How to Fix a Running Toilet Flapper: Replacement and Adjustment in 10 Minutes

A running toilet wastes about 200 gallons of water per day — roughly $1 per day in water costs, or $365 per year if left unfixed. The fix takes under 10 minutes and costs $5 to $15 in parts.

A running toilet wastes about 200 gallons of water per day — roughly $1 per day in water costs, or $365 per year if left unfixed. The fix takes under 10 minutes and costs $5 to $15 in parts. The flapper — the rubber seal at the bottom of the tank — is the culprit in about 80% of running toilet cases.

Here is how to confirm it and fix it.

The Dye Test: Confirm the Flapper Is the Problem

Before buying anything, spend two minutes confirming the flapper is actually leaking.

1. Remove the tank lid and set it aside.
2. Drop 5 to 10 drops of food coloring into the tank water. Any color works — red or blue shows up most clearly.
3. Wait 15 minutes. Do not flush.
4. Look at the toilet bowl water. If color has appeared in the bowl, the flapper is leaking.
5. If the bowl water is clear after 15 minutes, the flapper is fine. The problem is likely the fill valve or an overflow tube that is set too low — see the section at the end of this guide.

Flush after the test to clear the coloring.

Chain Length Adjustment: Try This First

Before replacing the flapper, check the chain. A misadjusted chain causes many running toilet problems and costs nothing to fix.

1. Remove the tank lid and look at the chain running from the flush handle arm down to the flapper.
2. Flush the toilet and watch the flapper: it should lift fully off the seat and then drop back down and seal completely before the tank refills.

If the chain is too short: The flapper is being held slightly open even at rest, letting water trickle through. Unhook the chain from the handle arm and move it to a link that gives about 1/2 inch of slack.

If the chain is too long: It can get pinched under the flapper when it closes, breaking the seal. Shorten it by clipping it to a higher link on the handle arm, or tuck any excess loops under the clip so they cannot reach the flapper seat.

Test by flushing several times. If the toilet stops running, you are done.

Choosing the Right Flapper

If the chain is fine but the flapper still leaks, you need a replacement. There are two approaches:

Universal flappers — A Korky universal toilet flapper fits most standard two-inch drain openings. These work well on American Standard, Eljer, and many other common brands. They are the fastest option when you do not know your toilet model.

OEM brand-specific flappers — Kohler, Toto, and some American Standard models use seats and flapper designs that do not seal reliably with universal replacements. If you have tried a universal flapper and the toilet still runs, you need the brand-specific part. Check the model number printed inside the tank lid or on the back of the toilet and look up the correct Kohler OEM flapper or equivalent part for your brand.

You can also find a toilet repair kit that includes both a flapper and fill valve if you want to replace both at once.

Step-by-Step Flapper Replacement

What you need:

• Replacement flapper
• Replacement chain if the existing one is corroded or the wrong length

Steps:

1. Turn off the water supply. Reach behind the toilet and turn the supply valve clockwise until it stops. This is a quarter-turn ball valve or a multi-turn compression valve. If the valve will not turn (common on older toilets), you can work without it — just move fast.

2. Flush the toilet. Hold the handle down to empty as much water from the tank as possible.

3. Remove the old flapper. Most flappers have two ears — one on each side — that hook over pegs on the overflow tube (the tall plastic tube in the center of the tank). Pinch and slide each ear off its peg. Then unhook the chain from the handle arm.

4. Check the flapper seat. Run your finger around the rim of the flush valve seat — the ring the flapper seals against. If you feel mineral deposits or rough spots, clean the seat with a damp cloth or fine steel wool. A pitted or damaged seat means the new flapper will leak too. In that case, you may need a seat replacement kit or a new flush valve.

5. Install the new flapper. Hook the ears of the new flapper onto the overflow tube pegs. Press them on firmly — they should snap into place and not wobble.

6. Attach the chain. Hook the chain to the handle arm. Leave approximately 1/2 inch of slack — enough that the flapper sits fully closed at rest, but lifts completely off the seat when you flush. Clip off excess chain length if it is long enough to get caught under the flapper.

7. Turn the water back on. Open the supply valve by turning it counterclockwise. Let the tank fill.

8. Test. Flush the toilet and watch the flapper close. Check that the flapper seats completely and the fill valve shuts off after the tank fills. Repeat the dye test if you want to confirm the seal is tight.

Total time: 5 to 10 minutes.

If Replacing the Flapper Does Not Fix It

If the toilet still runs after a new flapper and proper chain adjustment, the problem is elsewhere.

Check the overflow tube height. The overflow tube is the tall tube in the center of the tank. If water is flowing into the tube rather than out the bottom when the flapper is closed, the fill valve is filling the tank too high. The water level should sit about 1 inch below the top of the overflow tube. If it is too high, adjust the float on the fill valve — most modern fill valves have a clip or screw that raises or lowers the float.

Replace the fill valve. If the fill valve runs constantly even after adjusting the float, or if you can hear water hissing through it when the tank is full, the fill valve itself is worn. A Fluidmaster 400A fill valve is the standard replacement and fits nearly every toilet. Replacement takes about 15 minutes and costs $10 to $15.

For a complete walkthrough of fill valve replacement, see our guide on how to replace a toilet fill valve. If the toilet runs in other unusual ways, our full guide on how to fix a running toilet covers every scenario including phantom flushing. You may also find our guide on how to fix a phantom toilet flush useful if the running is intermittent.

Related Reading

• How to Fix a Running Toilet
• How to Replace a Toilet Fill Valve
• How to Replace a Toilet Flapper
• How to Fix a Phantom Toilet Flush