# Barlow's Equation

## What Does Barlow's Equation Mean?

Barlow's equation shows the relationship between a pipe's internal pressure, its allowable stress, the pipe's nominal thickness, and its diameter. Using three of these four variables enables you to calculate the fourth.

For instance, if you wish to know how thick a pipe's walls must be to withstand 12,000 p.s.i. and you know the allowable stress, the pipe's proposed diameter and the internal pressure of the materials passing through the pipe, you would use Barlow's equation to determine the pipe wall thickness.

## Trenchlesspedia Explains Barlow's Equation

Barlow's equation is expressed as P=2SI/D, where P is the pressure (p.s.i.), I is the wall thickness in inches or (for pipe walls less than one inch thick) the decimal equivalent of a fraction of an inch (0.125 in., for example), D is the outside diameter of the pipe and S is the allowable stress expressed in pounds per square inch. The allowable stress can be either the tensile strength of the pipe or the yield strength of the pipe; the choice between the two is dictated by the design's needs. 