# What Are Pipe-Fitting Formulas?

Credit: Michael Pereckas/CC-BY-SA 2.0

Pipe-fitting formulas allow pipe fitters and welders to determine how to fit a pipe so it can safely handle any internal pressure or stress from fluids within its wall. For example, Barlow's Formula is used by pipe fitters to predict the bursting pressure for a pipe.

Barlow's Formula, which is also called the Outside Diameter Formula, is determined by multiplying the total wall thickness of the pipe, denoted in inches, by two. The answer is then multiplied by the unit stress in pounds per square inch. This answer, which represents the internal units of pressure for the pipe, is then divided by the outside diameter of the tubing in inches. Therefore, the formula would appear as P = (2*T*S)/D, where P equals the internal units of pressure in pounds per square inch, S equals the unit stress in PSI, D equals the pipe's outside diameter in inches and T equals the pipe's wall thickness in inches.

Formulas are also helpful in aligning pipes correctly, as different fittings are used by pipe fitters and welders. In some instances, welding rings are used for alignment to ensure welding gaps are correct. Computation-based pipe installation tasks include aligning pipe components pipe-to-pipe, using a 45-degree elbow-to-pipe placement, incorporating a 90-degree elbow-to-pipe configuration and utilizing a tee-to-pipe design.

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