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Toilet keeps running or leaking into the pan? How to find and fix the faulty part

In short: Test for a silent leak with dry toilet paper or food colouring, then lift the cistern lid. Water at its normal line means the flush valve washer is leaking; water above it means the fill valve. Turn off the isolation valve before you fit a new part.

Co-authored by Sonia Abu Bakar✓, MSc Botany

By 6 min readPublisher: Saadat Rafiq

Last updated: 28 October 2026References

Set preferred
Illustration of a woman lifting the lid off a toilet cistern and looking inside, a folded towel on the floor ready for the lidIllustration

At a glance

Test
Dry toilet paper at the back of the pan, 1 hour to overnight (Bristol Water, WaterSafe)
Or
1 teaspoon of food colouring in the cistern, wait 10 to 15 minutes (NYC DEP)
Wasted
Up to 400 litres a day from one leaking cistern (WaterSafe)
Water level
About an inch (2.5 cm) below the overflow (HomeServe)
Advice from
WaterSafe, Bristol Water, NYC DEP, HomeServe, read 28 October 2026
Checked
October 2026

A toilet that keeps running, or trickles into the pan long after a flush, almost always comes down to one of two parts. Either the flush valve at the bottom of the cistern isn't sealing, or the fill valve isn't shutting off when the cistern is full. The checks below tell you which one, so you buy the right part. Many leaks are silent, so test even if you can't hear anything.

How to find and fix the leak

  1. Test for a silent leak.

    Illustration of a single dry sheet of toilet paper stuck across the back of a toilet pan just under the rim
    Illustration

    Toilet paper test: about 30 minutes after a flush, wipe the back of the pan dry and stick a dry sheet of toilet paper across it, just under the rim (WaterSafe, Bristol Water). Don't flush. Bristol Water says to leave it about an hour; WaterSafe says up to three hours or overnight.

    Food colouring test: put a teaspoon of food colouring in the cistern and wait 10 to 15 minutes without flushing (NYC DEP).

    You should see: dry paper or clear water if there's no leak. Wet or torn paper, or coloured water in the pan, means the cistern is leaking. The faster colour appears, the bigger the leak (NYC DEP).

  2. Lift the cistern lid and mark the water line.

    Illustration looking into an open toilet cistern, a hand drawing a short pencil line on the inside wall at the water surface, the overflow pipe and a fill valve visible
    Illustration

    Put the lid on a towel on the floor. Inside you will see a stain or "tide mark" showing the normal water level (Bristol Water). Mark the level the water is at now with a pencil (NYC DEP).

    Find the two parts: the fill valve where the water comes in, with a float on it, and the flush valve in the middle at the bottom, which opens when you press the button or lever. Most have an overflow: a pipe or opening that takes away water if the level gets too high.

    You should see: the tide mark, your pencil mark, and where both valves are.

  3. Flush, watch the refill and compare with the line.

    Illustration of two side-by-side views of a cistern: on the left water sitting at the tide mark with a trickle leaving through the bottom valve, on the right water above the mark running into the overflow pipe
    Illustration: left, flush valve leak; right, fill valve fault

    Bristol Water's rule: if the water sits at the tide mark, the flush valve is probably leaking and needs a new outlet washer. If it sits above the tide mark, the fill (inlet) valve is probably at fault and needs a new diaphragm washer.

    NYC DEP adds a second check. If the water rises above your pencil mark and then drops, suspect the flush valve or flapper. If it stops below the mark and then creeps above it, suspect the fill valve.

    You should see: which part is at fault. Use the table below if you're unsure.

  4. Turn off the water to the toilet and flush to empty the cistern.

    Illustration of a flat-head screwdriver turning the slot of a small isolation valve on the pipe below a cistern so the slot sits across the pipe
    Illustration

    Close the stop tap or isolation valve on the pipe next to the toilet. If there isn't one, use your home's main stop tap (Bristol Water). Then flush to empty the cistern (Bristol Water).

    Not sure where the main stop tap is? See how to turn off your main water valve.

    You should see: an empty cistern that doesn't refill.

  5. If the water was too high, lower the float.

    Illustration of two fill valve types side by side: a screwdriver turning the adjustment screw where a float arm meets the valve, and fingers pinching a clip to slide a cup-shaped float down a rod
    Illustration

    Sometimes the fill valve is fine and just set too high. HomeServe says the water should settle about an inch (2.5 cm) below the overflow.

    Float on an arm: turn the adjustment screw where the arm meets the valve anticlockwise to lower the level. Cup float on a rod, clip type: pinch the clip and slide the float down. Cup float, screw type: turn the screw clockwise to lower it (HomeServe). Turn the water back on, flush and check; adjust again if needed.

    You should see: the water stopping about 2.5 cm below the overflow, and no more running.

  6. Fit the new washer or flapper, following the maker's guide.

    Illustration of hands holding a flush valve lifted out of an empty cistern, a worn rubber ring being swapped for a new one
    Illustration

    Order the part that matches your toilet's make (Bristol Water). Washers and valves differ between makers, so follow the fitting guide from the maker or a trustworthy video tutorial (Bristol Water).

    Lever toilet with a chain and flapper: NYC DEP says to replace the flapper, make sure it suits a water-saving toilet if you have one, and check that the flapper, chain and lever line up with the flush valve. It suggests looking for a flapper with a 5 or 10 year warranty.

    Old ball float on a long arm: if you replace the whole fill valve, NYC DEP suggests a modern piston-type valve rather than another ball-float valve.

    You should see: the new part seated evenly, with nothing twisted or caught.

  7. Turn the water back on and test again.

    Illustration of a cistern refilling and stopping below the overflow, with a fresh dry sheet of toilet paper across the back of the pan
    Illustration

    Open the isolation valve, let the cistern fill and check underneath for drips. Then repeat the toilet paper or food colouring test from step 1. Bristol Water suggests checking for leaks regularly afterwards.

    You should see: the fill stopping by itself, no water reaching the overflow, and dry paper or clear water in the pan.

Which part is failing?

What you see in the cisternLikely partUsual fix
Water at the tide mark, trickling into the panFlush valve (Bristol Water)New outlet washer, or a new flapper on a lever toilet (Bristol Water, NYC DEP)
Water above the tide mark, running into the overflowFill valve (Bristol Water)Lower the float (HomeServe); if that fails, a new diaphragm washer or valve (Bristol Water)
Water rises above your pencil mark, then dropsFlapper or flush valve (NYC DEP)Replace it and check the chain lines up (NYC DEP)
Water stops below your mark, then creeps above itFill valve (NYC DEP)Replace it, or ask a plumber (NYC DEP)

How to tell it worked

  • The cistern refills and then goes quiet, with no hiss or trickle.
  • The water settles below the overflow, about 2.5 cm down (HomeServe).
  • A fresh toilet paper or food colouring test shows no leak (Bristol Water, NYC DEP).
  • No drips under the cistern or around the isolation valve.

If something goes wrong

What happensWhat it meansWhat to do
Still leaking after a new washerWrong part, or not seated evenlyCheck it matches your toilet's make (Bristol Water) and refit it
The cistern fills too slowly after lowering the floatThe isolation valve may be only partly openOpen it fully
The fill valve sticks open now and thenNYC DEP says pressure changes can leave a fill valve stuck openReplace the fill valve, or ask a plumber
The isolation valve won't turn, or dripsA stiff or worn valveDon't force it; turn off the main stop tap and call a plumber
You can't find a matching partOlder or unbranded cisternAsk a plumber; WaterSafe suggests a dual flush model if the cistern needs replacing

Questions readers ask

How much water does a leaking toilet waste?

A lot more than it looks. WaterSafe says a cistern leaking into the pan can waste up to 400 litres a day, about five full baths, and add around 300 pounds a year to a metered bill. NYC's water department says a fill valve stuck open can waste 3 to 5 US gallons a minute.

Can I get free leak test strips?

Some UK water companies offer them. WaterSafe suggests asking your water supplier, and Bristol Water, for example, sends a free leaky loo strip through its water-saving questionnaire.

Is it the flush button or the valve that's broken?

If the button sticks down or doesn't spring back, the button or its cable may be at fault, so check that first. If the button is fine, use the water-line check in this guide: water at the normal line points to the flush valve washer, water above it to the fill valve (Bristol Water).

How do I find a plumber I can trust?

In the UK, WaterSafe runs a register of approved plumbers you can search by postcode. Bristol Water suggests a suitably qualified professional if you aren't confident with DIY repairs.

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