The short answer
If your toilet keeps running after you flush, the cause is usually one of three things: a worn fill valve that will not shut off, a leaking flush valve seal that lets water trickle into the pan, or a float set too high so water spills into the overflow. All three are common, quick to fix and much cheaper than the water a running toilet can waste.
Find the cause in two minutes
- Take the lid off the cistern carefully.
- Look at the water level. If it is up to the overflow pipe, the fill valve or float is the problem.
- If the level is below the overflow but water still trickles into the pan, the flush valve seal is leaking.
- Put a few drops of food colouring in the cistern. If colour appears in the pan without flushing, the seal is leaking.
Causes and fixes
| Cause | Signs | Fix |
|---|---|---|
| Float set too high | Water runs into the overflow | Adjust the float down |
| Worn fill valve | Hissing, cistern never stops filling | Replace the fill valve |
| Leaking flush valve seal | Trickle into the pan, cistern refills every few minutes | Replace the seal or flush valve |
| Limescale on valves | Common in hard water areas | Clean or replace the valve |
Can I fix it myself?
Adjusting a float is a simple DIY job. Replacing a fill valve or flush seal is possible but usually means turning off the water, draining the cistern and sometimes removing it. If you are not confident, a plumber can do it quickly and check the rest of the toilet at the same time.
How much does it cost?
Checkatrade's 2026 figures put the average UK plumber at about £50 an hour, with London rates higher. Most running toilet repairs take under an hour plus the cost of a valve or seal. We give a written price first.
Why fix it quickly?
A running toilet can waste a lot of water every day. On a water meter, that can mean a much higher bill. It can also be a sign of wear that leads to a full failure.
Related service: tap, toilet and sink repairs.
A few common questions
Why does my toilet keep running after flushing?expand_more
Usually because the fill valve is worn and will not shut off, the flush valve seal is leaking water into the pan, or the float is set too high so water flows into the overflow. Lifting the cistern lid and checking the water level helps you find which one it is.
How do I stop my toilet running?expand_more
First check the float. If water runs into the overflow, adjust the float down. When water trickles into the pan, the flush valve seal needs replacing. And when the cistern hisses and never stops filling, the fill valve is worn. A plumber can replace these parts quickly.
How much does it cost to fix a running toilet?expand_more
Checkatrade's 2026 figures put the average UK plumber at about £50 an hour, and London rates are higher. Most running toilet repairs take under an hour, plus the cost of a new fill valve or seal. Ask for a written price before the work starts.
Does a running toilet waste a lot of water?expand_more
Yes. A constantly running toilet can waste a large amount of water every day. If you have a water meter, this shows up on your bill. Fixing the valve or seal quickly saves water, saves money and stops further wear on the parts.
Does hard water cause toilets to keep running?expand_more
It can. Limescale builds up on fill valves and flush seals, stopping them closing properly. This is common across London and the Home Counties. Cleaning or replacing the affected parts usually fixes the problem, and newer valves are often more resistant to scale.