Toilet Keeps Running? How to Find the Cause and Fix It

A toilet that keeps running is one of the most common household plumbing problems, and one of the most wasteful. Sometimes you hear a constant hiss. Sometimes the tank refills by itself every twenty minutes, even though nobody has flushed. Either way, water is being lost, and the good news is that the cause is almost always one of a few parts inside the tank (called the cistern in the UK and much of Europe).

This guide follows a simple logic: symptom → cause → part → fix. You don’t need to be a plumber. You do need about fifteen minutes, a towel, and a little patience.

How a toilet tank works (in 30 seconds)

Every gravity-flush toilet tank has two main valves:

• The fill valve (also called inlet valve, ballcock or float valve) lets water in after a flush and shuts off when a float rises to the set level.
• The flush valve (outlet valve) lets water out into the bowl. On many American-style toilets it is sealed by a rubber flapper. On many European push-button cisterns it is a canister or tower valve with a rubber seal at its base.

Most flush valves also include an overflow pipe. If the water rises too high, it spills into that pipe and down into the bowl, which protects your floor but wastes water silently.

So a running toilet has only two possible routes for the water: it is either coming in and not stopping (fill valve or water level problem), or leaking out into the bowl (flush valve problem). Your job is to find out which.

Safety first

• Know where the shut-off valve is. It is usually a small valve on the wall or floor behind the toilet (an angle stop or isolation valve). On most models you close it by turning clockwise; some quarter-turn types close with a 90° turn. If the valve is stiff, don’t force it with a big wrench, because old valves can crack. Use the main house stopcock instead.
• Lift the tank lid carefully and place it flat on a towel. Ceramic lids are heavy and break easily.
• If you see water on the floor, around the base of the tank or near the supply hose, turn the water off first and dry the area before you touch anything else.
• Never put your hand in a tank that contains strong chemical cleaning tablets without gloves.

Step-by-step diagnosis

Step 1: Look at the water level

Remove the lid and watch the tank after it finishes filling.

• If water is flowing into the top of the overflow pipe, the level is too high or the fill valve is not shutting off. Go to Step 2.
• If the level sits below the top of the overflow pipe but you still hear running water or see ripples in the bowl, the water is probably leaking through the flush valve. Go to Step 4.

Many tanks have a water line moulded or printed on the inside wall. If yours doesn’t, a common guideline is roughly 1–2.5 cm (½–1 inch) below the top of the overflow pipe. Always follow the toilet or valve manufacturer’s marking if there is one.

Step 2: Test the fill valve

Gently lift the float (the ball on an arm, or the cup that slides up and down the valve body) by hand.

• The water stops: the fill valve works, but it is set too high. Lower the float. See our guide on adjusting the toilet water level.
• The water keeps running even with the float held up: the valve is not sealing. This is usually caused by debris, limescale or a worn seal inside the valve head.

Step 3: Clean or replace the fill valve

1. Turn off the water at the shut-off valve.
2. Flush to empty the tank.
3. Many modern fill valves have a removable cap. Following the instructions for your model, remove the cap and check for grit or scale on the seal.
4. Hold a cup upside down over the open valve and briefly turn the water on and off to flush out debris (this prevents spraying).
5. Refit the cap, turn on the water and test.

If the valve still won’t shut off, or the plastic is cracked, replacement is usually the most practical fix. Follow our fill valve replacement guide.

Step 4: The dye test for flush valve leaks

If the water level is correct but the tank still refills by itself, water is leaking out.

1. Add a few drops of food colouring to the tank water. Don’t flush.
2. Wait 15–30 minutes.
3. If coloured water appears in the bowl, the flush valve seal is leaking.

Many water utilities recommend this exact test because a slow leak can be silent.

Step 5: Check the flapper or the dual flush seal

Lever toilets with a flapper:

• Check the chain. If it is too short, it holds the flapper slightly open. If it is too long, it can get trapped under the flapper. Leave a little slack.
• Rub the flapper with your finger. If it leaves black residue, feels soft or looks warped, it should be replaced.
• Check the seat (the ring the flapper rests on) for scale or rough spots. Clean it gently with a non-scratch pad.

Push-button cisterns with a canister/tower valve:

• With the water off and the cistern empty, many tower valves can be released with a quarter turn or by pressing clips. Check your model’s instructions.
• Lift the valve out and inspect the rubber seal at the bottom. Clean it and the seat underneath. Replace the seal if it is cracked, flattened or hard.
• Check that the push-button rods or cables are not holding the valve slightly open.

If the seat is damaged or the whole valve is worn, see our flush valve replacement guide.

Step 6: Check the refill tube

On many American-style fill valves, a small refill tube sends water into the overflow pipe to refill the bowl. It should be clipped above the overflow pipe, not pushed deep inside it. If it is pushed down below the water line, it can siphon water continuously.

Step 7: Test again

Turn the water back on, let the tank fill, flush two or three times and watch a complete cycle. Repeat the dye test the next day. Some leaks only show up at night when the mains pressure rises.

Quick symptom table

Symptom	Most likely cause	Typical part
Water flows into overflow pipe	Level too high / fill valve not sealing	Fill valve or its seal
Tank refills by itself every so often	Slow leak into bowl	Flapper or flush valve seal
Constant trickle in bowl, level normal	Flush valve seal or seat	Flapper, canister seal or flush valve
Hissing, very slow refill	Debris or worn fill valve	Fill valve
Must jiggle handle to stop	Chain, lever or button linkage	Chain, lever or button set

Frequently asked questions

Is a running toilet an emergency? Usually not, but it wastes water continuously and can raise your bill. If water is reaching the floor, turn off the supply immediately.

Can I fix it without turning off the water? Adjusting a chain or a float can often be done with the water on. Anything that involves removing a valve or a seal needs the water off and the tank empty.

Why does my toilet run at night but not during the day? Water pressure in many networks rises at night. A level set right at the limit, or a weak fill valve seal, can let water creep into the overflow pipe.

Should I replace the fill valve and flush valve together? If the toilet is old and both parts show wear, replacing them at the same time with a matching repair kit can save you a second repair. See our repair kit buying guide.

Do in-tank cleaning tablets cause leaks? Some toilet and valve manufacturers warn that strong chemical tablets can degrade rubber seals. Check your manufacturer’s guidance.

When should I call a plumber? If the tank itself is cracked, the shut-off valve won’t close, or water is leaking between the tank and bowl and you are not comfortable removing the tank.

For distributors and plumbing wholesalers

Running toilets are the reason most people buy replacement fill valves, flush valves and repair kits. If you sell plumbing spare parts in Europe or Latin America and are looking for a supplier of fill valves, flush valves and complete tank repair kits, including private-label options, visit our For Distributors page to send a wholesale or OEM inquiry. Please ask about the certifications your market requires, such as WRAS or ACS. We will confirm which documents apply to each product.