Barlow's Formula Calculator (Pipe Wall Thickness & Pressure)
By Santaji GadepatilinUpdated November 5, 2026
Barlow's formula relates the internal pressure a pipe can hold to its wall thickness, outside diameter and material strength. This Barlow's formula calculator gives the burst pressure and the design (allowable) pressure with a design factor, or the minimum wall thickness for a given pressure — a quick check before hydrostatic testing.
How to use the Barlow’s formula calculator
Enter the pipe material strength, the wall thickness and the outside diameter, plus a design factor. The calculator returns the pressure at which the hoop stress equals S and the design pressure. Clear the wall thickness and enter a design pressure to find the minimum wall.
Barlow’s formula
P = 2 S t ÷ D · Pdesign = 2 S t F ÷ D · t = P D ÷ (2 S F)
Worked example
6” API 5L X42 line pipe (SMYS 290 MPa), OD 168.3 mm, wall 7.11 mm, design factor 0.72:
- P at SMYS = 2 × 290 × 7.11 ÷ 168.3 = 24.50 MPa (245 bar)
- Design pressure = 24.50 × 0.72 = 17.64 MPa (176 bar)
Schedule 40 carbon steel pipe, S = 241 MPa (API 5L Gr B SMYS)
| Size | OD (mm) | Wall (mm) | P at SMYS (MPa) | P with F = 0.72 (MPa) |
|---|---|---|---|---|
| 2” Sch 40 | 60.3 | 3.91 | 31.3 | 22.5 |
| 3” Sch 40 | 88.9 | 5.49 | 29.8 | 21.4 |
| 4” Sch 40 | 114.3 | 6.02 | 25.4 | 18.3 |
| 6” Sch 40 | 168.3 | 7.11 | 20.4 | 14.7 |
| 8” Sch 40 | 219.1 | 8.18 | 18.0 | 13.0 |
| 10” Sch 40 | 273.0 | 9.27 | 16.4 | 11.8 |
| 12” Sch 40 | 323.8 | 10.31 | 15.3 | 11.0 |
Notes
- Barlow’s formula is a thin-wall approximation; it is conservative for thick walls (D/t below about 20).
- Always verify against the governing code (ASME B31.1, B31.3, B31.4, B31.8) before use.
Frequently asked questions
What is Barlow's formula?
P = 2 × S × t ÷ D, where S is the allowable or yield stress, t the wall thickness and D the outside diameter. It is the thin-wall hoop stress equation.
How do I find the wall thickness for a given pressure?
Rearrange to t = P × D ÷ (2 × S × F). In the calculator clear the wall thickness and enter the design pressure.
Which strength value should I use?
For gas pipelines (ASME B31.8) use the specified minimum yield strength with a design factor. For burst estimates use tensile strength with F = 1. For process piping use the code allowable stress.
Does Barlow's formula include corrosion allowance?
No. Add corrosion allowance and mill tolerance (often 12.5% of wall) to the thickness — ASME B31.3 uses t = PD ÷ 2(SEW + PY) plus allowances.