It runs all night. You know the sound. And you keep closing the bathroom door and hoping it fixes itself, which it won’t, because that’s not how toilets work.
The good news is that a running toilet is almost always the same problem. The flapper — the rubber seal at the bottom of the tank — has worn out and isn’t seating properly. Water leaks through, the tank never fills completely, and the fill valve keeps running trying to top it up. That’s the sound.
What you’ll need
A replacement flapper kit from any hardware store. Bring your phone into the bathroom, take a photo of the inside of your tank, and show it to someone at the store. Flappers cost between $8 and $15 and come in a few sizes. You want the right one. That’s it. No tools required beyond your hands.
What to do
Turn off the water supply valve behind the toilet — it’s the oval handle coming out of the wall near the floor. Turn it clockwise until it stops. Flush the toilet to empty the tank. Unhook the old flapper from the two pegs on either side of the flush valve and disconnect the chain from the flush handle arm. Hook the new one on, reconnect the chain with a little slack in it, turn the water back on, and let the tank fill. Flush once. Listen.
If it stops running, you’re done. If it still runs, the chain might be too short and catching under the flapper — adjust it. If it still runs after that, the fill valve itself may need replacing, which is a slightly bigger job but still well within reach.
Rather have someone show you in person? Find someone who can come over.