Why Is My Toilet Running? Diagnosis in 15 Minutes
A toilet that runs constantly — trickling into the pan, or refilling itself in the small hours — is wasting water around the clock, and on a meter it's doing so at your expense twice over. The good news: this fault has only a handful of causes, one free diagnostic test, and cheap universal parts at the end of it. Work through this in order.
The dye test: two minutes, definitive
Drop food colouring (or a dye tablet) into the cistern. Don't flush. Wait fifteen minutes. Colour appearing in the pan means water is passing the flush valve or siphon seal — the cistern-to-pan route — and that seal is your fault. No colour, but the cistern keeps topping up or the overflow runs? The fill valve is overfilling and your fault is on the inlet side. This single test correctly diagnoses the overwhelming majority of running toilets before anyone touches a tool.
Fault one: the flush valve or siphon seal
The seal between cistern and pan degrades — rubber hardens, scale scores the seating, debris lodges under it — and water trickles through continuously. Symptoms: dye test positive, faint ripple in the pan, hissing as the fill valve compensates. The fix is a new flush valve seal or siphon diaphragm: universal parts, modest cost. In hard-water areas this seal fails on a cycle, and a slightly better-quality valve is worth the extra pound.
Fault two: the fill valve
If the fill valve doesn't shut off cleanly, the cistern overfills until water escapes down the overflow (internal overflow tubes route this into the pan, mimicking a seal leak — the dye test still separates them, since overflow water enters visibly from the top). Causes: worn diaphragm washer, scale in the valve seat, or a float set too high. Adjusting the float is free; a replacement fill valve is a stocked universal part.
Ghost flushing and the night refill
A toilet that audibly refills itself every few hours with nobody near it isn't haunted — it's a slow flush-seal leak dropping the cistern level until the fill valve tops it up. Same diagnosis, same fix, but worth acting on: that cycle is 24/7 and the leak only widens. If a new seal doesn't cure it, the valve seating itself is scored or scaled and the full valve wants replacing.
| Symptom | Likely fault | Fix |
|---|---|---|
| Dye appears in pan (no flush) | Flush valve / siphon seal | Replace seal or valve |
| Overflow running / cistern overfull | Fill valve or float height | Adjust float or replace valve |
| Refills itself at night | Slow flush-seal leak | Replace seal; check seating |
| Hissing that never stops | Fill valve compensating for a leak | Dye test, then as above |
The cost of leaving it
Plumbing faults rarely improve with seasoning: drips scale up their seats and become replacements, small leaks soak fabric silently and become ceilings, and marginal components fail completely at the least convenient hour — usually a cold one. The economical moment to fix a diagnosed fault is while it's still minor, on a standard-rate weekday visit rather than the emergency call it's rehearsing to become.
Frequently asked questions
How much water does a running toilet waste?
A steady flush-seal leak commonly passes 200–400 litres a day — on a metered supply that's paid for as both water and sewerage. It's among the most expensive faults to ignore relative to its repair cost.
Can I fix a running toilet myself?
Adjusting a float and swapping an accessible fill valve are reasonable DIY with the water isolated. Concealed cisterns, siphon replacements (which need the cistern off on close-coupled suites) and anything seized are where the professional hour earns itself.
Why does it run only sometimes?
Intermittent running is usually debris or scale under the flush seal — sealing sometimes, passing sometimes — or a fill valve on the edge of its travel. Both are the same repairs, just earlier in their failure.
The toilet runs after every flush for too long — same problem?
Long refills are usually a partially blocked or failing fill valve rather than a leak — it can't pass water fast enough. Same universal replacement part, different symptom.
Where this guide's advice ends
Written guidance has a natural limit: it can tell you what usually causes a symptom, but only eyes on your actual pipework can confirm it. If what you're seeing doesn't quite match any pattern described here, or matches several at once, that ambiguity is itself diagnostic information worth giving an engineer. Our vetted local plumbers do this triage daily — and an honest one will tell you on the phone if your problem sounds like a cheap fix, a bigger job, or (occasionally) not a plumbing problem at all.