Toilet running constantly: how do you find the fault and fix it?

In short

A toilet that runs constantly can waste more than 600 liters (about 160 gallons) a day: this is not a fault to ignore.
There are only two suspects: the fill valve, which doesn’t shut off the water supply, or the flush mechanism, whose seal lets water leak into the bowl.
To tell which one it is, check whether water is going down the overflow tube, then do the dye test.
Often, adjusting the water level, descaling a part or replacing a €5 seal is all it takes.
Always shut off the water before putting your hands in the tank.

That little trickle of water running down the bowl, or the sound of the tank refilling on its own in the middle of the night: you almost get used to it. Bad idea: your water meter certainly hasn’t. The good news is that a toilet tank has only two parts that can leak, and you can find the culprit in a few minutes. Here’s how.

Through the overflow tube or under the flapper: a toilet tank can only leak in two ways

How does a toilet tank work?

Inside the tank, two parts share the work:

The fill valve refills the tank. A float rises with the water level and closes the supply when the tank is full, then drops and reopens it after each flush.
The flush mechanism empties the tank. When you press the button, it lifts a flapper at the bottom of the tank and the water rushes into the bowl. The flapper then drops back onto its seat to make the seal again. The mechanism also includes an overflow tube: if the level rises too high, the excess goes straight into the bowl instead of overflowing.

So a leak comes either from a water supply that doesn’t shut off, or from a flapper that no longer seals properly.

How can you tell where the leak comes from?

1. Look at the overflow tube

You don’t need to shut off the water for this first check: simply take off the tank lid (or the flush plate on a wall-hung toilet) and look at the overflow tube.

Water is flowing over the top of the overflow tube: the tank is overfilled. The fill valve is badly adjusted or no longer closes.
The level stays below the overflow tube, but water still runs into the bowl: the flush mechanism is leaking.

2. Do the dye test

If you’re in doubt, put a few drops of food coloring in the tank water without flushing. Wait at least 30 minutes, ideally a few hours without using the toilet. If the bowl changes color, water is getting past the flapper of the flush mechanism. If it stays clear, look at the filling side instead.

What should you do if water goes down the overflow tube?

Adjust the water level

This is often all that’s needed. The right level is about 2 cm (3/4 inch) below the top of the overflow tube.

On a recent fill valve, turn the adjustment screw to lower the shut-off point. The direction varies by model: go in small turns, then flush to check the new level.
On an older model with a metal rod, bend the rod slightly downward so the float shuts off the water earlier, trying several times.

The float stays at the bottom even though the tank is full

It is probably punctured and full of water: it no longer rises and no longer controls the shut-off. You need to replace it, or replace the whole fill valve.

The float is up, but water keeps coming in

The fill valve’s shut-off seal no longer seals, most often because of limescale: the seals are the first parts affected. Shut off the water, take apart the head of the fill valve following its instructions and soak the seal overnight in white vinegar. If that isn’t enough, some manufacturers sell the replacement diaphragm on its own; otherwise, replace the fill valve.

What should you do if water runs into the bowl?

The culprit is almost always the flapper seal of the flush mechanism: scaled, deformed or worn, it no longer seals.

Shut off the water at the shut-off valve, usually on the side or at the back of the tank.
Empty the tank by flushing.
Remove the flush button, then the lid.
Take out the mechanism: on most models, a quarter turn is enough.
Inspect the seal under the flapper. If it’s simply scaled, soak it for several hours in a half white vinegar, half hot water mix. If it’s cracked, porous or deformed, replace it with a new seal of the same diameter and thickness.
Reassemble, turn the water back on, and check over a full cycle that nothing runs once the tank is full.

Never force the nuts or the tank: ceramic cracks easily.

When should you replace the whole mechanism?

If the leak continues with a new seal, if a part is broken or if the mechanism is very old, replace it completely. Before buying:

note the brand and model number of the old mechanism, or take it with you to the store;
measure the height available in the tank, and the spacing of the fixing holes if you have to remove the tank;
for a fill valve, check the position of the water inlet: at the bottom, or on the side (top or bottom).

Flush mechanisms and fill valves are also sold together as a set. While you have everything apart, take the opportunity to change all the seals: they cost almost nothing. On a wall-hung toilet, access is through the flush plate, and the part has to match the in-wall carrier frame.

How much does it cost?

Prices noted at Castorama (France) in October 2026:

Flapper seal for a flush mechanism: about €4.50 to €7.90.
Replacement diaphragm for a fill valve: about €6 (depending on the model).
Fill valve: about €10.90 to €30.
Flush mechanism on its own: about €15.50 to €30.
Flush mechanism and fill valve together: about €26 to €42, more for a concealed tank.

On the other side of the balance, the leak is expensive: a leaking toilet can waste 45 to 220 m³ of water a year (roughly 12,000 to 58,000 US gallons). A leaflet from the City of Montpellier (France) puts the 220 m³ at about €900. Part prices vary from store to store: check them before you buy.

How can you stop it happening again?

Keep an eye on limescale: in hard-water areas, the seals of the fill valve and the flapper are the first to scale up. Treating the water at the source, for example with a magnetic limescale reducer, can extend their life.
Repeat the dye test from time to time: a small silent leak easily goes unnoticed.
Keep the bowl descaled: if limescale also builds up at the bottom, here is how to deal with stubborn limescale.

Frequently asked questions

Why does the toilet tank refill on its own at night?

It’s the sign of a small leak: water slowly seeps under the flapper of the mechanism, the level drops, and the fill valve reopens to top it up. The dye test will confirm it.

How do I know which mechanism to buy?

Find the brand and model number on the old mechanism, measure the height available in the tank, and note the position of the water inlet. If in doubt, take the old part to the store.

This article is part of our guide to cleaning your bathroom and toilet naturally, with the full daily, weekly and monthly routine.

So, was it the float or the seal? Tell us in the comments how you tracked down the fault: your experience will help the next reader woken up by a running toilet at three in the morning.