Using a 1 gallon container, time how long it takes to fill it with water. For the best accuracy measure the flow 3-to-4 times and average the times together. The formula to find GPM is 60 divided by the seconds it takes to fill a one gallon container (60 / seconds = GPM).

Example: The one gallon container fills in 5 seconds.

60 / 5 = 12 GPM.

(60 divided by 5 equals 12 gallons per minute.)

If you have a high flow you may need to use a larger container to get an accurate reading. To determine GPM using a larger container take the container capacity in gallons, divided it by the number of seconds needed to fill container, then multiply times 60. The result is the GPM.

Container size in gallons / Seconds to fill container x 60 = GPM

Example: Using a 5 gallon container it takes 14 seconds to fill the container.

5 / 14 x 60 = 21.4 GPM.

(5 divided by 14, then multiplied times 60, equals 21.4 GPM.)