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

There are several reasons to measure the flow rate of your home's faucets and showers. It can be a critical factor in sizing a new water heater, whether it's a tankless or traditional tank-style model. It can also be helpful information when you are buying or selling a house and are evaluating the efficiency of the plumbing fixtures. Perhaps most importantly, it can tell you definitively how much water a specific fixture is using, so you'll know exactly how much those long showers are costing in water use.

How Flow Rate Is Measured

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 showerhead or faucet. For water conservation purposes, the Federal Energy Policy Act of 1992 requires that all lavatory (bathroom) faucets sold in the U.S. have a flow rate of no more than 2.2 GPM at a water pressure of 60 psi, or pounds per square inch. According to the same law, showerheads may have a maximum flow rate of 2.5 GPM. The water pressure in your house may be higher or lower than 60 psi and, as a result, the water usage of each fixture may be below or above the product rating. That's why it's a good idea to measure flow rate at each fixture yourself.

How to Measure Flow Rate

You'll need a few basic supplies for measuring the flow rate of a faucet or showerhead:

  • Container for catching water
  • Timer or stopwatch
  • Measuring cup

A small pitcher is an ideal container for faucets because it makes it easy to pour out the water for measuring. A large bucket is best for a showerhead because you want to catch all of the water from the showerhead's spray. For a stopwatch, most people these days use a smart phone, or you can go old-school and use a watch or clock.

Follow these steps to measure the flow rate:

  1. Set the timer to 10 seconds.
  2. Turn on the cold water full-blast.
  3. Start the timer and simultaneously place the container under the water stream or spray, making sure all of the water is collected.
  4. Collect the water for exactly 10 seconds, then shut off the fixture.
  5. Measure the quantity of water in the container, using the measuring cup. You might want to note the cupfuls on a piece of paper so you don't lose track.
  6. Convert the measurement to gallons. For example, if you measure 2 quarts of water in your container, you have collected 1/2 gallon.
  7. 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 equals 3 gallons. Therefore, the flow rate is 3 GPM.

Changing a Fixture's Flow Rate

As a general guideline, faucet flow rate in the bathroom should be 1.5 GPM or less. This is the maximum flow rate established by the EPA's WaterSense program. And, frankly, that's more than enough water for a bathroom faucet. If you're concerned about water use, you can save more by installing a low-flow aerator that restricts the flow to 1.0 GPM or even less, and it's likely you'll never notice the difference.

Kitchen faucets typically have a maximum flow rate of 2.2 GPM. It might make sense to lower this to 1.5 GPM, using a low-flow aerator, but the tradeoff is that pots of water will take longer to fill. That said, if you rinse a lot of dishes, and particularly if someone in your house likes to wash or rinse dishes with the faucet on full-blast, it might make sense to reduce your kitchen faucet's flow rate.

Showerheads should have a maximum flow rate of 2.5 GPM. If your measured flow rate is any higher, simply replace the showerhead. The new unit will quickly pay for itself in water savings and, more significantly, in reduced water heating costs, since showers use about 70 percent hot water.