Why is my toilet running?
Updated August 2026
Start at the flapper: a warped rubber flapper that no longer seals against the flush valve causes most running toilets, and the part costs $5–25. Water slips past it into the bowl, the tank level drops, and the fill valve kicks on to top it up again, over and over. A toilet leaking that way can waste 200 gallons a day and add $60–200 or more to a monthly water bill. Confirm it with a dye tablet before you buy anything, then work down the causes below.
Safety first: a running toilet is a bill problem rather than a hazard, right up until the overflow tube is what fails. If water is rising past the top of that tube toward the tank rim, close the supply stop behind the toilet before you troubleshoot anything else. Do not wedge the flapper shut or tie the float down to quiet the noise, because that disables the overflow protection that keeps a stuck fill valve from putting water on your floor.
Causes, most likely first
| Cause | How likely | The fix | Typical cost |
|---|---|---|---|
| Worn or warped flapper | Most common | Shut the supply, drain the tank, and snap on a matching flapper. Take the old one to the store, since they are not universal. | $5–25 part |
| Fill valve set too high or failing | Common | Water pours nonstop into the overflow tube. Lower the float so the level sits 1 inch below the tube, or replace the valve. | $15–40 part |
| Chain snagged or wrong length | Common | A chain caught under the flapper, or one too short, props it open. Reset it with about half an inch of slack. | Free |
| Scaled or nicked flush valve seat | Occasional | Scale or a burr cuts a channel under even a brand-new flapper. Rub the seat smooth, or replace the flush valve assembly. | $0–35 |
| Cracked overflow tube or tank-to-bowl gasket | Rare | A split tube or a failing gasket bleeds water below the rim. Replace the flush valve assembly or the gasket set. | $15–60 |
Check these first
- Drop a dye tablet or ten drops of food coloring in the tank, do not flush, and check the bowl 15 minutes later. Color in the bowl means the flapper leaks.
- Look at the water level. Sitting at or above the top of the overflow tube points at the fill valve or float, not the flapper.
- Lift the flapper and run a finger around its rim and the seat below. Grit, scale, or a hardened, cupped edge all break the seal.
- Check the chain for about half an inch of slack, nothing pinched under the flapper, and no spare links resting on the seat.
- Read the water meter with every fixture off, wait an hour, and read it again. Any movement puts a gallons-per-hour number on the leak.
Prevent it next time
Fifteen minutes twice a year catches this before the bill does: run the dye test on every toilet on a set schedule and you find the leak while it is still a $5 part. Plan on a fresh flapper every three to five years, sooner if you use in-tank bleach tablets, which eat flapper rubber and fill valve seals faster than anything else you can drop in a tank. The rest of the routine is on the toilet maintenance checklist.
When to call a pro
Flappers, chains, and fill valves are homeowner parts, and a full rebuild kit with a new flush valve runs $20–40. Call a plumber when the supply stop is seized or weeping, when the tank-to-bowl bolts still leak after a retighten, or when a complete rebuild does not stop the running, which usually means a hairline crack in the tank. A plumber charges $100–250 to rebuild the tank guts and $250–600 to set a new toilet.
Most of the causes above are maintenance problems wearing a disguise. OnOtto keeps the boring prevention on schedule so the 2 a.m. version doesn't happen.
Stop chasing symptoms, free for 30 daysFrequently asked questions
How much water does a running toilet waste?
A steady leak past the flapper commonly runs about 200 gallons a day, and a wide-open fill valve passes far more. At typical 2026 water and sewer rates that is $60–200 or more on a monthly bill, enough to double it in a small household. The leak is usually silent, so the bill is how most people find out. Read your meter with every fixture off, wait an hour, and read it again; any movement is water you are paying for.
Why does my toilet run for 20 seconds every few minutes?
That is a phantom flush, and it is the signature of a slow flapper leak. Water seeps into the bowl until the tank drops far enough to trip the fill valve, which runs 10 to 30 seconds and shuts off again. The cycle repeats every few minutes to a few hours depending on how bad the seal is. A dye tablet confirms it and a $5–25 flapper ends it. A toilet does not refill on its own for a harmless reason.
Can I replace a toilet flapper myself?
Yes, and it is one of the friendliest repairs in the house. Close the stop valve behind the toilet, flush to empty the tank, unclip the old flapper, and snap the new one on: about ten minutes, no tools. A fill valve takes 20 to 30 minutes and one adjustable wrench. Bring the old part or your model number to the store, because flappers are not universal and a near-match leaks.
My toilet still runs after a new flapper. What now?
Three usual suspects. The flapper does not match your flush valve, common on 3-inch valves and older brands. The seat is scaled or nicked, so nothing seals against it; run a scouring pad around it and feel for a burr. Or the water level sits above the overflow tube, which quietly drains the tank through the tube while the flapper is fine. Set the level 1 inch below the top of the tube and watch again.