Barlow's Formula

Barlow's formula relates the internal pressure that a pipe can withstand to its dimensions and the strength of its material.

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 (typically, pounds per square inch)
S = allowable stress (typically, pounds per square inch, yield or tensile depending upon application)
t = wall thickness (typically, inches)
D = outside diameter (typically, inches)

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.