Toilet Flush Volume Calculator
Computes the volume of water used per flush by measuring the internal dimensions of the toilet tank. Ideal for identifying water-wasting fixtures, calculating household water consumption, or qualifying for rebate programs.
Last updated: October 2026
Flush Volume
3.74 gallons
Formula below · 2 sources (epa.gov, 19january2017snapshot.epa.gov) · Updated Oct 2026
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About this calculator
A toilet's flush volume is roughly the water held in the tank above the flush valve, so a tank measuring 18 × 8 inches inside with 6 inches of water released per flush uses about 3.7 gallons. The calculator multiplies the inside length, width and water depth in inches and divides by 231, the number of cubic inches in a U.S. gallon.
How it compares
The federal standard for new toilets is 1.6 gallons per flush (gpf), and EPA WaterSense-labeled toilets use no more than 1.28 gpf, 20% less. EPA notes that many older toilets use 3.0 to 7.0 gpf: models from before 1980 commonly used 5 or 7 gallons and those from the 1980s about 3.5. A result well above 1.6 gpf points to an older fixture worth replacing; see the table below.
Measuring accurately
Measure inside the tank, not the outside of the lid. For water depth, use the drop in water level during a flush rather than the full depth, because water below the flush valve stays in the tank: mark the waterline, shut the supply valve, flush, and measure how far the level falls. Bowl refill and the water that flows while the tank is refilling add a little more, and sloped or shaped tanks are not true rectangles, so treat the result as an estimate. For an exact figure, check the gpf stamped behind the seat hinge or inside the tank.
Saving water
Replacing a 3.5 gpf toilet with a 1.28 gpf WaterSense model saves 2.2 gallons on every flush. Check for a leaking flapper as well: a few drops of food coloring in the tank that show up in the bowl without flushing mean water is leaking through.
| Toilet | Gallons per flush | Liters per flush |
|---|---|---|
| Pre-1980 models | 5 to 7 | 19 to 26 |
| 1980s models | 3.5 | 13.2 |
| Federal standard for new toilets | 1.6 | 6.1 |
| EPA WaterSense maximum | 1.28 | 4.8 |
How to use
Measure the inside length and width of the tank in inches, then the depth of water that leaves the tank during a flush. The default, an 18 × 8-inch tank with a 6-inch drop, gives 18 × 8 × 6 ÷ 231 ≈ 3.74 gallons per flush, typical of a 1980s toilet. A drop of 2.5 inches in the same tank gives about 1.56 gallons, close to the 1.6 gpf federal standard.
Frequently asked questions
How many gallons per flush does a standard toilet use?
New toilets sold in the United States may use no more than 1.6 gallons per flush, and EPA WaterSense-labeled models use 1.28 gallons or less. Toilets from the 1980s typically used about 3.5 gallons, and many pre-1980 models used 5 to 7 gallons.
Why measure the drop in water level instead of the full depth?
Only the water above the flush valve leaves the tank. Water below the valve opening stays put, so the full depth overstates the flush. Mark the waterline, turn off the supply, flush and measure how far the level falls.
How can I use less water per flush without replacing the toilet?
Fix any leaking flapper first, then make sure the fill valve stops the water at the line marked inside the tank. Lowering the level below the manufacturer's mark can leave too little water for a clean flush, so replacing an old high-volume toilet with a WaterSense model is usually the better fix.
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