Short answerA toilet keeps running because water is leaving the tank when it should not: past a worn flapper, down the overflow tube because the fill valve is set too high, or because the flapper chain is too short. Lifting the tank lid shows which one in under two minutes.
Two-minute diagnosis
| What you see | Cause | Fix |
|---|---|---|
| Water spilling into the overflow tube | Fill valve set too high or failing | Lower the float; replace fill valve if it won't shut off |
| Water level below overflow, tank refills periodically | Flapper leaking | Replace flapper |
| Flapper not sealing, chain tight | Chain too short | Adjust chain slack |
| Hissing with valve shut off slowly | Worn fill valve | Replace fill valve |
Dye test
- Add a few drops of food coloring to the tank.
- Wait 15 to 30 minutes without flushing.
- Color in the bowl means the flapper is leaking.
When it's not a simple part
- Cracked flush valve seat
- Proprietary flush systems
- Corroded tank bolts that leak
How to replace a flapper
- Close the shutoff valve behind the toilet and flush to empty the tank.
- Unhook the chain and slide the old flapper off the overflow tube pegs.
- Match the new flapper to the flush valve size, usually 2 or 3 inches.
- Install it, leave about half an inch of slack in the chain, then turn water on and test.
Frequently asked questions
Is a running toilet expensive?
It can be. A continuously running toilet wastes a significant amount of water and is a leading cause of unexpectedly high water bills.
How long do flappers last?
Often several years, less with in-tank cleaning tablets or chlorinated water that degrades rubber.
