Pipe Tools & Calculators

Kelly Resources

Barlows Formula

Barlow’s Formula is used to determine:

  • Internal Pressure at Minimum Yield
  • Ultimate Bursting Pressure
  • Maximum Allowable Operating Pressure
  • Mill Hydrostatic Test Pressure

The formula is expressed as P=2St/D, where:
P = pressure, psig
t = nominal wall thickness, in inches(i.e. .375)
D = outside Diameter in inches
S = allowable stress in psi, which depends on the pressure being determined utilizing Yield or Tensile depending on what is trying to be determined

For example:
Internal Pressure at Minimum Yield:
S = SMYS – minimum yield for grade of pipe

Ultimate Bursting Pressure:
S = SMTS – minimum tensile strength for grade of pipe

Maximum Allowable Operating Pressure:
S = SMYS – reduced by a design factor

Mill Hydrostatic Test Pressure:
S = SMYS – reduced by a design factor depending on OD and grade


P = Pressure psig
t = Nominal Wall Thickness in.
D = Outside Diameter in.
S = Allowable Stress psi.

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