What is Barlow's formula?

Barlow's formula calculates minimum wall thickness for pipes under internal pressure: t = P × D / (2 × S × E), where P = internal pressure, D = outside diameter, S = allowable stress, E = weld joint efficiency factor.

What allowable stress should I use?

Allowable stress depends on material and temperature. For carbon steel (A106 Gr.B) at ambient: ~138 MPa (20,000 psi). For stainless 316 at ambient: ~138 MPa. Always refer to ASME B31.3 Table A-1 for design values.

What is the weld joint efficiency factor?

Weld efficiency (E) accounts for joint quality: E = 1.0 for seamless pipe, E = 0.85 for ERW (electric resistance welded), E = 0.60 for furnace butt welded. Higher E requires more inspection (radiography).

Do I need to add a corrosion allowance?

Yes. Add corrosion/erosion allowance (typically 1.5–3 mm for carbon steel in process service) to the calculated minimum wall thickness. Also consider manufacturing tolerance (typically -12.5% for seamless pipe).

What is the difference between Barlow and ASME B31.3?

Barlow's formula is a simplified version. ASME B31.3 uses t = PD/(2(SE+PY)), where Y is a temperature coefficient. For most cases at moderate temperatures, Y ≈ 0.4. The ASME formula gives slightly thinner walls.

Free Engineering Tool — #154

Pipe Wall Thickness Calculator — Barlow’s Formula

Calculate minimum wall thickness for pipes under internal pressure using Barlow’s formula: t = P×D/(2×S×E). Includes corrosion allowance and tolerance.

t = PD/(2SE)ASME B31.3Corrosion Allowance

Results

Minimum Required Wall Thickness (incl. corrosion & tolerance)

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Calculated t (Barlow)

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+ Corrosion Allowance

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Burst Pressure (2×S×E×t/D)

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Internal Diameter

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Barlow’s Formula

With Corrosion and Tolerance

P — design pressure
D — pipe outside diameter
S — allowable stress at design temperature
E — weld joint efficiency (1.0 seamless, 0.85 ERW, 0.60 FBW)
c — corrosion/erosion allowance
tol — manufacturing tolerance (typically 12.5%)

Example — DN100 at 10 MPa

Given: OD = 114.3 mm, P = 10 MPa, S = 138 MPa, E = 1.0

t = 10 × 114.3 / (2 × 138 × 1.0) = 4.14 mm

+ 1.5 mm corrosion = 5.64 mm

÷ (1 − 0.125) = 6.45 mm minimum

Select Sch 80 (8.56 mm wall) for this service.

⚠️ Note: This is Barlow’s simplified formula. For code compliance, use ASME B31.3 Eq. 3a which includes the Y factor. Always select the next standard pipe schedule above the calculated minimum.

Pipe Wall Thickness Calculator | Barlow’s Formula | Free Online Tool

Published by Nikolai Shelkovenko on February 15, 2026 February 15, 2026

Results

Barlow’s Formula

With Corrosion and Tolerance