How to use the flow rate calculator
- Choose whether you measured the container in gallons or liters.
- Fill a container of known size from the tap or hose with the valve fully open. Enter the volume and the seconds it took. Add more tests to average them.
- Optionally enter a volume you want to fill, such as a tank or trough, to see how long it takes at this flow.
- Read the flow in gallons per minute, liters per minute and gallons per hour, and the slowest and fastest tests.
Formula
flow per minute = total volume / total seconds x 60
liters per minute = gallons per minute x 3.785411784
gallons per hour = gallons per minute x 60
time to fill = volume to fill / flow per minute
All the volume is divided by all the time, so longer tests count for more than short ones.
Three tests with a 5 gallon bucket
The bucket fills in 30, 31 and 29 seconds. The total is 15 gallons in 90 seconds, which is 15 / 90 x 60 = 10 gallons per minute, or 37.85 liters per minute and 600 gallons per hour. The single tests are 10.34, 9.68 and 10 gallons per minute. A 500 gallon tank takes 500 / 10 = 50 minutes to fill.
Two tests of 20 liters in 40 and 38 seconds collect 40 liters in 78 seconds, or 30.77 liters per minute, which is 8.13 gallons per minute. A 1,000 liter tank takes 32.5 minutes.
Getting a clean test
Use a container with clear volume marks, hold it so the water does not splash, and time from the moment the water enters to the moment it reaches the mark. Repeat the test two or three times, since pressure changes from moment to moment. The page averages the tests for you.
Run the test the way the water will be used. If a sprinkler will be fed through a hose, measure through that hose, because it limits the flow. Other taps running at the same time reduce the flow.
What flow tells you
Flow limits how many sprinkler heads can run at once, how fast a tank fills and whether a fixture will perform as rated. Compare the number with the flow your equipment needs. For sprinklers, the Irrigation Zone Flow Calculator uses this flow to plan zones.
Related water tools
Plan watering time with the Sprinkler Run Time Calculator, find what a pipe holds with the Pipe Volume Calculator, and estimate a bill with the Water Bill Calculator.
Assumptions and limits
- The container is filled at a steady flow for the whole test.
- The volume you enter is the amount collected, not the size of the container.
- One US gallon is 3.785411784 liters.
- The result is the flow at the pressure you tested, which may change at other times of day.
Frequently asked questions
How do I measure gallons per minute at home?
Time how long a bucket of known size takes to fill. Divide the gallons by the seconds and multiply by 60. A 5 gallon bucket that fills in 30 seconds means 10 gallons per minute.
How many liters per minute is 10 gallons per minute?
Multiply by 3.785411784. Ten gallons per minute is 37.85 liters per minute.
Why does the page average several tests?
Pressure and timing vary from one test to the next. Adding the volume and time of all the tests gives a steadier number than any single test.
How long will it take to fill a tank?
Divide the volume of the tank by the flow per minute. A 500 gallon tank at 10 gallons per minute takes 50 minutes.