Quick answer: A toilet that keeps running is usually caused by a worn flush seal letting water leak from the cistern into the bowl. In most UK homes, this is either a deteriorated diaphragm washer (siphon toilets) or a perished rubber flapper (modern drop valve toilets). The fix takes 5 minutes and the replacement part costs under £10.
A running toilet wastes up to 400 litres a day — that's roughly £300 a year added to your water bill if you're on a meter. I fix these every week as an apprentice plumber, and 9 out of 10 times it's a £5 part that takes minutes to swap. Here's exactly how to fix it yourself before paying £80-£180 for a plumber to do the same thing.
First: Which Type of Toilet Do You Have?
This matters because UK toilets work differently from the American systems most online guides describe. You need to know which you have before you can fix it.
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Siphon Toilet (Most UK Homes Pre-2001)
Has a push-down or pull-up handle. Requires a firm press to flush. You can't do a "half flush" — it's all or nothing. Inside the cistern, there's a large plastic siphon unit in the centre. The most common failure point is a rubber diaphragm washer inside the siphon.
If your home was built before ~2001, you probably have this
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Drop Valve Toilet (Modern UK Homes)
Has dual flush buttons (small flush / big flush) on top of the cistern. Inside, there's a drop valve with a rubber seal at the bottom. The seal (sometimes called a "flapper" or "washer") perishes over time and lets water seep through constantly.
Dual flush buttons = drop valve system
The 3 Causes (and How to Fix Each One)
Cause 1: Worn Flush Seal or Diaphragm
This is the cause 60-70% of the time. The rubber seal that holds water in the cistern degrades over time. Water slowly leaks past it into the bowl, so the fill valve keeps topping up — creating that constant running sound.
How to tell: Put a few drops of food colouring in the cistern. Wait 15 minutes without flushing. If the colour appears in the bowl, water is leaking past the seal.
How to fix it:
1
Turn off the water supply
Find the isolation valve behind or beside the toilet. Turn it clockwise to shut off. If there's no isolation valve, turn off the mains at your stopcock.
2
Flush to empty the cistern
Hold the handle down to get as much water out as possible. Mop up any remaining water with a sponge or old towel.
3
Replace the seal
Siphon toilet: Disconnect the flush link from the handle. Lift the siphon unit out (or partially out — some have a split design so you don't need to remove the whole thing). The diaphragm washer is a flat rubber disc at the bottom. Pull it off and replace with a new one (£2-£5 from B&Q or Screwfix).
Drop valve toilet: Twist the top of the flush valve anti-clockwise and lift it out. The rubber seal is at the bottom. Pull it off and press the new one on. These cost £4-£8 and are available at any hardware store. Take a photo of your old one to match the size.
4
Reassemble and test
Put everything back, turn the water on, and flush. The running should stop immediately. If it doesn't, move to Cause 2.
Cause 2: Float Set Too High
This is the cause about 20% of the time. The float tells the fill valve when to stop filling. If it's set too high, water rises above the overflow tube and runs into the bowl constantly.
How to tell: Lift the cistern lid and look. Is water flowing over the top of the overflow tube? If yes, the float is too high.
How to fix it:
1
Identify your float type
Ball float: A plastic ball on the end of a metal or plastic arm. Float cup: A small cylinder that slides up and down on the fill valve shaft. Most modern UK toilets use float cups.
2
Adjust it downward
Ball float: Bend the arm gently downward, or turn the adjustment screw at the valve end anti-clockwise. Float cup: Find the metal clip or adjustment screw on the side of the fill valve. Squeeze the clip and slide it down about 1cm, or turn the screw to lower the float position.
3
Flush and check
The water level should now stop about 2-3cm below the top of the overflow tube. If it still overflows, lower the float a bit more.
Cause 3: Faulty Fill Valve
This is the cause about 10% of the time. The fill valve itself is broken and won't shut off regardless of the float position. Water keeps flowing in and overflowing.
How to tell: If you've adjusted the float and the water still won't stop filling, the valve is faulty.
How to fix it: Replace the fill valve. This is slightly more involved but still a DIY job — you need an adjustable spanner and about 20 minutes. Replacement fill valves cost £8-£15 from Screwfix or B&Q. Turn off the water, disconnect the supply hose from the bottom of the old valve, unscrew the locking nut, lift the old valve out, drop the new one in, and reconnect.
💡 Not sure which cause it is? Describe the problem to Reslvr — our AI will diagnose it in seconds and walk you through the exact fix for your specific toilet. Try it free →
Save Money vs Call a Professional
Fix
DIY Cost
Plumber Cost
You Save
Replace flush seal / diaphragm
£2 – £8
£80 – £150
£72 – £148
Adjust float level
£0 (free)
£80 – £120
£80 – £120
Replace fill valve
£8 – £15
£100 – £180
£85 – £165
Plumber costs include a typical UK callout fee of £60-£120 (2026 averages). Parts are the same whether you or the plumber buys them — the difference is entirely labour and callout charges.
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Water bill impact: A running toilet wastes 200-400 litres daily. On a water meter, that's an extra £200-£500 per year. Fixing it today pays for itself within days — even if you buy the parts.
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When to call a plumber: If you've replaced the seal AND the fill valve and the toilet is still running, there may be a crack in the cistern, a problem with the mains water pressure, or the entire siphon/flush valve unit may need replacing. At that point a professional diagnosis is worth the callout fee.
Something Else Broken?
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The three most common causes are a worn flush seal or diaphragm washer (60-70% of cases), a float set too high causing overflow (20%), or a faulty fill valve (10%). In UK homes with siphon toilets (most pre-2001 builds), a worn diaphragm washer is almost always the culprit. The fix takes 5 minutes and costs under £10.
A continuously running toilet wastes 200 to 400 litres per day — that's up to 146,000 litres per year. On a water meter, that adds roughly £200-£500 annually to your bill. Thames Water estimates an average of £300 per year for a typical running toilet. The BBC reported that UK toilets collectively leak 400 million litres daily.
In 2026, a plumber typically charges £80-£180 to fix a running toilet in the UK. This breaks down to a callout fee of £60-£120 plus 30-60 minutes of labour at £40-£60/hour. The parts themselves (seals, washers, valves) cost under £15 — so you're paying overwhelmingly for the callout and labour, not the fix.
Absolutely. This is one of the easiest plumbing fixes there is. You don't need any special tools — just a flat-head screwdriver for some cistern types. Replacement seals and valves are available at B&Q, Screwfix, and Toolstation for £2-£15. The most common fix (replacing the flush seal) takes about 5 minutes once you've identified your toilet type.
Phantom flushing — where the toilet randomly runs for a few seconds then stops — is caused by a slow leak past the flush seal. The cistern gradually loses water until the fill valve kicks in to top it up. The fix is the same: replace the flush seal or flapper. It's the exact same part and process as fixing a continuously running toilet.