If your toilet keeps running, one of two parts has almost always failed. The flapper at the bottom of the tank is no longer sealing, so water leaks into the bowl and the fill valve keeps topping the tank back up. Or the fill valve itself is not shutting off, so water runs into the overflow tube continuously. A two-minute test tells you which: mark the water level, shut the supply off, and wait. If the level drops, it is the flapper.
Nothing in that test requires a tool, and it decides which part you buy. Doing it first is the difference between one trip to the store and three.
The two-minute test that names the failed part
The manufacturer of the most common fill valves in North America publishes this method, and it is better than guessing. Fluidmaster's guidance on identifying flapper leaks, read on 2026-09-08, says to start with a full tank, turn the water off at the shutoff valve, and not to flush. Make a pencil mark in the tank at the current water level, then wait and check whether the level has moved, repeating until the level no longer changes.
Read the result like this:
- The water level drops with the supply off. Water is escaping from the tank into the bowl. That is the flapper or the flush valve seat it sits on.
- The level holds, but with the supply on the toilet keeps running or cycles on by itself. The fill valve is not shutting off, or the water level is set so high that it is spilling into the overflow tube.
- You can hear a faint hiss and see a thin ripple on the surface of the bowl water. Classic flapper seep. The ripple is water arriving continuously.
A dye test does the same job: a few drops of food coloring in the tank, no flushing, and color appearing in the bowl within some minutes means the flapper is passing water.
When the toilet keeps running because of the flapper
The flapper is a rubber or silicone disc on a hinge. It has one job, sealing an opening under water, and it wears out.
Age. Fluidmaster's own advice is direct: if the flapper is more than 5 years old, simply change it, because the time saved will outweigh the cost of fussing with an aged flapper. A flapper that has gone stiff, chalky or slightly warped will not seal no matter how it is adjusted.
Chain length. Fluidmaster specifies a quarter inch to half an inch of slack in the chain, and warns that too little slack may lift the flapper from the seat. Too much slack and the chain can fall under the flapper and hold it open. This one costs nothing to check and fixes a surprising number of running toilets.
Chlorine tablets in the tank. This is the cause people do not expect. Fluidmaster explicitly warns that in-tank chlorine products cause flapper warping and component damage, and states that using them will void the warranty. If there is a blue or white tablet in your tank and your flapper keeps failing, those two facts are related.
The seat, not the flapper. If a new flapper still leaks, run a finger around the rim of the opening it sits on. Mineral deposits, burrs or a rough patch on that seat will hold a perfectly good flapper a fraction off true.
When it is the fill valve instead
The fill valve is the tall assembly on one side of the tank that refills it after a flush.
It never shuts off. Debris or sediment in the valve cap is a common cause, especially after work on the water main, which pushes grit through the system. Most fill valves can be opened at the cap and flushed clean following the manufacturer's instructions for that model.
It shuts off, but only after cycling on and off. Look for the level. If the water is set high enough to reach the top of the overflow tube, water spills continuously down the tube and the valve keeps chasing it. The fix is to lower the set level to the mark on the tube or the tank, which on modern valves is a clip or an adjustment screw.
It runs for a long time and refills slowly. Often a supply or pressure issue rather than the valve.
The refill tube is in the wrong place. The small flexible tube from the fill valve should clip to the top of the overflow tube and discharge into it, not down inside it. A tube pushed down into the overflow can siphon and keep the toilet cycling.
When it is neither, and what that costs
A small number of running toilets are neither part.
A cracked or slipping float arm on older ballcock designs.
A flush valve that is worn where it meets the tank, which is a bigger job because it usually means removing the tank.
Phantom flushing, where the toilet refills itself for a few seconds every so often with nobody near it. That is a very slow flapper seep, and the pencil test catches it if you leave it long enough.
A toilet that keeps running after new parts. At that point you are looking at the seat, the flush valve or the tank itself, and it is worth a professional opinion before buying a third kit.
The reason not to leave it is measurable. The EPA's Fix a Leak Week page, updated March 2026 and read on 2026-09-08, states that the average household's leaks can account for more than 9,300 gallons of water wasted every year, and that household leaks nationwide waste nearly 1 trillion gallons annually. A toilet is usually the largest single contributor in a house because the loss is continuous and silent.
Hard water is why the same part fails twice
If you replace a flapper and it fails again within a year or two, look at your water rather than at the part.
Mineral scale builds on the flush valve seat and inside the fill valve, and both of them depend on a clean surface to seal against. That is the same process that furs up a kettle and leaves film on glassware. Getting a real answer about your supply, from your utility's water quality report rather than from a map, is covered in the signs of hard water in the house. If your kettle scales up within weeks of being cleaned, you already have your answer, and how often to descale a kettle explains why nobody publishes a single universal interval for that either.
The practical consequence is that in a hard-water house, flappers and fill valves are consumables on a shorter cycle, and the seat is worth cleaning gently whenever the flapper is changed.
Before you buy parts
Close the supply stop behind the toilet and flush to empty the tank. If that stop will not close, or will not turn, stop and book a licensed plumber. Working on a tank you cannot isolate is how a small job becomes a wet floor.
Find the brand and, if you can, the model. It is often stamped inside the tank lid or on the porcelain behind the seat. Flappers are not universal, and the two common sizes are two inch and three inch, which you can tell by looking at the opening in the bottom of the tank.
Buy the matching part, not the closest one. A universal flapper that does not match the seat profile is the most common reason a replacement still leaks.
Do not overtighten anything porcelain. Tank bolts and the supply connection are the two places where hand-tight plus a small amount is the whole instruction.
When a repair stops being worth it
There is a point at which a very old toilet stops being a candidate for another kit: cracked internal parts, a corroded flush valve, a rocking base, or repeated failures that outrun the cost of the fixture. The reasoning is the same one we apply to old machines elsewhere in the house, weighing the cost of the next repair against remaining life and what a replacement would actually solve, and we set that test out in repair or replace an aging appliance.
Replacing a toilet is a plumbing job with a wax seal and a floor flange involved, and if the flange or the floor around it is in poor condition, it is a licensed plumber's job rather than a weekend one.
Where this stops being yours
- A supply stop that will not close or will not turn. Licensed plumber, before any tank work.
- Water on the floor around the base. That can be a failed wax seal, a cracked bowl, or a leak at the tank-to-bowl connection. It is not a running-toilet problem, and a wet subfloor gets worse quietly.
- A toilet that rocks. The seal underneath is being worked loose every time somebody sits down.
- A crack in the porcelain of the tank or bowl. Replacement, not repair.
- Sewage backing up, gurgling in other fixtures, or several slow drains at once. That is a drain or main line problem, and it is a plumber's call rather than a tank part.
- A toilet that still runs after correct new parts are fitted properly. Get an opinion before buying a third kit.
FAQ
Why does my toilet keep running after I replaced the flapper? Usually because the seat the flapper sits on is scaled, pitted or rough, or because the replacement flapper does not match the seat profile. Check the rim of the opening with a finger, and confirm you have the right size, two inch or three inch.
How do I know if it is the flapper or the fill valve? Fill the tank, turn off the supply, do not flush, and mark the water level in the tank. If the level drops, the flapper is leaking into the bowl. If it holds and the toilet still cycles with the water on, the fill valve or the water level setting is the problem.
How often should a toilet flapper be replaced? Fluidmaster's guidance is that if a flapper is more than five years old, replace it rather than trying to adjust it. In hard-water households it is often sooner.
Do those blue tablets in the tank damage the toilet? Fluidmaster warns that in-tank chlorine products cause flapper warping and component damage and that using them voids the warranty. If you are losing flappers quickly and there is a tablet in the tank, remove it.
How much water does a running toilet waste? More than any other common household leak, because it runs continuously. The EPA puts the average household's leaks at more than 9,300 gallons a year, and a running toilet is frequently most of that.
Can I just turn the water off to the toilet until I fix it? Yes, close the stop behind the toilet and flush to empty the tank. If that stop will not close, that is the first thing to have a licensed plumber deal with, before the tank parts.
