Tips to Determine the Gallons-Per-Flush Volume
Look for marks or labels near the toilet seat hinge. A 5-gallon per flush (gpf) toilet might be shown as 22.73 liters, a 3.5 gpf as 15.91 liters. All 1.6 gpf (6.1 liters) toilets should be marked at this location. If no markings, check the underside of the tank lid or the tank's back inside wall for a date stamped in the porcelain. Toilets produced before 1985 are 5-7 gpf, a date between 1985 to 1991 would be 3.5 gpf. All toilets made after 1991 are 1.6 gpf.
There are two ways to conduct a Toilet Water Use Test:
1. The most accurate way is by measuring the tank.
Measure the length of the tank.
Measure the width of the tank.
Measure the full water level in the toilet tank (depth 1).
Flush the toilet and measure the drop at the lowest level (depth 2).
Subtract depth 2 from depth 1. This will give you the "drop" measurement.
Multiply the length times the width, times the "drop" measurement to determine the volume of cubic inches of water used per flush.
Divide the volume by 231 to get the number of gallons per flush.
2. The second way to conduct a Toilet Water Use Test is by reading your water meter.
Read your water meter. Make sure all water-using fixtures in your home are turned off (reading 1).
Flush the toilet and then read the meter again (reading 2). Make note of where the sweephand is for gallons up to 10.
Subtract reading 1 from reading 2 to determine the amount of water used in a flush.