How to Fix a Running Toilet
Nine times out of ten a running toilet is a worn flapper or a float set too high. Both are ten-minute fixes with no special tools.
Co-authored by:
Tom Becker
Licensed Plumber & Home Repair Writer
Steps
3 methods1.Finding Out What Is Wrong
- 1
Listen and look for the source
Take the tank lid off and watch. Water trickling continuously into the bowl means the flush valve is leaking. Water running into the overflow tube means the fill level is set too high.
If nothing moves but the tank refills itself every few minutes, the leak is slow and intermittent — the dye test below will confirm it.
- 2
Do the dye test
Put a few drops of food colouring, or a dye tablet, into the tank and wait fifteen minutes without flushing. Colour appearing in the bowl proves the flapper is leaking, even when you cannot hear it.
Tip: A silent leak can waste several hundred litres a month, so do not skip this test.
- 3
Check the handle and chain first
Jiggle the handle. If the water stops, the flapper chain is too short or too long, or the handle is not returning fully. That is a free fix before you buy anything.
The chain should have about 1 cm of slack when the flapper is closed, and never catch under the flapper as it drops.
2.Replacing the Flapper
- 1
Shut off the water and empty the tank
Turn the valve on the wall behind the toilet clockwise, then flush to drain the tank. Sponge out the remaining water into a bucket so you can see the seat clearly.
- 2
Remove the old flapper
Unhook the chain from the flush lever, then slide the flapper off the two pegs on the overflow tube or unscrew it from the flush-valve nut, depending on the type.
Take the old flapper with you to the shop. Sizes and seat shapes differ, and the wrong one will leak on the first flush.
- 3
Clean the seat and fit the new flapper
Wipe the flush-valve seat with a cloth and mild vinegar to remove the mineral film that stops a seal. Do not sand or scrub it with an abrasive pad — a scratched seat will leak with any flapper.
Attach the new flapper, set the chain with about 1 cm of slack, and let it settle flat on the seat before you turn the water back on.
- 4
Turn the water back on and test
Open the shutoff valve, let the tank fill, then flush and watch for three cycles. Mark the water line with a pencil and check it an hour later — if it has dropped, the seal is still wrong.
3.Adjusting the Fill Valve and Float
- 1
Set the correct water level
The water should stop about 25 mm below the top of the overflow tube, or at the mark moulded into the tube. If water is running into the tube, the level is too high.
On a ball-and-rod float, bend the rod slightly downward or turn the adjustment screw. On a cup-style float, squeeze the clip and slide the cup down.
- 2
Replace a sticky or cracked fill valve
If the valve keeps running or never fully shuts, it is worn. Turn off the water, flush, then unscrew the supply nut under the tank and lift the old valve out.
Set the new valve height so the top sits about 10 cm above the overflow tube, tighten the nut hand-tight plus a quarter turn, and reconnect the supply line.
Tip: Do not overtighten plastic nuts; they crack and create a new leak under the tank.
- 3
Check the base for a second leak
Water on the floor around the base is not a running-toilet problem — it is usually a failed wax ring or a loose tank bolt. If the base is wet, tighten the tank-to-bowl bolts slightly; if it still leaks, reseal the base.
Tips
- Take a photo of the linkage before you disconnect anything; reassembly is much faster.
- A rubber flapper lasts about four to five years. If it is soft and swollen, it has been soaking in mineral-heavy water.
- Avoid in-tank drop blocks with bleach — they chew through rubber seals and shorten the life of the whole mechanism.
- Keep a spare flapper and a universal fill valve in the house; both cost less than a single service call.
- Flush twice a week in guest bathrooms so the seals do not dry out.
- If the tank water level is right but the bowl keeps refilling, check for a hairline crack in the overflow tube.
Warnings
- Toilet water is not clean. Wear gloves, and wash hands and any tools thoroughly after the job.
- Lift porcelain tank lids with two hands on a clear floor — they are heavy, slippery, and break instantly.
- If you are not comfortable working with the water supply, or you find a crack in the porcelain, stop and call a plumber. A cracked tank cannot be repaired safely.
Community Q&A
Why does my toilet run only at night?+
Pressure in the supply line rises when few taps are open, which can push a marginal flapper past its seal. Replacing the flapper usually fixes it.
How much water does a running toilet waste?+
A continuously running toilet can waste 200–400 litres a day, which is enough to show clearly on a water bill.
Can I just adjust the chain instead of buying parts?+
Often yes. If the flapper is still firm and flat, setting the chain to about 1 cm of slack solves the problem for free.
Are all flappers the same size?+
No. The common sizes are 2 inch and 3 inch, and some valves use a tower-style seal. Measure the flush-valve opening or bring the old part with you.
My toilet still runs after replacing the flapper. What now?+
Check that the seat is clean and unscratched, the chain is not catching, and the water level is below the overflow tube. If all three are right, the fill valve is likely the culprit.
How do I know if the tank is cracked?+
Look for a hairline crack with the tank empty and dry, and for mineral streaks running down the porcelain. A cracked tank must be replaced, not sealed.
References
- U.S. Environmental Protection Agency — WaterSense toilet leak detection
- U.S. Department of Energy — fixing leaky faucets and toilets in the home
- Manufacturer flush-valve installation guides (Fluidmaster / Korky)
About This Article
Written by Site Admin. A subject-matter reviewer checked the steps before publication. Last updated Oct 27, 2026. 5,980 readers have viewed this guide.
91% of readers found this article helpful.
74 votes - 91%
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Peter V.
Oct 19, 2026
“Dye test, new flapper, twenty minutes. My water bill dropped by a third the next month.”
