Barlow’s Formula

Barlow’s Formula relates the internal pressure that a pipe can withstand to its dimensions and the strength of its materials. The formula is P=(2*t/D), where:

  • P = pressure
  • S = allowable stress
  • t = wall thickness
  • D = outside diameter

To use our Barlow’s Formula calculator, select values for three of the four variables, and the calculator will provide you with a number for the fourth variable that you are trying to calculate:

P (pressure): psigS (allowable stress): psi.

t (wall thickness): in.

D (outside diameter): in.

More about Barlow’s Formula

Barlow’s Formula is commonly used in the piping industry to verify that pressure vessels and pipe used for gathering, transmission and distribution lines can safely withstand operation pressures. Today, we also see a modification of Barlow’s Formula that incorporates a safety factor into the calculation: P=(2*S*T)/(D*SF)), where SF is the safety factor. In DOT 192, the federal government has defined the numerical values of the safety factor based on the physical location of the pressure vessel.

Our Barlow’s Formula calculator is a highly useful tool in determining the requirements of your pipe. But ultimately, the design of complex pressure containment systems involves much more than the application of Barlow’s Formula. For almost all pressure vessels, ASME code stipulates the requirements for design and testing. If you have any further questions, our piping product experts can assist you. To speak with a pipe expert, click here, email us at, or call us at 1-800-316-5737.