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ASME B31.3 Pipe Wall Thickness Calculator

Pressure design thickness per paragraph 304.1.2, Eq. (3a), with corrosion allowance, mill tolerance and schedule selection per ASME B36.10M.

Units

Nominal pipe size (NPS)

Outside diameter D

Design pressure P (gauge)

MPa

Allowable stress S

MPa

At design temperature, from ASME B31.3 Table A-1 for your material.

Joint quality factor E

Weld joint strength reduction factor W

—

1.0 at moderate temperatures (para. 302.3.5).

Coefficient Y

—

0.4 for ferritic steels below 482 °C (Table 304.1.1).

Allowance c (corrosion, threads, grooves)

Mill tolerance (%)

12.5% for ASTM A106 and A53.

Reference result. Verify S, E, W and Y in the ASME B31.3 edition that applies to your project; this calculation does not replace review by a responsible engineer.

t = P·D / [2·(S·E·W + P·Y)]

Pressure design thickness per paragraph 304.1.2, Eq. (3a), with corrosion allowance, mill tolerance and schedule selection per ASME B36.10M.

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How pipe wall thickness is calculated per ASME B31.3

For straight pipe under internal pressure, ASME B31.3 (paragraph 304.1.2) defines the pressure design thickness with Eq. (3a). It is valid when the calculated thickness is less than D/6; above that limit the code requires special analysis.

t = P·D / [2·(S·E·W + P·Y)]

Symbol	Meaning
P	Internal design gauge pressure
D	Outside diameter of pipe
S	Allowable stress at design temperature (Table A-1)
E	Longitudinal joint quality factor (Table A-1A / A-1B)
W	Weld joint strength reduction factor (302.3.5)
Y	Coefficient from Table 304.1.1

The thickness you buy is not t. First add the allowance c for corrosion, erosion or thread and groove depth to get the minimum required thickness tm. Then divide by (1 − mill tolerance), because the material standard allows the wall to come out thinner than nominal. Use that value to pick the first ASME B36.10M schedule that covers it.

Pipe wall thickness FAQ

Which equation does ASME B31.3 use for wall thickness?

Eq. (3a) in paragraph 304.1.2: t = P·D / [2·(S·E·W + P·Y)], valid for straight pipe under internal pressure when t is less than D/6.

Which value of Y should I use?

For ferritic steels below 482 °C, Y = 0.4. For other temperatures and materials, see ASME B31.3 Table 304.1.1.

Why divide by 0.875?

Because ASTM A106 and A53 allow a 12.5% mill under-tolerance. Dividing by 0.875 ensures the actual wall still meets the minimum thickness.

What if t is greater than D/6?

Eq. (3a) no longer applies. Per 304.1.2(b), thick-wall pipe design requires special consideration such as failure theory, fatigue and thermal stress.

Does it work for ASME B31.1 or B31.4?

Not directly. Each code has its own equation and factors. This tool is for ASME B31.3 only; PipeStamp includes calculators for B31.1, B31.4, B31.8 and more.