Determining Gallons Per Minute (GPM) of Water Flow from Household Fixtures

The flow rate of a faucet or shower head is measured in gallons per minute (GPM) and is a critical factor in sizing a Bosch tankless water heater, or for evaluating if an older fixture is wasting water.

The Federal Energy Policy Act of 1992 required all faucet and shower fixtures made the USA to have a flow rate of no more than 2.2 GPM at 60 PSI. Often you can get below 2.2 GPM with low flow aerators. However before 1992, older fixtures 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. If any fixture is much more than 2.2 GPM, it is a good candidate 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:

Turn the fixture on to its normal position

Place a container under the fixture and collect the water for 10 seconds

Measure the quantity of water in the container and convert the measurement to gallons (e.g., 0.25 gallons)

Multiply the measured quantity of water by 6 to calculate the flow rate in gallons per minute (0.25 gal x 6 = 1.5 GPM).

For sizing of a tankless water heater you would want to determine the most applications (uses) at one time you would be using in the worst cases scenario.  As an example, You normally run 2 showers at the same time and the laundry going too.  Each shower flows 2.0 GPM and the Laundry flows 1.5 GPM so that would be a total of 5.5 gpm.  Using this calculation you can then find a unit that would give you at least 5.5 gpm with an output temperature of 116 degrees or better.  

You can call us at 888-726-9752 for help with sizing a Bosch tankless water heater.

Determining Your Water Flow Rate for Tankless Water Heaters

Selecting tankless hot water heaters isn't very difficult. Select one based on the maximum amount of hot water to meet your peak demand.

Selecting Tankless Water Heaters

Selecting tankless hot water heaters isn't very difficult. Select one based on the maximum amount of hot water to meet your peak demand. Use the following assumptions on flow for various appliances to find the size of unit that is right for your purposes: Faucets: 1.0 gallons (3.78 liters) to 2.5 gallons (9.46 liters) per minute.

Low-flow showerheads: 1.0 gallons (3.78 liters) to 2 gallons (7.57 liters) per minute. Older standard shower heads: 2.5 gallons (9.46 liters) to 3.0 gallons (11.36 liters) per minute.

Clothes washers and dishwashers: 1 gallon (3.79 liters) to 2 gallons (7.57 liters) per minute.

Unless you know otherwise, assume that the incoming temperature is 55° F (12.78° C). You will want your water heated to 110° F (43.33° C) for most uses. To determine how much of a temperature rise you need, subtract the incoming temperature from the desired output temperature. In this example, the needed rise is 55° F (12.78° C).

List the number of hot water devices you expect to have open at any one time, and add up their flow rates. This is the desired flow rate for the heater. Selecting a tankless water heater is nearly impossible without knowing the flow rate you need.

As an example, assume the following conditions: One hot faucet open with a flow rate of 1.0 gallons (3.79 liters) per minute. One person bathing using a shower head with a flow rate of 2.5 gallons (9.46 liters) per minute. Add the two flow rates together. If the inlet water temperature is 55° F (12.78° C), the needed flow rate through the heater would need to be no greater than 3.5 gallons (13.25 liters) per minute. Faster flow rates or cooler inlet temperatures will reduce the temperature at the most distant faucet.