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Pipe Wall Thickness Calculator

ASME B31.3 pressure design thickness for straight pipe under internal pressure — with schedule check.

Pipe size (NPS)

Design pressure (barg)

Design temperature (°C)

Corrosion / mechanical allowance (mm)

Typical: 0 for stainless/clean service, 1.5–3 mm for carbon steel water/process lines.

Pipe material

Custom allowable stress S (MPa)

Longitudinal joint quality factor (E)

Custom E value

Include 12.5% mill under-tolerance (standard practice for ordering pipe)

Minimum nominal thickness needed

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Pressure design thickness (t)

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Allowable stress used (S)

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Schedule	Nominal thickness	Sufficient?
Schedule 40 / STD	–	–
Schedule 80 / XS	–	–

t = P × D / (2 × (S × E + P × Y))

⚠ Calculated thickness exceeds D/6 — this pipe falls into the "thick wall" category where ASME B31.3's alternate thick-wall formula (Eq. 3a, para. 304.1.2) applies instead of the standard thin-wall equation used here. Have this case checked directly against the code.

⚠ Design temperature is outside the reference allowable-stress table range for this material — the value shown is clamped to the nearest table point. Consult ASME B31.3 Table A-1 directly for this temperature.

Why this matters: pipe wall thickness isn't just about withstanding pressure today — it has to survive corrosion and wear over the design life too. The pressure design thickness (t) is the bare structural minimum; the corrosion/mechanical allowance (c) is what's sacrificed to wear over time, and the 12.5% mill under-tolerance accounts for the fact that "nominal" pipe wall is allowed to run thinner than stated straight from the mill. Skipping any of these three is how pipe ends up under-designed.

How this is calculated

This tool uses the ASME B31.3 straight-pipe pressure design formula (para. 304.1.2): t = P × D / (2 × (S × E + P × Y)), where t is the pressure design thickness, P is internal design pressure, D is pipe outside diameter, S is the material's allowable stress at design temperature, E is the longitudinal joint quality factor, and Y is a wall-thickness coefficient fixed here at 0.4 (valid for ferritic/austenitic/ductile materials below 482°C / 900°F, which covers the great majority of process and water piping). The minimum required thickness adds the corrosion/mechanical allowance (t + c), and — since mills are permitted to supply pipe up to 12.5% thinner than nominal — the minimum nominal thickness you should order is (t + c) ÷ 0.875. Allowable stress values are drawn from ASME B31.3 Table A-1 reference data for common materials (A106/A53 Grade B carbon steel, 304/304L and 316/316L stainless) and linearly interpolated between listed temperature points. Standard Schedule 40/STD and Schedule 80/XS wall thicknesses are per ASME B36.10. This is a preliminary sizing tool — final wall thickness selection for fabrication or code-stamped work should be verified against the current B31.3 edition, or reviewed by a qualified engineer.