How to fix a toilet that keeps running

There are only two ways a toilet can run forever, and once you can picture the inside of the tank you can tell them apart in about ninety seconds.

Either water is leaking out of the tank into the bowl, so the tank never stays full and the fill valve keeps topping it up — or the tank is being overfilled, so water pours into the overflow tube and down the bowl continuously. The first is a flapper problem. The second is a float problem. Everything else is detail.

Inside the tank: flapper, chain, float and overflow tube

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Step 1 Take the lid off and find the water line Rest the lid flat on a towel, never on its edge. Look at the tall open pipe in the middle — the overflow tube. If water is running into the top of it, the tank is simply overfilling.

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1. Step 1: Take the lid off and find the water line
2. Step 2: Do the dye test
3. Step 3: Free the chain and reseat the flapper
4. Step 4: Lower the float so the fill stops early
5. Step 5: Flush and listen

The four parts you need to recognise

Lift the lid and ignore everything except these:

• The flapper. A rubber flap at the bottom of the tank covering a hole about the size of a fist. When you flush, it lifts and the tank empties. It is the only thing standing between the tank and the bowl.
• The chain. Links the flush handle to the flapper. Length is everything.
• The overflow tube. The tall open pipe in the middle. Its job is to send water safely into the bowl if the tank overfills, instead of onto your floor. Its top edge is also your target water level.
• The fill valve and float. The tall column at one side, with either a cup-shaped float around it or an arm with a ball on the end. It refills the tank and shuts off when the float rises far enough.

The dye test does the diagnosis for you

This is the step people skip and then spend an hour guessing.

Add ten drops of food colouring to the water in the tank — not the bowl. Then do nothing for twenty minutes. No flushing, no using the toilet.

• Colour appears in the bowl → the flapper is leaking. Go to the flapper section below.
• The bowl stays clear → the flapper is fine. The tank is overfilling. Go to the float section.

That’s it. One test, two possible answers, no dismantling.

If it’s the flapper

Turn the water off at the shut-off valve behind the toilet, flush to empty the tank, and look at the flapper closely.

Check the chain first, because it’s free. Lift the flapper and let it drop. It should fall flat and stay there. Two common failures:

• Too tight. The chain is holding the flapper a millimetre off its seat. You often cannot see the gap, but water finds it. Move the clip one or two links along.
• Too long. The slack chain slides under the flapper as it closes and props it open. Same fix, other direction. Aim for around 12 mm of slack.

Then check the seal. Run a finger around the plastic seat the flapper lands on. Grit, mineral scale or a stray hair will hold it open. Wipe it clean with a cloth.

Then check the rubber itself. Press the flapper with a gloved finger. Good rubber is springy. If it feels hard, or if it leaves black marks on your glove, it has perished and no amount of adjustment will save it. Replacements are a few pounds, take about five minutes, and require no tools — unhook the old one, hook the new one on, reattach the chain.

If it’s the float

Look at where the water settles. It should sit roughly 25 mm below the top of the overflow tube. If it is level with the tube’s rim, water is quietly pouring in and the fill valve can never reach its shut-off point.

How you lower it depends on the age of the toilet:

• Modern cup float (a plastic cylinder that slides up and down the fill valve): turn the adjustment screw on top anticlockwise, or pinch the spring clip on the metal rod and slide the whole cup down.
• Older ball float (a ball on a long arm): most have an adjustment screw where the arm meets the valve. If not, you can bend the arm gently downward — a small bend goes a long way.

Turn the water back on, let the tank fill, and check the new level. Adjust in small increments; overshooting downward gives you a weak flush.

If it still runs after all that

The fill valve itself is worn and no longer shutting off. You’ll usually hear a faint continuous hiss coming from the valve column rather than from the flapper area. A universal fill valve costs about the same as two coffees, and swapping it means turning off the water, disconnecting one supply hose, unscrewing one plastic nut under the tank, and reversing the process. It’s a genuinely easy job — but it’s a different job, and worth its own fifteen minutes.

When to call a plumber

• Water is appearing on the floor rather than inside the bowl.
• The shut-off valve behind the toilet won’t turn, or drips when you try it.
• The tank is cracked, or the porcelain is wet where it shouldn’t be.
• The toilet rocks when you sit on it, or there is a smell at floor level.

Those are seal and mounting problems, not tank-mechanism problems, and they get expensive if you improvise.

The steps, in writing

Same sequence as the animation — handy if you'd rather read it, or print it and take it with you.

1. Take the lid off and find the water line Rest the lid flat on a towel, never on its edge. Look at the tall open pipe in the middle — the overflow tube. If water is running into the top of it, the tank is simply overfilling.
2. Do the dye test Add a few drops of food colouring to the tank and do not flush for twenty minutes. If colour appears in the bowl, the flapper at the bottom is not sealing and water is leaking straight through.
3. Free the chain and reseat the flapper A chain that is too short holds the flapper open a fraction; one that is too long tangles under it. Set about 12 mm of slack, clear any grit off the seat, and check the flapper drops flat.
4. Lower the float so the fill stops early Turn the adjustment screw, or squeeze the clip on the float rod, until the water stops about 25 mm below the top of the overflow tube. That gap is what stops the endless trickle.
5. Flush and listen The tank should refill and then go completely silent. If it is still hissing after two minutes, the fill valve itself is worn and is a ten-dollar replacement.

Questions people actually ask

How much water does a running toilet actually waste?

A silent leak past a flapper commonly runs into the hundreds of litres a day, and a badly seated one can waste far more than that. It is usually the single largest wasteful leak in a home, and because it makes almost no noise it can go on for months before anyone notices it on a bill.

The dye test came back clear but it still runs.

Then the flapper is fine and the tank is overfilling. Water is spilling into the overflow tube, so the fill valve never senses "full" and never shuts. Lower the float. If lowering it does not stop the flow, the fill valve is not closing and should be replaced.

How do I know which flapper to buy?

Take the old one to the shop. Flappers vary in diameter and in how they hook onto the overflow tube, and a near-miss will leak. Note the toilet brand stamped inside the tank or on the bowl behind the seat while you are at it.

Can I just shorten the chain by cutting it?

You do not need to. Move the clip to a different link instead, which is reversible if you overshoot. Aim for a small amount of slack: enough that the flapper sits flat with no upward pull, but not so much that the chain can slide under the flapper edge.

The tank refills briefly every few minutes with no one using it.

That is called phantom flushing and it is a slow flapper leak. The bowl loses water, the tank level drops, and the fill valve tops it up. It is the same fix: dye test, then flapper.