How to Fix a Running Toilet: Float, Flapper and Fill Valve

If your toilet hisses, trickles or refills every few minutes when nobody's used it, something inside the cistern isn't shutting off. The good news is that there are only a handful of likely causes, and the first checks need no tools at all.

Here's how I'd explain it, based on guidance from WaterSafe, the UK plumber register, and the Homebuilding advice site. Work through the checks in order, from the simplest to the most involved, and stop as soon as the noise does.

Why a toilet keeps running

WaterSafe lists three main suspects inside the cistern:

• The flapper or flush valve seal. If the rubber seal is damaged, worn out or dirty, it doesn't close fully and water leaks from the cistern into the bowl.
• The overflow tube. If it's too short or out of position, water can flow into the bowl constantly.
• The fill valve. If it malfunctions, it may not cut off the supply once the cistern is full.

Homebuilding adds a few more: a float set higher than the top of the overflow pipe, dirt and scale on the fill valve, debris on the flush valve seat, and a flush chain that's been adjusted badly. It describes a worn seal or flapper as the most common cause.

Look first: lift the lid and watch a flush

WaterSafe's first step is simply to remove the cistern lid, look at the parts, then flush and watch what the water does. Put the lid somewhere safe where it can't be knocked and cracked.

Most UK toilets use a float valve with a float cup, according to Homebuilding. Spotting which parts you have will tell you what to adjust.

Check the water level and adjust the float

Water that sits too high spills into the overflow pipe and runs away. WaterSafe says the level should sit just below the overflow pipe, and Homebuilding says the float cup should end up about an inch below it. To lower it, Homebuilding says to turn the adjustment screw clockwise and slide the float cup down the rod.

If the level is too low instead, WaterSafe says to adjust the water valve to let more water in. Flush and watch again after each small change.

Inspect the flapper and the flush chain

Both sources point to the flapper next. Replace it if it's warped, dirty or damaged. Check the chain too: if it's tangled or too tight, adjust it so the flapper can close fully.

Before you take anything out, isolate the water. Homebuilding says to turn the shut-off valve, usually on the floor or wall behind the toilet, clockwise, and then flush to drain the cistern. To swap a flapper, it says to disconnect the chain from the overflow pipe and lever, remove the old flapper, fit the new one and reconnect the chain.

Replace the flush valve seal if the flapper isn't the problem

Some cisterns have a flush valve unit with a seal at the bottom. Homebuilding says replacing the seal, or the whole unit, can stop a leak into the bowl. Most flush valve units are 50mm, which is about 2 inches, and it suggests measuring the old one where it joins the cistern drain hole. If you're unsure, take the old part to a plumbers' merchant to match it.

Clean or replace the fill valve

Dirt and scale can stop the fill valve closing. Homebuilding says cleaning it removes that debris. WaterSafe says that if the fill valve is faulty, it should be replaced.

A quick checklist for a running toilet

What you notice	Likely cause	First thing to try
Water spilling into the overflow pipe	Float or water level set too high	Lower the float cup
Trickle into the bowl, level looks fine	Worn or dirty flapper or seal	Inspect and replace it
Flapper won't sit down	Chain tangled or too tight	Adjust the chain
Water keeps coming in after the cistern is full	Fill valve not shutting off	Clean it, or replace it

Why it's worth fixing and when to call a plumber

Homebuilding says a constant small trickle can let up to 200 litres pass through a toilet every 24 hours. That's a lot of clean water going straight down the drain.

If you've been through the checks and it still runs, WaterSafe says the job may be better left to a professional, and it has a tool for finding an approved contractor. A plumber can replace worn parts and help prevent leaks later.

Other leaks to fix while you're at it

A running cistern is a good prompt to look at the rest of your water use. A dripping tap wastes water in the same quiet way, and I've covered it in how to fix a dripping tap. Pipes in cold places are also worth checking before winter, and how to insulate pipes explains the basics.

Plumbing knowledge worth sharing

Clear, practical advice is what helps people sort their own plumbing faster, and trade expertise is exactly what readers look for.

Trade professionals with advice to share can find plumbing sites open to outside articles here.

Frequently asked questions

Why does my toilet keep running after I flush?

WaterSafe says the usual causes are a worn or dirty flapper, an overflow tube that's too short or out of position, or a fill valve that doesn't shut off once the cistern is full.

Should the water level be near the overflow pipe?

It should sit just below it, according to WaterSafe. Homebuilding says to aim for the float cup to be about an inch under the overflow pipe.

Do I need to turn the water off first?

For anything beyond looking and adjusting the float, yes. Homebuilding says to isolate the supply at the shut-off valve behind the toilet and flush to drain the cistern.

How much water can a running toilet waste?

Homebuilding says a constant small trickle can let up to 200 litres pass through a toilet in 24 hours.

When should I call a plumber?

If the checks above don't stop it, WaterSafe says the problem may be better left to a professional and points to its list of approved contractors.