Fix a Toilet That Won't Stop Running
A toilet that runs on its own is almost always a rotten flapper or a tired fill valve — two cheap parts inside the tank. Start with the 15-minute food-coloring test so you replace the right one.
What is actually broken
A running toilet is almost always one of two cheap rubber parts inside the tank: the flapper, which rots and stops sealing the flush hole, or the fill valve, which no longer shuts off at the right water level. Water silently leaks into the bowl 24/7 — a bad flapper can waste 200 gallons a day.
Safety first
- Tank water is clean supply water — no gloves needed, but wash your hands after; old flappers leave black residue.
- Don't overtighten anything on the tank. Porcelain cracks, and a cracked tank is a new toilet.
The fix, step by step
1 Do the food-coloring test
Take the tank lid off. Put a few drops of food coloring in the tank, don't flush, and wait 15 minutes. If color appears in the bowl, the flapper is leaking — that's your fix. If the toilet instead cycles on and off or water runs over the overflow tube, it's the fill valve or its float setting.
Tip: This 15-minute test prevents replacing the wrong part, which is the most common DIY toilet mistake.
2 Shut off the water and flush dry
Turn the oval valve behind the toilet clockwise. Flush and hold the handle down to drain the tank, then sponge out the last inch of water into the bowl or a bucket.
Tip: Note the water level before you drain — it should sit about an inch below the top of the overflow tube. You'll reset to that line later.
3 Swap the flapper
Unhook the old flapper's chain, unclip its ears from the overflow tube pegs, and snap the new one on the same way. Reattach the chain with about half an inch of slack — a taut chain holds the flapper open, and a very loose one gets sucked under it.
Tip: Take the old flapper to the store. Universal flappers fit most toilets, but the old one confirms the size (2" vs 3").
4 Or replace the fill valve
If the fill valve is the problem: put a cup under the tank, unscrew the supply line from the valve's tail, unscrew the plastic nut under the tank, and lift the old valve out. Set the new valve's height so its critical-level mark sits at least an inch above the overflow tube, install, and reconnect hand-tight plus a quarter turn.
Tip: A complete fill valve is about $12 and takes 15 minutes — often smarter than nursing a 15-year-old valve.
5 Refill, set the level, test
Turn the water back on and let the tank fill. The water should stop about an inch below the overflow tube top — adjust the float screw or clip if needed. Flush three times, watch the flapper drop cleanly each time, and confirm the refilling stops completely and silently within a minute.
Tip: Repeat the food-coloring test after the repair. No color in the bowl after 15 minutes = sealed, done.
Stop and call a pro if…
Call a plumber if water is leaking from the base of the toilet onto the floor (that's a wax-ring/sewer issue, not a tank issue), or if the shut-off valve behind the toilet won't turn or leaks.
Your work, your responsibility
If you do this repair yourself: you attempt it entirely at your own risk. This guide is general information, not professional advice, and it can be wrong or incomplete. ClickTheFix is not responsible for injury to you or anyone else, or for damage to your property, that results from following — or misfollowing — this guide. Follow the safety warnings, use proper protective gear, follow the manufacturer's instructions and your local codes, and stop if any step is beyond your skill or comfort.
If a contractor does the work: any contractor you hire is an independent professional working under an agreement between you and them alone. ClickTheFix does not provide repair services, does not supervise or control their work, and does not guarantee or warranty it. ClickTheFix is not liable for a contractor's errors, omissions, poor workmanship, property damage, or injuries.