Maintenance procedure · Written to ASD-STE100 Issue 9 · Formatted to ANSI Z535

Replacing a Toilet Flapper

Tank-type toilets — the dye test, flush valve size, and setting the chain
ApplicabilityTank-type toilets with a flapper on the flush valve and a stop valve below the tank
Task timeat least 37 min to 39 min (35 of 36 steps estimated) editorial estimate, not a measurement
PersonnelOne
Revisionv1 · 2026-07-29
BEFORE YOU BEGIN

Before you begin. This procedure is general reference information, not professional advice. Equipment varies by manufacturer, model, and year. Always follow the documentation supplied with your equipment — where it differs from this page, follow the manufacturer. Verify that you have the tools, the working conditions, and the ability to do this task safely. If anything is unclear, if the equipment is damaged, or if the task exceeds your experience, stop and use a qualified professional. You are responsible for your own safety and for the outcome. This site accepts no liability for any loss, damage, or injury arising from the use of this information.

Specifications marked unverified could not be traced to a manufacturer source. Do not rely on them — obtain the value from your equipment documentation.

Summary

A toilet that hisses long after the flush, refills by itself in the middle of the night, or simply costs more than it used to, is usually failing at one rubber part: the flapper that seals the bottom of the tank. EPA's WaterSense programme puts most toilet leaks down to old or worn-out flappers, and reckons a leaking household loses more than 9,300 gallons a year. The part costs a few dollars, the job needs no tools, and the whole thing is over in about half an hour.

What this page will not do is tell you which flapper to buy. Flappers come in a standard and a large size, keyed to the flush valve underneath them, and the fit is further keyed to the toilet — Korky's own answer is that its repair kits are universal across brands, immediately followed by an instruction to check what size your toilet uses. So this page teaches you to read the size off the toilet in front of you and to take the worn part with you, rather than printing a number that is right for someone else's toilet. It also does not print a chain length or a tank water level, because both are set by fit against your own hardware and neither can be traced to a source that covers it.

Safety

NOTICE

DO NOT PUT A CHEMICAL TABLET IN THE TANK.
The chemical decreases the life of the flapper and of the other rubber parts.

Necessary equipment

ItemNote
Rubber glovesThe gloves keep your hands dry.
SpongeUse the sponge to remove the last water from the tank.
Old towelsPut the towels below the stop valve.
ClothUse the cloth to clean the seat of the flush valve.
Flashlight optionalThe inside of a tank is a dark space.
Mobile phone with a camera optionalUse the camera to make a record of the parts before you remove them.
New flapper of the correct size for your flush valve materialThe new flapper must be the same size as the used flapper.
Food coloring materialUse the food coloring for the dye test.

Do the dye test

Estimated 12 min

NOTEA toilet can lose water with no sound. The dye test finds a leak that you cannot hear.
Water level in the tank
Unverified — do not rely on this value
CONSULT THE DOCUMENTATION FOR YOUR TOILET. MANY TANKS AND OVERFLOW PIPES CARRY A MARK

No tier-1 source was found for this value. Take it from the documentation supplied with your equipment.

WARNING

PUT THE TANK LID ON THE FLOOR AWAY FROM YOUR FEET.
The lid is heavy china. A broken lid has sharp edges that can cut your hand.

  1. Remove the tank lid with two hands.
    <1m
  2. Look at the water level in the tank.
    • Water that goes into the overflow pipe points to the fill valve and not to the flapper.
    <1m
  3. Put some drops of food coloring into the water in the tank.
    <1m
  4. Do not flush the toilet for ten minutes.
    10m
  5. Look into the bowl after the ten minutes.
    <1m
  6. If the water in the bowl has color, the flapper does not seal.
    <1m
  7. If the water in the bowl stays clear, the flapper is not the cause. Stop this procedure.
    <1m

Find the size of the flush valve

Estimated 4 min (partly estimated)

NOTEThe flapper manufacturers make two sizes. Korky names a standard 2 inch valve and a large 3 inch valve.
Flapper part for your toilet
Unverified — do not rely on this value
MATCH THE USED FLAPPER AT THE PARTS COUNTER, OR CONSULT THE PARTS LIST FROM THE TOILET MANUFACTURER

No tier-1 source was found for this value. Take it from the documentation supplied with your equipment.

Volume of water in one flush for your toilet
Unverified — do not rely on this value
READ THE VOLUME FROM THE MARK ON THE TOILET, OR CONSULT THE TOILET MANUFACTURER

No tier-1 source was found for this value. Take it from the documentation supplied with your equipment.

  1. Look at the flapper on the bottom of the tank.
    <1m
  2. Compare the flapper with an orange and with a grapefruit.
    • Korky gives an orange or a baseball for the standard 2 inch valve.
    • Korky gives a grapefruit or a softball for the large 3 inch valve.
    <1m
  3. Look at the opening below the flapper. This opening is the flush valve.
    <1m
  4. Make a record of the make and the model of the toilet.
    • The make is usually on the front of the bowl or on the tank.
    • The model number is usually inside the tank, above the water level.
    2m
  5. Get a new flapper of the same size.
    • Take the make, the model and a photograph with you.

Close the stop valve and empty the tank

Estimated 5 min

  1. Find the stop valve on the wall below the tank.
    <1m
  2. Put the old towels below the stop valve.
    <1m
NOTICE

TURN THE STOP VALVE GENTLY.
An old stop valve can break and let water onto the floor.

  1. Turn the stop valve clockwise until the valve stops.
    <1m
  2. Flush the toilet to empty the tank.
    <1m
  3. Make sure that the tank does not fill again.
    • A tank that fills again shows that the stop valve does not close.
    1m
  4. Remove the last water from the tank with the sponge.
    2m

Remove the used flapper

Estimated 4 min

  1. Remove the chain from the arm of the flush lever.
    <1m
  2. Remove the two ears of the flapper from the pegs on the overflow pipe.
    • Some flappers have a ring that goes around the overflow pipe.
    1m
  3. Remove the used flapper from the tank.
    <1m
NOTICE

DO NOT USE A TOOL ON THE FLUSH VALVE.
A crack in the flush valve makes the repair much larger.

  1. Clean the seat of the flush valve with the cloth.
    • The seat is the flat ring that the flapper touches.
    2m
  2. Examine the seat for damage.
    • A seat with a crack or a rough area does not seal.
    • A flush valve with damage is a job for a plumber.
    <1m

Install the new flapper

Estimated 4 min

NOTICE

USE A FLAPPER OF THE SAME SIZE AS THE USED FLAPPER.
An incorrect flapper does not seal and changes the quantity of water in a flush.

  1. Put the new flapper and the used flapper together.
    <1m
  2. If the two flappers are not the same size, stop. Get the correct flapper.
    <1m
  3. Put the two ears of the new flapper on the pegs.
    1m
  4. Make sure that the flapper sits flat on the seat.
    <1m
  5. Attach the chain to the arm of the flush lever.
    1m

Set the chain

Estimated 2 min to 3 min

Chain length between the flush lever and the flapper
Unverified — do not rely on this value
SET THE CHAIN BY FIT. CONSULT THE INSTRUCTIONS THAT COME WITH THE NEW FLAPPER

No tier-1 source was found for this value. Take it from the documentation supplied with your equipment.

  1. Set the chain with a small quantity of slack.
    • A tight chain holds the flapper away from the seat.
    • If the chain goes below the flapper, the flapper cannot seal.
    2m

Do the final checks

Estimated 6 min

  1. Turn the stop valve counter-clockwise to open it.
    <1m
  2. Let the tank fill.
    2m
  3. Flush the toilet. Look at the flapper.
    <1m
  4. Make sure that the flapper falls onto the seat.
    <1m
  5. Make sure that the water stops when the tank is full.
    1m
  6. Do the dye test again after one hour.
    2m
  7. Install the tank lid with two hands.
    <1m

Terminology

This page uses one term for each item and holds it throughout, as ASD-STE100 requires. Equipment sellers and other guides use the names below for the same parts.

flapper
Also called: toilet flapper, flapper valve, flush valve seal, tank ball, flush ball, toilet flap, the rubber thing in the toilet tank, valve seal
flush valve
Also called: flush valve seat, tank outlet, flush valve opening, the hole in the bottom of the tank, outlet valve
stop valve
Also called: shutoff valve, shut off valve, angle stop, toilet shutoff, supply valve, isolation valve, the little tap behind the toilet
overflow pipe
Also called: overflow tube, standpipe, the open tube in the middle of the tank
flush lever
Also called: flush handle, trip lever, toilet handle, the handle you push
fill valve
Also called: ballcock, ball cock, float valve, filler valve, the float thing
ghost flush
Also called: phantom flush, toilet flushing on its own, toilet refills by itself, running toilet

When to use a professional

Stop if the stop valve does not close. Stop if the stop valve leaks when you turn it. Speak to a plumber.

Stop if the seat of the flush valve has a crack. Stop if the tank has a crack. Stop if water is on the floor around the base of the toilet. These faults are not on this page.

Do not do work on the supply pipe above the stop valve. Do not remove the tank from the bowl.

This page does not give a part number for your flapper. This page does not give a chain length. This page does not give a water level for your tank. Ask the manufacturer of your toilet for these three answers.

Common questions

Why does my toilet keep running after I flush?

Nine times out of ten the flapper is not sealing, so water keeps trickling from the tank into the bowl and the fill valve keeps topping the tank back up. EPA's WaterSense programme says most toilet leaks come down to old or worn-out flappers. The other common cause is the water level sitting too high, so it spills continuously into the overflow pipe — look at where the water is going before you buy a part. If the water surface is well below the top of the overflow pipe and the toilet still runs, the flapper is your answer.

How do I tell if my toilet is leaking when I cannot hear anything?

Use the dye test, which is EPA's own method: put a few drops of food coloring in the tank, wait, and look in the bowl. EPA puts the wait at ten minutes — if any color shows up in the bowl in that time, you have a leak. Nothing else is needed and it costs nothing. A silent leak is worth finding: EPA estimates household leaks at more than 9,300 gallons a year, and reckons toilets are often the culprit.

What size toilet flapper do I need, 2 inch or 3 inch?

The number refers to the flush valve opening underneath the flapper, not to the width of the flapper itself, which is why people order the wrong one. Korky's own rule of thumb is to look at the old flapper: about the size of an orange or a baseball means a standard 2-inch seal, and more like a grapefruit or a softball means a large 3-inch seal. Korky also says you can simply measure the flush valve opening. Most toilets made before 2000 take the 2-inch; the 3-inch turned up on some models after that. We are not going to guess which yours is — look in the tank, or take the old one to the counter.

Are toilet flappers universal?

Sort of, and the caveat is the important half. Korky says its toilet repair kits are designed to be universal and repair all toilet brands, and then immediately says to be sure to check what size flapper and flush valve your toilet uses. So "universal" means it is not tied to a brand; it does not mean one part fits every toilet. The size split is real and a wrong-size flapper simply will not seal. Take the worn one with you, and note the toilet's make and model while you are at it.

How often should a toilet flapper be replaced?

EPA's WaterSense guidance is to check the flapper periodically and replace it at least every five years to keep a good seal. Korky, which makes them, is less willing to name an interval and says the life of a flapper depends on external factors such as the kind of water you have and whether you use chemical tablets in the tank; it puts a five-year warranty on most of its flappers. These are not in conflict — one is a maximum interval, the other is what actually wears the part out. The practical version: run the dye test once a year, and drop the in-tank cleaning tablets entirely.

Will a new flapper change how much water my toilet uses per flush?

It can, and this is the reason to match the size rather than fit whatever is on the shelf. The flapper controls how long the flush valve stays open, so a flapper that is the wrong size or the wrong weight for your toilet changes the flush volume. For scale, WaterSense labelled toilets are certified at 1.28 gallons per flush or less against a federal standard of 1.6, and the whole gap between those two numbers is smaller than the error a badly matched flapper can introduce. Your toilet's rated flush volume is usually stamped on the bowl behind the seat hinge or inside the tank.

Sources

Every specification on this page is traced to one of these sources. Tier 1 is a manufacturer document, a published standard, or a government safety authority. See Standards for the full tiering rule.

  1. Tier 1Fix a Leak WeekWaterSense, United States Environmental Protection Agency · accessed 2026-07-29 · re-verification due 2027-01-25
  2. Tier 1Residential ToiletsWaterSense, United States Environmental Protection Agency · accessed 2026-07-29 · re-verification due 2027-01-25
  3. Tier 1How to Determine What Size Flapper You NeedKorky (Lavelle Industries, Inc.) · accessed 2026-07-29 · re-verification due 2027-01-25
  4. Tier 1Frequently Asked QuestionsKorky (Lavelle Industries, Inc.) · accessed 2026-07-29 · re-verification due 2027-01-25
  5. Tier 1TM 3-34.70 / MCRP 3-17.7E, Plumbing, Pipe Fitting, and SewerageHeadquarters, Department of the Army (Army Publishing Directorate) · accessed 2026-07-29 · re-verification due 2027-01-25

Revision history

Current version 1 · updated 2026-07-29 · next review due 2027-07-29

  1. v1 · 2026-07-29First draft. Dye test and the five-year check from EPA WaterSense; flush valve sizing and the universal-but-check-the-size caveat from Korky's own support pages; flush volume figures from EPA WaterSense residential toilets. Sequence cross-read against TM 3-34.70 chapter 4, which is archaic and publishes no value here. No part number, chain length or tank water level publishes as a verified value. Written independently in Simplified Technical English.

Corrections to this page are listed in the public corrections log. Reports that are credible and not yet verified are listed in the public errata register, and an open one renders above the step it concerns.

Comments and corrections

Found an error? A correction with a manufacturer source is applied and logged publicly. You can also write to corrections@fixchecklist.com.

Every step above carries its own report control, which tags the report with that step so a correction reaches the right place. For anything about the procedure as a whole — the order of the sections, a step that is missing, something that did not match your equipment — use this: report something about this whole procedure.

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