Guides · Plumbing

How to fix a running toilet

A toilet that keeps running or refills on its own wastes water all day. The cause is almost always inside the tank: the flapper does not seal, the chain holds it open, or the fill valve lets water reach the overflow tube. Each fix takes minutes and no special tools.

RoomTag Guides · November 1, 2026

Time

15 to 60 min

Skill

Beginner

Water off

At the toilet

• Replacement flapperFlappers come in 2 in and 3 in sizes. Check the size of the hole at the bottom of the tank, or bring the old one to the store.
• Fill valve kitOnly if the fill valve is the problem. Universal kits fit most tanks; check the box.
• Adjustable wrenchFor the supply line nut, if you replace the fill valve.
• Sponge, small bucket and towelsTo soak up what is left in the tank.
• Rubber glovesTank water is clean, but the old parts are slimy.

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1. Find the cause

Take the tank lid off and set it somewhere flat and safe; it is heavy and breaks easily. Then watch the water.

• Water runs over the top of the overflow tube (the open pipe in the middle): the fill valve shuts off too late. Go to step 3.
• Water level is below the tube but the tank keeps refilling: water is leaking past the flapper into the bowl. Go to step 2.
• Not sure? Put a few drops of food colouring in the tank and do not flush for 15–30 minutes. Colour in the bowl means the flapper leaks.

2. Fix the flapper and chain

1. Check the chain. With the flapper closed, the chain should have a little slack, about half an inch. Too tight holds the flapper open; too long can get trapped under it. Move the clip one link at a time.
2. Clean the seal. Shut off the water at the valve behind the toilet (turn clockwise), flush to empty the tank, and wipe the flapper and the rim it sits on.
3. Replace the flapper if it is warped, stiff or still leaks. Unhook it from the ears on the overflow tube, unclip the chain, fit the new one the same way, then set the chain slack as above.
4. Turn the water back on and test a few flushes.

3. Adjust or replace the fill valve

1. Lower the water level. Most fill valves have an adjustment: a screw or clip on the float, or a float arm you can bend gently on older types. Set it so the water stops about an inch below the top of the overflow tube, or at the line marked inside the tank.
2. Still running or hissing? The valve is worn. Shut off the water, flush, sponge out the rest, and disconnect the supply line under the tank.
3. Swap the valve following the kit's instructions: undo the nut under the tank, lift the old valve out, fit the new one, set its height, and tighten by hand plus a little more. Do not overtighten; the tank is porcelain.
4. Reconnect, turn on, check for drips under the tank with a dry paper towel, and set the water level.

When to call a plumber

• The shut-off valve will not turn or does not stop the water.
• Water leaks between the tank and the bowl, or around the base of the toilet.
• The tank or bowl is cracked.

Related: How to replace a bathroom faucet · The starter tool kit.