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Toilet leak cost.

Test first: put a few drops of food colouring in the tank, wait 10 minutes without flushing, and if colour reaches the bowl the toilet leaks (EPA). Then size the leak here from how far the tank level drops, how often it refills on its own, or a rate you already know, and see the gallons and cost. Runs in your browser; nothing is sent or stored.

Your numbers

How do you know the leak size?

Tank length, side to side (inches)

Measure inside the tank. Example value.

Tank width, front to back (inches)

Measure inside the tank. Example value.

Level drop (inches)

Shut the supply valve, mark the water level, wait, then measure the drop. Example value.

Minutes between the two readings

Refills per day

Times the tank tops itself up with nobody flushing. Example value.

Gallons per refill

The toilet's flush volume; 1.6 gallons is the US federal standard per the EPA. Check the tank or bowl marking.

Leak (US gallons per day)

For example from a meter test. Example value.

Water price (include sewer if billed per volume)

Example value. Use the rate on your own bill.

Water price is per

Result

Fill the form and press Calculate.

Worked example

A tank 18 inches long and 8 inches wide whose level falls 0.5 inch in 60 minutes loses 18 x 8 x 0.5 = 72 cubic inches, which is 72 / 231 = 0.31 gallon an hour (231 cubic inches make a US gallon), or 7.5 gallons a day. That is 2,730 gallons a year, $21.84 at an example $8 per 1,000 gallons, or $1.82 a month. If instead the tank refilled itself 6 times a day at 1.6 gallons, the leak would be 9.6 gallons a day, 3,504 gallons and $28.03 a year. Example inputs: replace them with your own.

How it works

Dye test. The US EPA describes it this way: place a few drops of food colouring in the toilet tank; if any colour shows up in the bowl after 10 minutes, you have a leak. Flush right afterwards to avoid staining. The EPA says old or worn flappers are a common cause.

Sizing the leak. By level drop: gallons per day = length x width x drop (inches) / 231 x 1,440 / minutes. By refills: refills per day x gallons per refill. Cost per year = gallons per year / 1,000 x price per 1,000 gallons.

Not covered. A leak that is intermittent or that varies with water pressure will differ from a one-off measurement. Include sewer charges in the price if your utility bills sewer by volume. A tank with a sloped or odd shape needs a rougher estimate.

Sources and basis (pages opened 2026-10-08): US EPA WaterSense, Fix a Leak Week; US EPA WaterSense, Residential Toilets.

This is a planning estimate and not professional plumbing, electrical or financial advice. Rates, fixtures and equipment vary, so confirm important decisions with your utility, the manufacturer or a licensed professional.

FAQ

How do I do a toilet dye test?

Put a few drops of food colouring in the tank, do not flush for 10 minutes, and check the bowl. Colour in the bowl means the flapper or valve seal is leaking. Flush right after to avoid staining.

How much does a running toilet cost per month?

It depends on how fast it leaks and your water price. Enter the leak rate here: gallons per month divided by 1,000 and multiplied by your price per 1,000 gallons.

What commonly causes a toilet leak?

The EPA says old or worn-out toilet flappers (valve seals) are a common cause of toilet leaks, and a worn flapper can silently leak thousands of gallons a year.

How big is a typical household leak?

The EPA says the average household's leaks can account for more than 9,300 gallons of water wasted every year, and nine percent of homes have leaks that waste 50 gallons or more per day.

How do I measure a slow leak with the tank level?

Close the supply valve, mark the water level, wait an hour or so without flushing, then measure how far the level dropped and enter the tank dimensions.