A toilet that runs continuously is the most expensive small fault in a house. A steady trickle can waste hundreds of gallons a day without ever making enough noise to feel urgent. The causes are few, cheap and almost always DIY-fixable — the trick is diagnosing in the right order rather than replacing parts hopefully.
First, Confirm It Is Actually Leaking
If the toilet refills periodically without anyone flushing it — the “phantom flush” — water is escaping from the tank into the bowl. Confirm it with the dye test:
- Take the tank lid off and add a few drops of food colouring to the tank water.
- Do not flush. Wait twenty to thirty minutes.
- Look in the bowl. Colour in the bowl means water is passing from tank to bowl, and the flapper or flush valve seat is your culprit.
If the bowl stays clear but the toilet still runs, the water is going over the overflow tube instead, which points at the fill valve or float.
Check 1: The Flapper
By far the most common cause. The flapper is the rubber seal at the bottom of the tank that lifts when you flush and drops back to seal.
Lift the tank lid and look at it. Rubber perishes: it goes hard, warps, or develops a rough film that stops it seating cleanly. Run a finger around the sealing edge and around the seat it lands on — grit or mineral scale on either will hold it open a fraction.
Clean both surfaces first. If the flapper is stiff, warped or visibly degraded, replace it. They cost very little and take two minutes.
One preventable cause worth naming: in-tank bleach tablets destroy flappers. If you use them, expect to replace the flapper every year or two instead of every five.
Check 2: The Chain
The chain connecting the handle to the flapper needs a small amount of slack — roughly half an inch. Too tight and it holds the flapper fractionally open permanently. Too loose and it can slip under the flapper and prop it open.
Adjust the chain at the handle arm so there is a little slack with the flapper closed, and trim any long tail so it cannot tangle.
Check 3: The Water Level and Overflow Tube
The overflow tube is the open vertical pipe in the middle of the tank. It exists to stop the tank overfilling — excess water goes down it into the bowl.
The water level should sit roughly an inch below the top of that tube. If the level is at or above the top, water is running down it continuously, and the toilet will never stop filling.
Adjust the float down. On a modern column-style fill valve there is usually an adjustment clip or a screw on the top; on an older ball-float arm you bend the arm gently downward or turn the adjustment screw. Lower it until the level sits an inch below the overflow, then flush and watch it refill to check.
Check 4: The Fill Valve
If the flapper seals, the chain is right, the level is correct, and it still runs, the fill valve itself is failing to shut off. Seals inside harden with age and mineral content.
Some fill valves can be rebuilt with a cheap seal kit. Most people replace the whole valve, which is a straightforward job — see how to replace a toilet fill valve for the full sequence.
If You Have an Older Ball-Float System
The diagnosis is the same but the adjustment differs, and a ball float can also fail by filling with water and sinking, which stops it ever signalling “full”. Unscrew the ball and shake it; if there is water inside, replace it. Our guide to fixing a running toilet without a ball float covers the modern equivalents.
What It Is Probably Not
A running toilet is almost never a problem with the bowl, the trapway or the drain. If your complaint is a weak or slow flush rather than continuous running, that is a different fault entirely — start with how to fix a slow flushing toilet.
And if there is water on the floor rather than inside the tank, see toilet leaking at the base instead.
Worth Fixing Today
All of the above are inexpensive parts and short jobs, and the water they waste is continuous. Our guide to toilet hardware repair covers the same components in more depth, and how long a toilet lasts explains the replacement intervals worth planning for.
