Increasing competition for water resources has made water conservation a priority. Measuring the flow rate of water is the first step to good water management and can also be very important when installing lawn sprinklers.

The easiest way to measure flow rate is with a flow rate meter. However, if one is not available, there are ways to measure the cubic feet per second (cfs) flowing through your pipes by using formulas or tanks and stopwatches.

To make sure everyone is starting on the same page. We’re using the calculation that one cubic foot equals 7.48 gallons. So a cubic foot per second is approximately 450 gallons per minute.

Measuring Discharge by Time and Volume

Depending on your system, use a bucket and a stopwatch to measure flow. Attach a tube or hose onto your spigot and time how long it takes to fill a 5 gallon bucket. If your spigot can fill a 5 gallon bucket in 47 seconds, you can figure out the flow rate with the following formula.

Flow rate = volume / time.

Formulaic Equation for Finding Volume in a Pipe

If you know the radius (or diameter) and length of your pipe, you can decipher the volume without getting your hands wet using this formula:

Pi x  radius squared x length

For a 20 foot length of 10” PVC pipe (using the diameter)

3.1416  x .5 (squared) x 20  = 15.71 cubic feet

There are 7.48 gallons in a cubic foot, so 15.71 cubic feet x 7.48 = 117.5 gallons can be carried by the pipe

Need more help? Ask an Expert. We’re happy to find the answers you need.

Sources: Oregon State University Extension Service and Goulds Water Technology