Pipe Pressure Rating Calculator (Barlow)
Working or burst pressure of a pipe from outside diameter, wall thickness and allowable stress.
Results
What this tool does
Barlow's formula is the thin-wall relationship that every pipe catalogue is built on: pressure times diameter equals two times stress times wall thickness. It is deliberately conservative because it uses the outside diameter rather than the mean, and it ignores the axial and radial stresses entirely. For thick-walled pipe, where the wall is more than about a twentieth of the diameter, the Lame equations are the correct tool instead.
Formula
P = 2 S t ÷ D (Barlow's formula)
Variables
| Symbol | Meaning | Unit |
|---|---|---|
od | Outside diameter | mm |
wt | Wall thickness | mm |
ss | Allowable stress | MPa |
df | Design factor | — |
PB | Allowable pressure | bar |
PM | Allowable pressure | MPa |
HS | Hoop stress at that pressure | MPa |
ID | Inside diameter | mm |
Worked example
- Outside diameter114.3 mm
- Wall thickness6.02 mm
- Allowable stress245 MPa
- Design factor1
- Allowable pressure258.1 bar
- Allowable pressure25.808 MPa
- Hoop stress at that pressure245.00 MPa
- Inside diameter102.26 mm
Limitations
- For work that must comply with a standard or be signed off, check the result against the applicable code and have it reviewed by a qualified engineer.
- Mixing units is the most common source of error. Convert every input to the units shown next to each field before calculating.
Frequently asked questions
Which stress value should I put in?
It depends on what you are asking. Put in the ultimate tensile strength and you get the theoretical bursting pressure, which is not a rating. Put in the allowable stress from the design code — typically a fraction of yield, with further factors for temperature, weld joint quality and corrosion allowance — and you get a working pressure. Only the applicable code decides those factors, never this page.