Enter the GPM (gallons per minute) and the velocity of the flow (ft/min) into the calculator to determine the pipe size.

GPM to Pipe Size Formula

The following formula is used to convert GPM to pipe size, in this case, a diameter. 

D= SQRT(4*(GPM/7.481)/V/PI)
  • Where D is the diameter of the pipe (ft)
  • GPM is the flow rate through the pipe (gallons per minute)
  • V is the velocity of the flow (ft/minute)

To calculate pipe size, divide the GPM by 7.481, then again by the floe velocity and again by pi. Multiply this result by 4, then take the square root.

GPM to Pipe Size Definition

Converting a GPM to pipe size is done using the formula Q=V*A, where Q is the volumetric flow rate (GPM), V is the velocity, and A is the cross-sectional area.

In this case, we know Q and V, but not the area (aka pipe size). Solving for the area we get a formula of A= Q/V. Substituting area with the formula of the area circle and solving for D we get the formula above.

Example Problem

How to calculate Pipe Size from GPM?

First, determine the volumetric flow rate of the fluid through the pipe. In this case, we want the flow rate to be in gallons per minute. After measuring we determine the flow rate to be 40 gallons per minute.

Next, convert the flow rate into feet cubed (ft^3) per minute. This is so the units match up with the area in the next step. To do this, divide 40 by 7.481

= 40 / 7.415 = 5.3944 ft^3/min

Next, determine the linear velocity of the fluid. Measuring the flow yields a velocity of 4 ft/min.

Finally, calculate the pipe diameter using the formula above:

D= SQRT(4*5.3944)/4/3.14159)

= 1.310

Similarly, the cross-sectional area can be calculated as:

A = Q/V

= 5.3944/4

= 1.336 ft^2