Toilet keeps running: causes and fixes

Lift the lid: the answer is in the tank.

The answer in 2 lines. Close the supply valve behind the toilet and lift the tank lid. Either water rises above the overflow tube and spills into it (the fill valve doesn't shut off), or it leaks past the flapper at the bottom (a worn, warped or dirty flapper, or a chain that catches). Both are usually easy, cheap do-it-yourself fixes. A cracked tank, a leaking supply valve or water on the floor that won't stop: call a licensed plumber, or your landlord if you rent.

The first 3 steps

Turn off the supply valve on the small pipe behind or under the toilet (clockwise), or the main water valve if there isn't one.
Lift the tank lid and look: is the water above the top of the overflow tube (the open pipe in the middle), or below it?
Flush to empty the tank, and take a photo of the parts and any brand name on them: it helps you buy the right replacement.

Stop: danger

Water on the floor near an outlet, power strip or electric heater: don't touch anything wet. Turn off power at the breaker panel only if it's dry and you can reach it without stepping in water; otherwise leave the room and call 911.
Upflush or macerating toilet (a box behind the bowl, plugged into the wall): unplug it before anything else and don't open it yourself. Follow the maker's manual or call a plumber.

What not to do

The porcelain lid and tank are heavy and brittle: set the lid on a towel on the floor. A cracked tank can't be patched safely; it gets replaced.
Forcing a stuck supply valve with pliers: if it breaks, you'll need to close the main valve. Use the main valve instead.

Where the water goes: two problems, two signs

Water rises into the overflow tube, the tank hisses or keeps refilling: the fill valve isn't shutting off. It may be set too high, worn, or blocked by grit or mineral buildup. Usually a DIY fix.
Water stays below the tube but a trickle runs into the bowl: the flapper isn't sealing. Do the dye test: a few drops of food coloring (or a dye tablet from your water utility) in the tank, no flushing; if color shows in the bowl after 15 to 30 minutes, the flapper leaks (Denver Water explains the test). Usually a DIY fix.
The toilet refills by itself every so often ("phantom flush"): a slow leak past the flapper. Same fix.
Water on the floor at the base or under the tank: the tank-to-bowl gasket, the supply line connection, or a cracked tank. The supply line, yes; a cracked tank: plumber.

What you can do yourself, and where to stop

Fill valve: adjust the float (a screw, clip or knob depending on the model) so the water stops about 1 inch below the top of the overflow tube, or at the water line marked inside the tank (how to adjust a fill valve). If it still runs, replace the whole fill valve: universal kits come with instructions and fit most toilets.
Flapper: water off and tank empty, wipe the flapper and the seat it closes on. If the rubber is warped, stiff or slimy, replace it. Check the size (most are 2 or 3 inches; the drain opening tells you) and take the old one to the store. Leave a little slack in the chain, about half an inch (Pinellas County, the leaky flapper).
Worn-out mechanism: a complete fill-valve-and-flapper kit is replaced with the water off, following the maker's instructions.
Stop there and call a licensed plumber for a cracked tank, a floor leak that continues, a supply valve that leaks or won't close, an in-wall tank you can't reach comfortably, or an upflush toilet.

Common causes

"I hear water running at night": the fill valve doesn't shut off, or a slow flapper leak keeps topping up the tank.
"There are stains or mineral streaks in the bowl": a constant trickle, often the flapper.
"My water bill jumped": a running toilet is the first thing to check. See our guide on high water bills and hidden leaks.
"It started after plumbing work or a water shut-off": grit or scale is stuck in the fill valve or under the flapper.

Who pays, who to tell

Renters: tell your landlord or building manager in writing. Who pays for the parts and the wasted water depends on your state, city and lease; many leases make the landlord responsible for plumbing repairs. See our guide on repairs between tenant and landlord.
The water bill: some utilities offer a one-time leak adjustment once the leak is fixed. Call the number on your bill and ask; it's not a right everywhere.
Water leaking into the unit below: tell building management and your insurer as soon as possible.

Preventing the next one

Do the dye test twice a year, and after any water shut-off. EPA's Fix a Leak Week, every March, is a good reminder.
Avoid bleach or chlorine tablets in the tank: some toilet makers warn they wear out the flapper and seals. Check your toilet's manual.
Read your water meter once a month: a jump without explanation is a warning sign.

In Columbus, Dana hears water running at night. She closes the valve behind the toilet and lifts the lid: the water is above the top of the overflow tube. She turns the float screw down a few turns and reopens the valve: the tank now stops filling an inch below the tube. The next day, the dye test leaves the bowl clear. The noise is gone.

A made-up story, for illustration.

The numbers (latest published)

Nearly 10,000 gallons a year: the water an average US household loses to leaks, according to EPA WaterSense figures repeated for Fix a Leak Week, March 16 to 22, 2026 (Michigan EGLE, March 17, 2026). It's an estimate, not a new measurement.
Running toilets are among the most common household leaks, and a badly running one can waste hundreds of gallons a day; the often-quoted figure is up to 200 gallons a day (Marketplace, 2010; an old order of magnitude, not a recent measurement). Seattle Public Utilities ran a reminder on running toilets in March 2026 (Is your toilet running non-stop?).

Bobby

Sources, verification, confidence

Verified on November 1, 2026, for the United States. Confidence: high for the dye test and the repair steps, which are standard and published by water utilities; medium for the water-loss figures (EPA estimates repeated by utilities in 2026, not new measurements) and for who pays in a rental, which depends on your state and lease. This guide describes common cases, not a diagnosis.