How to Measure the GPF in Toilets


The amount of water a toilet uses is measured in gallons per flush, or GPF. As toilet designs have become more efficient, the amount of water needed to produce an adequate flush has declined remarkably, from more than 7 gallons in the 1950s to 3.5 gallons in the 1980s to the 1.6-gallon flush that has been required by law in the United States since 1995. Most toilets have a GPF figure written or stamped somewhere. However, if you can't find a label -- or you don't trust it -- there's a quick and easy way to measure the GPF yourself.

Things You'll Need

  • Waterproof marker, pencil or grease pencil
  • Large measuring cup
  • Turn off the water to the toilet. Look for the supply line running from the bottom of the tank to the wall. There should be a valve on it. Turn the valve counterclockwise all the way to shut off the water.

  • Remove the top of the toilet tank by lifting it off. Set it aside.

  • Use a waterproof marker, pencil or grease pencil to mark the water line inside the tank.

  • Flush the toilet as normal. Don't hold down the lever any longer than you usually would. Wait for the water to drain out and for the flap to drop back onto the outlet tube at the bottom of the tank. Since the water to the toilet has been shut off, the tank will not automatically refill.

  • Fill a large measuring cup with water from the bathroom sink. Fill it to a measured line, so you can keep track of how much water you're pouring into the tank.

  • Pour the water in the measuring cup into the tank. Continue measuring and pouring water into the tank until the water level reaches the line you marked earlier.

  • Add up the number of cups of water you poured into the tank. Divide this number by 16 to arrive at your toilet's GPF measurement.

  • Replace the toilet tank lid and turn the water back on.

Tips & Warnings

  • If there's no valve on your toilet's water supply line, you'll have to shut off the main water supply. Look for the main shutoff valve at the point where the water line enters the house from the street. Since this will also cut off water to your bathroom sink, fill a large bucket with water ahead of time to use to fill your measuring cup.

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  • Photo Credit Toilet bowl and bidet in a toilet image by terex from
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