A running toilet wastes a surprising amount of water and usually comes down to one of three parts failing. Here's how to diagnose and fix it yourself.
Lift the tank lid and look at the flapper, chain, and fill valve. Watch where water is moving — that points directly to the failed component.
If it looks warped, cracked, or stiff, replace it. A worn flapper allows water to slowly leak into the bowl, forcing the toilet to refill constantly.
If the chain is too tight it holds the flapper open. If too loose it tangles under the flapper. Adjust so there's a small amount of slack when the flapper is closed.
If water continues rising and spilling into the overflow tube, the fill valve is failing and needs replacement.
Shut off the supply valve clockwise. Flush to empty the tank. Replace the flapper or fill valve per kit instructions. Turn water back on and confirm the tank fills and stops correctly.
Replace flappers every 2-3 years. Avoid drop-in tank tablets — they damage rubber seals faster.