Quick answer: To replace a toilet flapper, shut off the supply, drain the tank, remove the old flapper, clean the flush-valve seat, install a correctly sized new flapper (usually a 2″ or 3″ class), set about ½ inch of chain slack so it seals closed, restore water, and confirm the seal with a dye test. Wrong size or a hang-open chain is a common reason a “new” flapper still leaks.
These steps cover sizing and installing a flapper so it seals correctly — not shopping comparisons. For product selection context, see best toilet flapper for a running toilet. If the toilet runs or refills on its own, also use why does my toilet keep running and toilet randomly refills (phantom flush).
When a flapper swap is the right job
- Positive dye test with color reaching the bowl without flushing
- Warped, cracked, slimy, or mineral-crusted flapper that will not seat flat
- Chain tangled, too short, or too long so the seal hangs open
- You already confirmed the tank waterline is below the overflow (so the “leak” is not constant overflow spill)
If you have a canister or tower flush system (a cylinder that lifts instead of a rubber flapper), do not force a flapper onto it — use the maker’s seal/tower kit instead.
Educational illustration — diagram for teaching, not a photo of a real repair.
How to choose the correct size
Most common residential tanks use a 2-inch or 3-inch flush opening class. Manufacturer sizing guidance (for example Fluidmaster) commonly recommends measuring the old flapper or the drain hole rather than guessing by toilet brand alone:
- 2″ class: old flapper about 3 inches across, or a drain hole around 2.5 inches — typically sold as a 2″ flapper
- 3″ class: old flapper about 4 inches across, or a drain hole around 3.5 inches — typically sold as a 3″ flapper
Also match frame style (solid vs flexible), how the flapper attaches (posts/ears), and whether your kit is adjustable. Bring the old flapper when shopping if you can. Prefer the sizing guide printed in the kit over any “one size fits all” assumption.
Reference: Fluidmaster — determining 2″ or 3″ flush valve and flapper and choosing the right flapper.
Tools and materials
- Correct-size replacement flapper (and chain/clip if not included)
- Towels and a sponge
- Optional: flashlight to inspect the seat; soft cloth for mineral wipe-down
- The manufacturer’s install sheet for your exact kit
Safety first
- Shut off water at the toilet stop. Do not force a frozen or corroded valve.
- Protect the floor; expect leftover tank water.
- Do not overtighten anything on porcelain while “just changing a flapper.”
- Do not mix bleach, acids, or drain chemicals in an open tank.
- Stop if the flush-valve seat is cracked, the tank is cracked, or hardware is seized.
Educational illustration — diagram for teaching, not a photo of a real repair.
Step-by-step replacement
- Shut off the supply. Flush and hold the handle to drain most tank water. Sponge out the rest so you can see the seat.
- Unhook the chain from the trip lever. Note how the old flapper attaches (snap arms on posts, or hooked ears).
- Remove the old flapper. Lift it off the mounting posts or unhook the ears.
- Clean the seat. Wipe mineral grit and slime from the flush-valve rim so the new seal can land flat. Skip harsh chemical cocktails.
- Install the new flapper. Snap arms onto the posts (or seat ears as designed) so the seal sits centered and flat.
- Attach the chain to the lever directly above the flapper. Leave roughly ½ inch of slack when the flapper is closed (a small “j” shape is a common manufacturer cue). Too tight holds the seal ajar; too loose may not lift fully.
- Restore water. Open the shutoff slowly. Confirm the waterline stops about ½ inch below the top of the overflow (or at the tank mark). Flush once and watch the flapper drop and seal.
- Prove the seal. Run a dye test. No color in the bowl during the wait means the seal held for that window.
Kit example: Fluidmaster PerforMAX-style flapper sheets describe measuring the old flapper, attaching arms for the correct size class, and setting roughly ½ inch of chain slack.
Educational illustration — diagram for teaching, not a photo of a real repair.
What not to do
- Do not install a 2″ flapper on a 3″ opening (or the reverse) and hope the rubber “will settle.”
- Do not leave the chain so short that the flapper cannot close.
- Do not push a refill tube deep underwater in the overflow — that can create siphon/phantom refill cycles unrelated to the flapper rubber itself.
- Do not force corroded tank bolts while chasing a flapper leak.
When to call a plumber
- Flush-valve seat is cracked, deeply pitted, or will not clean up enough to seal
- Canister/tower system needs a brand-specific rebuild, not a flapper
- Shutoff seized, spraying, or spinning
- Cracked tank porcelain or active tank-to-bowl seepage
- Leak continues after correct size, clean seat, and proper chain slack
If the flush valve itself is the failure (warped seat, broken overflow), see how to replace a toilet flush valve.
FAQ
How do I know 2″ vs 3″? Measure the old flapper across or the drain hole diameter using manufacturer sizing cues (about 3″ across / ~2.5″ hole for many 2″ parts; about 4″ across / ~3.5″ hole for many 3″ parts).
Why does my new flapper still leak? Wrong size, dirty seat, chain too tight, waterline at the overflow, or a damaged flush-valve seat.
Do adjustable flappers matter? Some kits let you tune how long the flapper stays open. Follow the printed settings for your toilet — do not invent settings that empty the tank early every flush.
Is this the same as replacing the flush valve? No. The flapper is the seal; the flush valve is the assembly the seal sits on (and usually requires lifting the tank).
Sources
- Fluidmaster — 2″ or 3″ flush valve and flapper
- Fluidmaster — choosing the right flapper
- Fluidmaster flapper kit install guide (PDF)
- EPA WaterSense — residential toilets