My toilet started running at 11 p.m. on a Saturday. Not a leak — just that constant hissing sound of water cycling through the tank every five minutes. By Sunday morning I was ready to call someone. The emergency plumbing rate in my area is \$150 minimum. For a toilet. On a Sunday.

I opened the tank, stared at the mechanism for maybe three minutes, watched a YouTube video, and fixed it with a \$5 part from the hardware store the next morning. Here is exactly what I learned.

Diagnose First — It Is Usually the Flapper

Take the lid off the tank and watch what happens. If water keeps trickling from the tank into the bowl, the flapper — that rubber flap at the bottom — is not sealing. This is the most common cause and the easiest fix.

The flapper gets old and warped. Over time it stops making a tight seal against the flush valve opening. Water slowly leaks out, the water level drops, the fill valve kicks on, and the cycle repeats forever. Your toilet “runs” because it is constantly trying to refill.

Replace the Flapper (10 Minutes)

1. Turn off the water supply valve behind the toilet — twist clockwise.
2. Flush to empty the tank.
3. Unhook the old flapper from the flush lever chain and the pegs at the bottom of the overflow tube.
4. Take the old flapper to the hardware store to match the size. There are 2-inch and 3-inch flappers — you need the right one.
5. Hook the new flapper onto the pegs, attach the chain to the flush lever with about half an inch of slack — too tight and it will not seal, too loose and the flush will be weak.
6. Turn the water back on and test.

If It Is Not the Flapper

Check the fill valve. If water is running out of the top of the fill valve into the overflow tube and never shutting off, the fill valve is stuck or failed. A new fill valve costs about \$15 and replaces in 20 minutes — unscrew the old one, screw in the new one, attach the refill tube.

Also check the float. If the float is set too high, water overflows into the overflow tube before the fill valve shuts off. Adjust the float screw or bend the float arm down slightly to lower the water cutoff level.

The Cost Comparison

Plumber visit: \$150 minimum. Flapper: \$5. Fill valve: \$15. The knowledge that you fixed it yourself: free and surprisingly satisfying. I have since fixed two more toilets at friends’ houses. Same flapper issue. Five minutes each.