How to Fix a Running Toilet
Updated September 18, 2026
Can I do this myself?
Safe to DIY
Yes. Everything that makes a toilet run lives in the tank, uses clean supply water, and can be fixed with the water shut off at the valve behind the toilet. The main risks are a cracked tank lid or an overflow, not injury.
A running toilet is the kind of problem that seems minor until you notice how much water it is quietly sending down the drain. A tank that never stops filling, or one that tops itself up every few minutes, can waste more water in a day than the rest of the household uses. The good news is that the fix almost always happens inside the tank, with parts that are cheap, standard and designed to be replaced by hand.
Two parts do nearly all the work in a toilet tank. The flapper (or on some toilets, a tower-style flush valve seal) at the bottom holds water in the tank until you flush. The fill valve on the left side refills the tank and shuts off when the float rises to the right level. A running toilet means one of these two is failing: either water is leaking out past the flapper, or the fill valve is not shutting off and water is spilling into the overflow tube.
This guide shows you how to tell which one it is in under a minute, then walks through adjusting or replacing the flapper, chain, float and fill valve in the order a plumber would check them.
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Signs of the problem
- You can hear water trickling or hissing long after the flush has finished
- The toilet refills briefly on its own every few minutes or hours (called 'phantom flushing')
- Water is flowing into the top of the overflow tube in the middle of the tank
- The handle has to be jiggled or held to make the toilet stop running
- The flush is weak or the tank never seems to fill completely
- Your water bill or water meter shows use when nothing is turned on
Common causes
Worn or warped flapper
Most commonThe rubber flapper at the bottom of the tank hardens, warps or collects mineral scale over time. Once it no longer seals tightly against the flush valve seat, water slowly leaks into the bowl and the fill valve kicks on to replace it. This is the single most common cause of a running toilet and of phantom flushing.
Chain too short, too long or tangled
Most commonThe lift chain connects the handle arm to the flapper. If it is too tight, the flapper cannot close fully. If it has too much slack, it can get trapped under the flapper and hold it open. A little slack, roughly 1 cm (half an inch), is correct.
Float set too high
Most commonThe float tells the fill valve when to stop. If it is set too high, the water level rises above the top of the overflow tube and runs straight into the bowl, so the valve never shuts off. The correct level is usually marked by a line inside the tank, about 2.5 cm (1 inch) below the top of the overflow tube.
Failing fill valve
SometimesThe fill valve has a seal inside its cap that wears out or gets clogged with grit and mineral deposits. A failing fill valve may hiss constantly, refill very slowly, or keep running even when the float is correctly set. Cleaning the seal sometimes helps, but a full replacement is quick and usually the better fix.
Cracked overflow tube or damaged flush valve seat
Less commonRarely, the overflow tube cracks below the water line, or the plastic or brass seat the flapper rests on is chipped or rough. Water then drains into the bowl no matter how good the flapper is. Fixing this means replacing the entire flush valve, which involves removing the tank.
Safety first: before you start
- Critical:Close the shut-off valve on the wall or floor behind the toilet (turn clockwise) before removing any parts, then flush to empty the tank.
- Critical:Set the porcelain tank lid down flat on a towel on the floor, away from where you will step. Lids are heavy and crack easily if dropped or knocked.
- Do not use drop-in chemical tank tablets or bleach while working; they degrade rubber parts and irritate skin.
- Keep a towel and small bucket nearby. There will always be a little water left in the tank and supply line.
- Wear rubber gloves if you prefer. Tank water is clean supply water, but the bowl and rim are not.
Tools & materials
Tools
- Towel and small bucket or bowl
- Sponge (to soak up the last water in the tank)
- Adjustable wrench or large slip-joint pliers
- Flathead screwdriver
- Food colouring or dye tablets (for the leak test)
- Scissors (for trimming a new fill valve's refill tube, if needed)
Materials
- Replacement flapper sized to your flush valve (usually 5 cm/2 inch or 7.5 cm/3 inch)
- Universal fill valve kit, if the fill valve is the problem
- Replacement lift chain (often included with a new flapper)
Step-by-step fix
- 1
Lift the lid and watch the tank
Remove the tank lid and place it safely on a towel. Watch where the water is going. If water is spilling into the top of the overflow tube (the open pipe in the middle), the fill valve or float is the problem. If the water level is below the overflow tube but the fill valve keeps turning on, water is leaking past the flapper.
Tip: Taking a quick photo of the inside of the tank helps you remember how everything was connected.
- 2
Do the dye test for a leaking flapper
Add a few drops of food colouring to the tank water and do not flush. Wait 15 to 20 minutes. If coloured water appears in the bowl, the flapper or flush valve seat is leaking. Flush afterwards so the dye does not stain.
- 3
Check and adjust the chain
With the flapper closed, look at the lift chain. It should hang with about 1 cm (half an inch) of slack. If it is taut, move the clip one or two links down the chain. If there is a long loose tail that could get caught under the flapper, trim it or hook the clip higher. Flush a few times and watch whether the flapper drops cleanly and seals.
- 4
Adjust the float height
If water is flowing into the overflow tube, lower the float. On a modern float-cup fill valve, squeeze the metal clip on the vertical rod and slide the cup down about 1 cm, or turn the adjustment screw on top counter-clockwise. On an older ball-and-arm float, turn the screw where the arm meets the valve, or gently bend the brass arm downward. Aim for the water to stop about 2.5 cm (1 inch) below the top of the overflow tube, or at the fill line marked on the tank.
- 5
Shut off the water and drain the tank
If adjustments did not help, you will need to replace a part. Turn the supply valve behind the toilet clockwise until it stops. Flush and hold the handle down to drain as much water as possible, then soak up the rest with a sponge.
Warning: If the valve will not turn or starts dripping from the stem, do not force it. Old valves can snap. Shut off the main water supply to the house instead.
- 6
Replace the flapper
Unhook the chain from the handle arm. Slide the flapper's side ears off the pegs on the overflow tube, or slip the ring over the top of the tube if it uses one. Before fitting the new flapper, run your finger around the flush valve seat. If it feels gritty or slimy, clean it with a non-scratch scrub pad. Fit the new flapper, reconnect the chain with slight slack, and check that it sits centred on the seat.
Tip: Take the old flapper to the store. Match the size (2 inch or 3 inch) and style, and check the packaging for your toilet brand if listed.
- 7
Replace the fill valve if needed
Put a bucket under the tank. Unscrew the supply line coupling from the bottom of the tank (counter-clockwise when looking up). Unscrew the plastic locknut underneath and lift the old fill valve out. Set the height of the new valve according to its instructions, insert it with its rubber washer inside the tank, and hand-tighten the locknut plus about a quarter turn with pliers. Reconnect the supply line hand-tight plus a quarter turn, and clip the refill tube onto the overflow tube so it sits above the water line.
Warning: Plastic tank fittings crack if overtightened. Tighten just enough to stop leaks; you can always snug it a little more later.
- 8
Turn the water back on and test
Open the supply valve slowly, counter-clockwise, and watch the tank fill. Check the supply connection and locknut underneath for drips with a dry tissue. Let the fill valve shut off, then check the water level against the fill line and adjust the float if needed.
- 9
Flush several times and repeat the dye test
Flush five or six times, checking each time that the flapper drops and seals and the fill valve shuts off completely. Repeat the dye test to confirm nothing is seeping into the bowl, then replace the lid carefully.
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How to check it's fixed
- The tank refills and goes completely silent within about a minute of flushing
- Dye added to the tank does not appear in the bowl after 20 minutes
- The water level stops below the top of the overflow tube
- No drips at the supply line or fill valve locknut under the tank
When to stop and call a professional
- The shut-off valve behind the toilet is seized or leaks when you turn it
- The tank or bowl is cracked, or water is leaking from the bolts between tank and bowl
- The flush valve seat is chipped or the overflow tube is cracked and you are not comfortable removing the tank
- Water is leaking onto the floor from underneath the toilet
- The toilet still runs after replacing both the flapper and fill valve
How to prevent it
- Avoid drop-in bleach or chlorine tablets. They shorten the life of flappers and seals.
- Do a dye test every six months or so to catch a slow flapper leak early.
- Check that the chain has a little slack whenever you notice the handle feels different.
- In hard-water areas, wipe mineral scale off the flush valve seat when replacing the flapper.
Frequently asked questions
Why does my toilet run for a few seconds on its own every so often?
That is called phantom flushing, and it almost always means water is slowly leaking from the tank into the bowl past the flapper. When the level drops enough, the fill valve briefly turns on to top it up. Replace the flapper and clean the seat it rests on.
Is a running toilet an emergency?
No, but it is worth fixing soon. It wastes a lot of water and can raise your bill. It only becomes urgent if water is overflowing the bowl or leaking onto the floor. In that case, close the valve behind the toilet straight away.
Should I replace the flapper or the whole fill valve?
Start with the cheapest and most likely fix. If the dye test shows a leak into the bowl, replace the flapper. If water is running into the overflow tube and adjusting the float does not help, replace the fill valve. Many people replace both at once on an older toilet.
Are toilet repair parts universal?
Most fill valves are universal and adjust to fit different tank heights. Flappers come in a few standard sizes, mostly 2 inch and 3 inch, but some toilets use brand-specific seals. Check the model number stamped inside the tank or take the old part with you.
My toilet has a button on top instead of a handle. Is the fix the same?
Dual-flush and push-button toilets usually use a tower-style flush valve instead of a hinged flapper. The cause is the same (a worn seal at the bottom), but the replacement seal is specific to the brand and model. Twist or unclip the tower per the maker's instructions and replace its rubber seal.
This guide is general information, not professional advice. Homes differ, so follow your product manuals and local codes. If anything looks different from what's described, or you feel unsure, stop and call a qualified professional. In an emergency, leave the area and call your local emergency number.
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