How to Fix a Running Toilet in Under an Hour

A running toilet is one of those household problems that people tend to ignore because it seems minor. It is not. A toilet that runs continuously can waste anywhere from 200 to 400 litres of water per day — that is enough to fill a bath twice. Over a year, that adds up to a significant amount on your water bill, particularly if you are on a meter. The good news is that the fix is almost always straightforward, inexpensive, and well within the ability of a competent DIYer.

This guide walks you through diagnosing the cause of your running toilet and fixing it yourself in under an hour, using tools and parts available from Screwfix or B&Q for typically under £20.

Understanding How a Toilet Cistern Works

Before diving into the fix, it helps to understand the basics. A standard UK toilet cistern contains two key components:

• The fill valve (ballcock or float valve): Controls the water entering the cistern after a flush. When the water level rises to the correct point, the valve shuts off.
• The flush valve (flapper or siphon): Controls the water leaving the cistern when you flush. It should seal completely between flushes.

Running toilets almost always come down to one of three problems: the fill valve is not shutting off properly, the flush valve is not sealing properly, or the overflow pipe is draining continuously. The first step is diagnosing which one is at fault.

Step 1: Diagnose the Problem

Remove the cistern lid (it simply lifts off) and observe what is happening inside. Here is what to look for:

Water Running Into the Bowl Constantly

If you can see or hear water trickling into the toilet bowl continuously — even when the cistern appears to be full — the flush valve (flapper or siphon) is not sealing properly. This is the most common cause of a running toilet.

A quick test: put a few drops of food colouring into the cistern water. Wait ten minutes without flushing. If the colour appears in the bowl, the flush valve is leaking.

Water Running Into the Overflow

Most UK cisterns have an internal overflow: a pipe that runs down inside the pan and drains excess water into the bowl if the water level rises too high. If the cistern is filling beyond the overflow point, you will see water constantly trickling from the bottom of the pan — not from the main flush, but from inside the trap.

This indicates the fill valve is not shutting off correctly, either because the float is set too high, the valve is worn, or the float itself is waterlogged (common with older ball-float designs).

Water Running Out of an External Overflow Pipe

Older properties may have an external overflow pipe exiting through the wall outside. If water is dripping from this pipe, the fill valve is at fault — same diagnosis as above.

Step 2: Fixing the Fill Valve (Ballcock)

If the cistern is overfilling, tackle the fill valve first.

Adjusting the Float Level (Older Ball Float)

Traditional ballcocks have a long arm with a hollow plastic or copper ball on the end. As the water rises, the ball rises and eventually pushes the valve closed. If the arm is set too high, the water level rises above the overflow point before the valve closes.

On older metal arm designs, you can gently bend the arm downward to lower the shut-off point. On plastic arms, there is usually an adjustment screw where the arm attaches to the valve body. Turn this to lower the effective float position. You are aiming for the water level to sit approximately 25mm below the overflow pipe.

Replacing a Modern Fill Valve

Newer toilets use a compact fill valve (sometimes called a Fluidmaster or bottom-entry valve) rather than a traditional ball arm. These have a small float that travels up a vertical spindle. There is typically an adjustment collar or clip that sets the water level.

If adjustment does not solve the problem — or if the valve is worn and dripping even at the correct level — replacement is the answer. A Fluidmaster fill valve (Screwfix stock several types, typically £8–£15) is an almost universal fit and takes around 20 minutes to swap over.

To replace the fill valve:

1. Turn off the water supply to the cistern. The isolation valve is usually on the water supply pipe below the cistern — turn it clockwise with a flathead screwdriver until the slot is at right angles to the pipe. If there is no isolation valve, turn off the main stop cock under the kitchen sink.
2. Flush to empty the cistern, then use a sponge to mop out any remaining water.
3. Disconnect the water supply hose from the bottom of the cistern (have a small bowl ready — a little water will drain out).
4. Unscrew the plastic nut on the underside of the cistern that holds the fill valve in place. The valve can then be lifted out.
5. Insert the new valve, tighten the nut by hand (do not over-tighten), reconnect the supply hose, and turn the water back on.
6. Adjust the water level using the valve's adjustment mechanism, then test with a flush.

Step 3: Fixing the Flush Valve

If water is leaking past the flush valve into the bowl, you have two options: replace the rubber seal, or replace the entire flush valve assembly.

Replacing the Flapper Seal (Push-Button Cisterns)

Modern dual-flush toilets (the type with two push buttons on top) use a flush valve with a rubber seal or diaphragm at the bottom. Over time, this seal degrades and allows water to seep through.

The simplest fix is to replace the flush valve unit entirely. Units from manufacturers like Siamp, Geberit, or Grohe cost £12–£25 and are widely available. The process involves:

1. Turning off the water supply and emptying the cistern.
2. Removing the push-button assembly (usually a quarter-turn anti-clockwise to release).
3. Unscrewing the collar holding the flush valve to the cistern base.
4. Lifting out the old valve, dropping in the new one, and reassembling in reverse.

Replacing a Siphon (Older Toilets)

Toilets made before the mid-1990s typically use a siphon flush mechanism rather than a flapper. The siphon does not leak in the same way as a flapper — instead, a running toilet on a siphon-flush design almost always indicates the fill valve is the culprit. However, if the siphon diaphragm is split, the flush will feel weak or incomplete. Replacement siphons (£8–£15 at Wickes) require fully emptying and partially dismantling the cistern.

Step 4: Check the Toilet Pan Seal

If the toilet appears to rock slightly or there is evidence of water around the base of the pan, the wax ring seal (or in modern installations, a foam gasket) between the pan and the drain may be compromised. This is a different issue from a running cistern, but worth checking while you have the lid off and are already in "maintenance mode". A replacement pan-to-soil-pipe connector costs around £5–£10.

Tools and Parts You Will Need

• Adjustable spanner
• Flathead screwdriver
• Sponge and small bucket
• Replacement fill valve (if needed) — Fluidmaster 400A or equivalent, £8–£15
• Replacement flush valve or seal (if needed) — £12–£25
• PTFE tape (£1–£2) for any threaded connections

When to Call a Plumber

The vast majority of running toilet issues can be resolved with the steps above. However, there are situations where a professional is advisable:

• The cistern is a concealed in-wall (Geberit) unit — accessing the mechanism requires removing a wall panel, and the parts can be model-specific
• There is evidence of water damage or mould beneath the floor around the toilet base
• You cannot locate or operate the isolation valve and the main stopcock does not fully close
• The toilet is very old and parts are no longer available (some pre-1980s cisterns)

For straightforward repairs in a standard UK bathroom, expect to pay £80–£150 if you do call a plumber — which makes the DIY route all the more appealing when the parts cost less than £20 and the job takes under an hour.

Preventing the Problem in Future

Running toilet problems are often caused by mineral scale build-up (particularly in hard water areas like the South East and Midlands) causing valves to stick, or by rubber seals degrading over time. Fitting an in-line scale filter on the cistern supply can prolong valve life significantly. Brands like FreshFlow and Fernox make products suited to this.

Checking your toilet cistern every year or two — simply lifting the lid and observing the water level and valve operation — takes two minutes and can catch problems before they become costly water waste issues. Given that the average metered household water bill in England is around £440 per year, a toilet wasting 200 litres a day could be adding an extra £150–£200 annually. The hour you invest in this repair pays dividends immediately.