How to Fix a Running Toilet in the UK – Step by Step Guide

A running toilet is one of those household problems that’s easy to ignore because it seems minor – a quiet hiss or trickle that you stop noticing after a few days. But a toilet that runs continuously wastes between 200 and 400 litres of water every single day, which adds meaningfully to your water bill and, if you’re on a water meter, costs real money every month you leave it unfixed.

The good news is that the vast majority of running toilet problems come down to one of four things – an overflowing cistern, a worn flapper valve, a faulty fill valve or a misadjusted float. All of them are DIY-fixable without calling a plumber, usually for under £15 in parts.

Diagnosing the problem first

Before buying any parts, spend five minutes diagnosing exactly what is happening. Lift the cistern lid and observe. This tells you everything.

Fixing an overflowing cistern

If water is running into the overflow pipe – the pipe that exits through the bottom of the cistern directly into the toilet pan – the water level in the cistern is too high. This is usually caused by the float being set too high or the fill valve failing to shut off properly.

Look at the water level relative to the overflow pipe opening. If water is at or above the overflow pipe level, the float needs adjusting downward. On older UK sloshing ball-type floats, you simply bend the float arm slightly downward to lower the water level. On modern fill valves with a vertical float, there is usually an adjustment screw or clip that lowers the float position. The correct water level is approximately 25mm below the overflow pipe.

Fixing a faulty flapper or flush valve

The flapper is the rubber seal at the bottom of the cistern that opens when you flush and seals the cistern from the bowl when it closes. Over time the rubber deteriorates, warps or accumulates limescale deposits and stops sealing properly. This is the most common cause of a running toilet in the UK.

1. 1
   Turn off the water supply The isolation valve is usually on the pipe entering the bottom of the cistern. Turn it clockwise with a flathead screwdriver until it stops. If there’s no isolation valve, turn off the mains stopcock under the kitchen sink.
2. 2
   Flush to empty the cistern This empties the cistern so you can work in it. Mop up any remaining water with a sponge or old towel.
3. 3
   Remove the flapper Unhook the flapper from the side pegs of the flush valve seat and disconnect the chain from the flush handle arm. Take the old flapper with you to the plumber’s merchant or DIY store – UK cisterns use various sizes and you need an exact match.
4. 4
   Fit the new flapper Hook the new flapper over the flush valve seat pegs and reconnect the chain to the flush handle arm. Leave about 1-2cm of slack in the chain – too tight and the flapper won’t seal, too loose and it may tangle.
5. 5
   Turn water back on and test Open the isolation valve, allow the cistern to refill, then flush several times. Listen carefully – the running sound should be gone.

Replacing the fill valve

If adjusting the float doesn’t stop the cistern overfilling, or if the cistern takes a very long time to refill, the fill valve itself may need replacing. This is a slightly more involved job but still well within DIY capability and the parts cost around £12-15 from any plumber’s merchant or DIY store.

Turn off the water supply and empty the cistern as above. Disconnect the water supply pipe from the bottom of the cistern (have a towel ready – some water will come out). Unscrew the locknut holding the fill valve to the cistern base – this usually turns anticlockwise by hand or with an adjustable spanner. Remove the old valve and fit the new one in reverse order. Universal fill valves fit most UK cisterns – check the inlet thread size before buying (most UK toilets use a 1/2 inch BSP thread).

Adjusting the float

If the cistern is not overflowing but keeps running, the float level may need fine adjustment. On modern vertical fill valves, there is typically a clip or screw that adjusts how high the float rises before shutting off the water. Turn the adjustment screw clockwise to lower the shutoff point. The water should stop filling when the level is approximately 25mm below the top of the overflow pipe.

When to call a plumber

Most running toilet problems are DIY-fixable, but some situations warrant professional help:

• Water leaking from the base of the toilet – this is a pan-to-floor seal problem, not a cistern issue, and requires lifting the toilet
• Cracked cistern – a cracked cistern body needs full replacement, not repair
• Leak from the cistern-to-pan connector – the large rubber washer where the cistern connects to the pan can deteriorate; replacement requires removing the cistern
• No isolation valve and you can’t locate the stopcock – never work on plumbing without being certain you can turn off the water supply

A good plumber will fix any of these in under two hours – always get a fixed price before they start rather than agreeing to an hourly rate. For more practical home repair guides, read our guide on how to fix a dripping tap UK – the same isolation valve and water supply skills apply.