tolerance
- Fully substituted formula for the calculation record
Formula overview
SI throughout: P and S in MPa, D and t in mm. E, W, and Y are dimensionless; mill tolerance is a percentage.
ASME B31.3 para 304.1.2, Eq. (3a):
t = (P × D) / [2 × (S × E × W + P × Y)]
tm = t + c
t_nom = tm / (1 − millTol%)Worked example
2" pipe, P = 2 MPa, D = 60.3 mm, S = 138 MPa, E = 1.0, W = 1.0,
Y = 0.4, c = 1.6 mm, mill tolerance 12.5%
t = (2 × 60.3) / [2 × (138 × 1.0 × 1.0 + 2 × 0.4)]
= 120.6 / 277.6 = 0.43 mm
tm = 0.43 + 1.6 = 2.03 mm
t_nom = 2.03 / (1 − 0.125) = 2.33 mm
2" Sch 40 is 3.91 mm > 2.33 mm → acceptable. Corrosion allowance,
not pressure, governs this line.Common mistakes
- Stopping at t. The number you order against is t_nom — add the corrosion allowance, then divide by (1 − mill tolerance), in that order.
- Reading allowable stress S at ambient instead of at the design temperature. S falls sharply with temperature, and using the wrong row of Table A-1 is the most common non-conservative error here.
- Leaving E = 1.0 for a longitudinally welded pipe, or leaving W = 1.0 above about 400 °C where the weld strength reduction factor applies.
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