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

Water running from a showerhead

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There are several reasons to measure the flow rate of your home's faucets and showers. It can be a critical factor in choosing the size for 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 shower head 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 (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 the flow rate at each fixture yourself.

Tools and Supplies You Will Need

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

  • Bucket or another 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 shower head because it makes it easier to catch all of the water from the shower head's spray. For timing, most people use the timer feature on a smartphone, or you can go old-school with a watch or clock.

Instructions

Follow these simple steps to measure water flow and calculate the flow rate.

Collect the Water

Set the timer to 10 seconds. Turn on the cold water full-blast. Start the timer and simultaneously place the container under the water stream or spray, making sure all of the water is collected. Collect the water for exactly 10 seconds, then shut off the fixture.

Measure the Water

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. Convert the measurement to gallons. (There are 16 cups, or 4 quarts, in a gallon.) The measurement can be expressed as a simple fraction. For example, 8 cups equals 8/16, or 1/2 gallon.

Calculate the GPM Flow Rate

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.

Tips for Changing a Fixture's Flow Rate

In most cases, the reasons for changing the flow rate on a faucet or shower is to reduce water usage.

Bathroom Faucets

As a general guideline, the 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 generally speaking, 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 unlikely you'll notice the difference.

Kitchen Faucets

Kitchen faucets typically have a maximum flow rate of 2.2 GPM. It might make sense to lower this to 1.5 GPM by using a low-flow aerator, but the tradeoff is that it will take longer to fill pots of water. If you rinse a lot of dishes with the faucet on full-blast, it might make sense to reduce your kitchen faucet's flow rate to save water.

Showers

Shower heads should have a maximum flow rate of 2.5 GPM. If your measured flow rate is any higher, replace the shower head. The new unit will quickly pay for itself in water savings and, more significantly, in reduced water heating costs—for a typical user, the shower flow consists of about 70 percent hot water.