How to Fix a Running Toilet

Why Toilets Run Constantly

A toilet runs when water continuously flows from the cistern into the bowl. The three most common causes are: a faulty flapper valve (doesn't seal properly), a faulty float (lets water level rise too high), or a worn fill valve. Each has a different fix.

Diagnosing the Problem

Lift the cistern lid and observe. If water is flowing into the overflow pipe (the pipe in the back of the cistern), the float is set too high or the fill valve is faulty. If water trickles into the bowl constantly, the flapper valve is worn. Add a drop of food colouring to the cistern — if colour appears in the bowl without flushing, the flapper is leaking.

Fix 1 — Adjusting the Float

The float controls when the cistern stops filling. If it's set too high, water runs into the overflow. On a ball float, bend the arm slightly downward to lower the water level. On a modern float valve, adjust the screw or clip on the side of the fill valve.

Fix 2 — Replacing the Flapper Valve

The flapper is a rubber seal at the bottom of the cistern. Turn off the water supply, flush to empty the cistern, unhook the flapper from its mounting, and take it to a DIY store for an exact replacement. Fit the new flapper and turn water back on.

Fix 3 — Replacing the Fill Valve

If adjusting the float doesn't work, the entire fill valve may need replacing. Turn off the water supply at the isolation valve, flush to empty, disconnect the water supply pipe, unscrew the locknut underneath the cistern, and fit a new fill valve (available for £10–£20 from any plumbers' merchant).

When to Call a Plumber

If you've tried all fixes and the toilet still runs, or if the cistern itself is cracked, call a plumber. A toilet repair typically costs £60–£150 for a plumber.