How to measure water flow rate (in gpm)

Flow rate is very important when sizing a tankless water heater. Although we can estimate a flow rate based on the number of bathrooms in your home, it is always best if you can provide an actual flow rate measurement. Follow the instructions below to measure your water flow rates.

Required Equipment

Instructions for measuring the water flow rate at a faucet or shower

  1. Turn on the water.
  2. Time how long it takes to fill the container using your stop watch or phone (in seconds).
  3. Divide 60 by the number of seconds it took to fill your container (ie. if it took 30 seconds to fill then 60 / 30 = 2 gallons per minute)
  4. The result is the flow rate of that application in gallons per minute.

If you would like to run multiple applications at once (ie. two showers at once) with your tankless water heater you will need to measure each application and add the flow rates together. As individual flow rates can decrease when multiple applications are running at the same time it is a good idea to turn the water on at all applications you want to run at the same time before measuring the individual flows.

If you have any questions please call our tankless experts toll free at 1-877-374-2696 and they would be happy to help.