Why does my toilet keep running every few minutes when nobody flushed it?

Concerns & risks

You were lying in bed, the house totally quiet, when the toilet suddenly hissed and refilled all by itself. Nobody flushed it, and it keeps happening every few minutes like a ghost is using your bathroom. It's annoying at night, and it quietly wastes water around the clock.

It's annoying, it wakes you up, and it quietly burns water money every single day.

Causes & evidence

Water is sneaking past the flapper, so the tank slowly drains and the fill valve tops it up again. Full anatomy here: how a toilet tank works.

What to do

1. Drop 5 drops of food coloring in the tank. Don't flush. Wait 10 minutes.
2. Color in the bowl? The flapper leaks. Turn off the valve behind the toilet and flush to empty the tank.
3. Unhook the old flapper, take it to the store, buy the matching one (most are 2-inch or 3-inch).
4. Clip it on, leave about half an inch of slack in the chain, turn the water back on. Silence!

Common mistakes: The big one is swapping parts before testing. Do the dye test first, so you know whether the flapper or the fill valve is the real problem. Another is buying the wrong flapper size: take the old one to the store or check the size printed on it. Also don't over-tighten the chain. Leave a little slack so the flapper can seat fully. And don't use in-tank bleach tablets if your toilet maker warns against them, because they can wear out rubber parts faster.

Speed Man tip: If the new flapper doesn't stop it, scrub the valve seat with a scouring pad. Mineral crust breaks the seal too.

Wrap-up: A toilet that refills by itself almost always has a leaky flapper. Do the food-coloring test: color in the bowl in 10 minutes means replace the flapper. Bring the old flapper to the store so the new one fits.

Want the why behind it? Read Household Water Leaks and the Water Meter Test in the wiki.