Why Does My Toilet Keep Running? Check These in Order
A toilet that runs and runs is almost always one of three small problems inside the tank. The good news: you can find which one it is before you buy a single part. Work through the checks on this page in order — cheapest and easiest first.
Everything here is inside-the-tank work. You take the lid off, you look, you touch. If anything goes beyond that, the red box near the bottom tells you when to stop and call a plumber.
Start here: what do you notice?
Pick the one that sounds most like your toilet. You will get the likely cause, the first check to run, and the fix if that check proves it.
Tap one of the three above to see what is probably wrong and what to check first.
A — It refills by itself: a slow leak past the flapper
What is probably happening
The flapper is the rubber plug at the bottom of the tank. It is supposed to seal tight after a flush. If it leaks even a little, tank water slowly slips into the bowl. The tank level drops, the fill valve wakes up, and the tank refills — over and over, all day and night. That is the “phantom flush.”
First check
Run the dye test (Check 2). It takes about 10 minutes and proves whether water is leaking past the flapper — no guessing, no parts bought yet.
The fix, if the test shows color in the bowl
First, check the chain (Check 1) — a tangled chain can hold a good flapper open. If the chain is fine, feel the flapper (Check 3). If it is warped, stiff, slimy, or pitted, replace it. A flapper is a cheap part; take the old one to the store so you get the right match.
B — It never stops after a flush: flapper not seating, or fill valve not shutting
What is probably happening
After a flush, two things have to happen: the flapper drops flat and seals, and the fill valve shuts off when the tank is full. A hiss or trickle that never ends means one of those two is not happening — the flapper is being held open or is not landing flat, or the valve keeps letting water in.
First check
Take the lid off and watch one full flush (Check 1). Watch the flapper: does it drop flat and cover the hole? Is the chain tangled, caught under the flapper, or pulled so tight it holds the flapper up?
The fix, if the check shows it
Chain too tight, loose, or tangled? Adjust it so it has about ½ inch of slack with the flapper closed — that fix costs nothing. Flapper lands crooked or feels worn? Clean it, and replace it if it is warped or stiff (Check 3). If the flapper seals fine but the water still runs, go to the fill valve (Check 5).
C — Water spills into the overflow tube: the water level is set too high
What is probably happening
The overflow tube is the tall open tube in the tank. It is the safety drain: if the tank fills too high, extra water runs down it into the bowl. If you can see water trickling into that tube, the float is set too high — or the fill valve is not shutting off when it should.
First check
Check the water level (Check 4). After the tank fills and stops, the water should sit about 1 inch below the top of the overflow tube. If it reaches the top and spills over, the level is the problem.
The fix, if the level is too high
Lower the float — it costs nothing and takes a minute. Cup floats have an adjustment screw or a squeeze clip on the side of the fill valve; ball floats have a screw at the top of the fill valve. Lower it a little, flush, and watch where the water stops. If the water rises past the right level and keeps going, the fill valve itself is failing — see Check 5.
The parts you will see with the lid off
The five checks, in order
Do them in this order. Each one is free or nearly free, and each one rules something out before you spend money on the next.
Watch one flush with the lid off
Lift the tank lid and set it down somewhere safe — it is heavy and it can crack. Then flush and watch.
- Does the flapper lift, then drop back down flat over the hole?
- Look at the chain. With the flapper closed, the chain should have about ½ inch of slack — a little looseness, not pulled tight (that figure comes from Fluidmaster’s 400A instructions).
- Is the chain tangled, twisted, or caught under the edge of the flapper?
If the chain is the problem: a chain pulled too tight holds the flapper open so water runs straight through. A chain that is too loose or tangled can catch under the flapper and break the seal. Unhook it, untangle it, and re-hook it so there is about ½ inch of slack. Flush again and watch it seat. Cost: nothing.
The dye test — is water leaking past the flapper?
This is the leak-check method from the EPA’s WaterSense program. It proves a silent leak you cannot hear.
- Put a few drops of food coloring in the tank water. Dark color works best.
- Wait 10 minutes. Do not flush.
- Look in the bowl. Then flush right away, either way — the EPA warns the coloring can stain the tank if it sits.
If color shows up in the bowl: tank water is leaking past the flapper. That is your leak — go to Check 3. If the bowl water stays clear, the flapper is sealing and your problem is more likely the water level or the fill valve (Checks 4 and 5).
Feel the flapper
- Turn the water off at the toilet’s shutoff valve — the tap on the wall or floor behind the toilet. Turn it gently; if it will not turn, stop and read the red box below.
- Flush to empty the tank. Hold the handle down to drain as much as you can.
- Unclip the flapper from its pegs and lift it out.
- Run your fingers over it. Feel the edge that seals against the hole.
Replace the flapper if it feels warped, stiff, slimy, or pitted, or if the sealing edge is rough or crumbly. Flappers wear out slowly in treated water — this is the most common cause of a running toilet, and the part is cheap. Take the old flapper to the store with you so you can match the size and style exactly, as Korky’s flapper guidance advises. While the tank is empty, also wipe the rim of the hole the flapper sits on — grit or slime there can break the seal too.
Check the water level
Turn the water back on and let the tank fill. When the fill valve shuts off, look at the water line. The water should stop about 1 inch below the top of the overflow tube. Many tanks also have a “water line” mark stamped inside the back wall — the water should stop at that mark.
If the water sits too high (or spills into the overflow tube): lower the float. On a cup float, turn the adjustment screw on the fill valve, or squeeze the spring clip and slide the cup down a little. On a ball float (a ball on a metal arm), turn the screw at the top of the fill valve where the arm attaches. Lower it a bit, flush, and check the level again. Repeat until the water stops about an inch below the top of the tube.
The fill valve — the last suspect
If the chain is right, the flapper seals, and the water level is set right — but the valve still never shuts off, or the toilet still runs after Checks 1–4 — the fill valve itself is the likely part. Fill valves wear out, and grit from the water line can stop them closing.
Replacing a fill valve is a common homeowner job — but only if your shutoff valve works (turns easily and fully stops the water; see the red box below). The job is: water off, tank emptied, old valve unbolted from under the tank, new valve in its place. Follow the instructions in the new valve’s box step by step — they are written for your exact valve, and brand instructions (Fluidmaster’s are a good example) walk through it one step at a time. If anything under the tank looks corroded, cracked, or stuck, that is plumber territory, not a Saturday project.
Why it is worth fixing
A running toilet is not just a noise. It is clean, treated, paid-for water going straight down the drain, all day and all night.
A silent flapper leak can waste up to 30 gallons a day.
And a fill valve that never shuts off wastes far more than that. One cheap flapper — or one float adjustment that costs nothing — can stop most of it.
Call a plumber instead if…
- The shutoff valve behind the toilet won’t turn, or it leaks when you turn it. Forcing an old valve can break it — and then the water cannot be stopped and the room floods.
- Water is leaking from the supply line, or from under the tank or the base of the toilet.
- The tank or the bowl is cracked.
- You have done the checks above and it still runs.
Never ignore a leak you can see on the floor — water damage costs more than the plumber.
Sources: Fluidmaster 400A fill valve installation instructions (chain slack and water-level settings); Korky flapper guidance (flapper wear and matching a replacement); EPA Fix a Leak Week and WaterSense (the food-coloring leak-check method).
EPA-attributed content checked against the EPA on November 3, 2026.