Toilet Running Constantly: Flapper, Fill Valve, or Float

A continuously running toilet is the most expensive small fault in a house. At the high end it is 200 gallons a day, silently, on your water bill.

Almost every case is one of three parts, and a dye test tells you which.

The dye test

Take the tank lid off. Drop food colouring into the tank — not the bowl — and do not flush. Wait 20 minutes.

Colour appears in the bowl → water is leaking past the flapper at the bottom of the tank.
Tank level stays high and water trickles into the overflow tube → the fill valve or float is not shutting off.
Nothing happens and it still runs → check whether the sound is coming from a different toilet or a supply line.

That single test splits the diagnosis cleanly.

Flapper leaks

The flapper is a rubber seal that lifts when you flush and drops to hold water in. It fails by hardening, warping, or getting mineral deposits on its seat.

Check in order:

Chain length. Too short and the flapper cannot seat; too long and it tangles under the flapper. You want about half an inch of slack.
The seat. Run a finger around the rim the flapper lands on. Grit or mineral scale there will hold it open. Clean it with a scouring pad — do not use anything that scratches.
The flapper itself. If it is stiff, cracked, or slimy and misshapen, replace it. Take the old one to the shop; there are several standard sizes (2-inch and 3-inch being the common ones) and they are not interchangeable.

While you are there: in-tank cleaning tablets destroy flappers. If you have one in the tank, that is your cause.

Fill valve and float

If the tank overfills into the overflow tube, the valve is not shutting off at the right level.

Adjust the float first. On a modern column-style valve, a clip or a screw on the float sets the shutoff height. Set the water level about an inch below the top of the overflow tube.
A float that is waterlogged (older ballcock designs) will not rise enough. Replace it.
A valve that hisses and never fully stops has a worn seal. Fill valves are inexpensive, universal, and replaceable in about 20 minutes with the supply shut off and the tank sponged dry.

Phantom flushing

The tank refills briefly every few minutes with no one using it. That is a slow flapper leak — the level drops enough to trigger the fill valve, it tops up, and the cycle repeats. Same fix as above.

Worth checking while the lid is off

The overflow tube height. If water sits above it, the tank was set up wrong.
The flush valve gasket between tank and bowl, if you see water on the floor behind the toilet rather than in the bowl.
The supply line connection, hand-tight plus a gentle turn; overtightening cracks the plastic nut.

What is not a running toilet

Water on the floor around the base, with the tank behaving normally, is the wax ring or the flange. That is a bigger job — the toilet has to come off — and it is worth doing promptly because the subfloor absorbs whatever leaks.

Replacing the flapper, step by step

Once the dye test points at the flapper, the repair is about fifteen minutes.

Shut off the supply at the valve behind or beside the toilet. If it is seized — common on old chrome stops — shut off the house supply instead rather than forcing it.
Flush to empty the tank, and sponge out the last inch.
Unhook the chain from the flush lever arm and note which hole it was in.
Slide the flapper’s ears off the pegs on the overflow tube. Some clip on, some are a ring that lifts over the tube.
Take the old one to the shop. Match the diameter — 2-inch and 3-inch are both common and they are not interchangeable — and the mounting style.
Fit the new flapper, hook the chain with about half an inch of slack, and turn the supply back on.
Test with dye again after 20 minutes to confirm.

If a new flapper still leaks, the seat is the problem. Seat repair kits exist — a flat ring bonded on with sealant — and they work, but at that point replacing the whole flush valve is more durable. That job means removing the tank from the bowl, which is a step up in effort.

Replacing a fill valve

Also about twenty minutes, and a universal part fits nearly everything.

Shut off and drain as above, unscrew the supply line from the tank underside, then the plastic locknut holding the valve through the tank floor. Lift the old valve out, fit the new one with its washer inside the tank, hand-tighten the locknut — plastic threads, so no wrench — reconnect the supply, and set the height so the water level sits about an inch below the top of the overflow tube.

The one thing worth care over: the tank-to-supply connection is hand-tight plus maybe a quarter turn. People crack the nut by treating it like a plumbing fitting.

What it is costing you

A flapper leak that refills every ten minutes can waste 100–200 gallons a day. Depending on local water rates, that is meaningful money per month for a part that costs about the same as a sandwich.

If you suspect a leak but the dye test is ambiguous, read your water meter with everything in the house off, wait an hour, and read it again. Any movement is a leak somewhere, and a toilet is the most likely candidate.

Other running-water sounds that are not the toilet

A hissing from the wall with no toilet running — a supply leak inside the wall. Turn off the main and call a plumber.
A brief refill every few hours with a dry dye test — a leaking pressure tank or an expansion vessel on a heating system, not the toilet.
A running sound that stops when you close the shower valve — a failing diverter or mixer cartridge.
Water hammer, a bang when a valve closes — air chambers or an arrestor, unrelated to the toilet’s operation.

Preventing the repeat

Two habits: keep chemical tablets out of the tank, because they destroy the flapper and the fill valve seals; and once a year, drop dye in the tank as a routine check. Catching a flapper before it becomes a continuous leak is the whole point.