How to Measure the GPM Flow Rate of a Faucet or Shower

Water running from showerhead
Shower heads since 1992 are required to have a flow rate of no more that 2.2 GPM. Nick Koudis/Photodisc/Getty Images

Although it's not a common task, there may be instances where you need to measure the flow rate of your home's faucets and showers. For example, it can be a critical factor in sizing a tankless water heater, or for evaluating if an older fixture is wasting water. It's also sometimes a helpful measurement to know when you are buying or selling a house and are evaluating the efficiency of the plumbing fixtures.

 

The standard of measurement for water flow in plumbing fixtures is Gallons per Minute (GPM). Sometimes you will see a rating for 'flow rate' printed on the packaging for a shower head or faucet. For water conservation purposes, the Federal Energy Policy Act of 1992 required that all faucet and shower fixtures made the US have a flow rate of no more than 2.2 GPM at 60 PSI. 

Fixtures that are exceeding the 2.2 GPM guideline can often be brought into compliance by adding low-flow aerators to the faucet spout. But older fixtures dating before 1992 often used much more water than 2.2 GPM, sometimes upward of 10 GPM. So, before any calculations for water heater sizing can properly be done, an accurate measurement of a fixture's flow rate should be done. The sum flow rate for fixtures served by a water heater is a key measurement for selecting the proper size heater.

If any of your fixtures measure much more than 2.2 GPM, they are good candidates for replacement with a new code-compliant water efficient fixture.

Here's how to quickly determine flow rate for a specific faucet or shower head:

  1. Turn the fixture handle fully on to its normal position.
  2. Place a container under the fixture and collect the water for exactly 10 seconds.
  3. Measure the quantity of water in the container and convert the measurement to gallons . For example, if your measure 2 quarts of water in your container, you have collected 1/2 gallon. 
  1. Multiply the measured quantity of water by 6 to calculate the flow rate in gallons per minute. In our example, 1/2  gallon multiplied by 6 calculates as 3 gallons. A flow rate of 3 GPM indicates a fixture that is not meeting Federal standards.