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Confirm the valve before buying: Fluidmaster 502P21 for a compatible 2-inch valve; Korky 100BP as an adjustable 2-inch alternative; Fluidmaster 5403 for a compatible 3-inch valve. A canister needs its model-specific seal, not one of these flappers.
A running toilet does not prove the flapper is bad. Check where the water is going, identify the tank model and inspect the valve seat before ordering. Universal means fits many models, not every toilet.
A replacement helps only when the old flapper is the cause and the new one fits the flush valve. Use the toilet’s tank model, flush-valve size and manufacturer compatibility information as the buying checks. Guessing from toilet age or buying a larger flapper because it looks more substantial can leave the same running problem.
| Product | Best For |
|---|---|
| Fluidmaster 502P21 PerforMAX Flapper | Best Overall |
| Korky 100BP Universal Flapper | Best Universal Fit |
| Fluidmaster 5403 3-Inch Flapper | Best for 3-Inch Flush Valves |
Fluidmaster 502P21: Adjustable 2-Inch Choice
Fluidmaster’s 502 instructions describe a solid-frame adjustable flapper for most 2-inch flush valves. The guide identifies the 502 for 1.6 or 1.28 gpf toilets and directs owners of older brass flush valves to model 504 instead. That exclusion matters more than the word universal on a listing.
Choose it when your exact valve and toilet requirements match the maker’s application. The adjustable design lets the installer tune how the flapper closes, following its settings. It does not repair a damaged seat or convert a 3-inch valve into a 2-inch one.
Skip it if: the toilet has a canister/tower, a 3-inch valve, an excluded brass valve, or a model-specific part requirement. Match the existing mounting arrangement before opening the package. The maker warns against in-tank drop-in cleaners containing bleach or chlorine; resistance language is not permission to use them.
Korky 100BP: Check the Tank Compatibility List
Korky’s model 100 product page lists 100BP among its alternate package numbers. It describes an adjustable flapper for most 2-inch flush valves, includes a stainless chain and lists a five-year warranty. Its compatibility tool identifies tank models; use that information rather than treating universal as a guarantee.
The maker lists adjustment for 1.28, 1.6, 3.5 and 5 gpf toilets. Compare that stated range with the toilet’s original requirements. Do not claim that it fixes an out-of-round or pitted valve seat: a flexible seal is not proof that damaged hardware can be kept in service.
Skip it if: the valve is 3-inch, uses a tower/canister seal, or its tank model requires another part. A warranty period is not a predicted service life in your water conditions. We have not run a durability comparison against Fluidmaster.
Fluidmaster 5403: Adjustable 3-Inch Choice
The 5403 is Fluidmaster’s universal 3-inch water-saving adjustable flapper. Its instructions describe adjustable arms and flush settings, so both mounting and adjustment need attention. The same nominal valve size does not remove the need to confirm the toilet application.
Choose it for a documented compatible 3-inch flapper valve. Skip it for a standard 2-inch valve or a canister. Follow its own chain instructions: Fluidmaster specifies 1/2 inch of slack with the flapper closed. Korky’s instructions use different wording, so do not present one chain setting as a rule for all products.
Before buying: identify the old valve and mounting points, compare the instructions, and confirm that a flapper is the failed part. A larger opening does not automatically mean the toilet is newer or that every 3-inch replacement fits it.
Korky or Fluidmaster? Both brands are in the picks above. See Korky vs Fluidmaster for a part-by-part comparison of their flappers and fill valves.
Replaced the Flapper and It Is Still Running?
Stop buying flappers until you have identified the leak path. A compatible new flapper cannot solve every fill-valve, overflow or flush-valve problem. Check that the selected part matches the tank and that the chain is not holding it open or catching underneath it.
Inspect the surface the flapper seals against and the mounting alignment. Follow the toilet and replacement-part instructions for cleaning, chain placement and adjustment. If the seat is damaged, another flapper may not be the repair. If water is going into the overflow, investigate the fill system rather than assuming the flapper caused it.
The next step depends on that observation. Our flapper vs fill-valve guide distinguishes the two tasks; the post-replacement running-toilet guide covers further checks. The aim here is to choose the correct replacement after diagnosis, not promise a cure from the symptom alone.
Before Replacing the Part
Have the tank model, old part and replacement instructions available. Confirm the water supply can be shut off normally, empty the tank as its maker instructs and compare the mounting arrangement before removing the old part. If a shutoff or component will not operate normally, stop rather than forcing it.
Use the selected model’s installation steps. For the 5403, follow its arm and chain setup; for Korky 100, follow its chain attachment and numbered adjustment guidance. Do not copy a universal chain-slack number across different designs.
After fitting, check that the part lifts and returns to its seat without the chain catching. Observe a complete fill and flush cycle and verify the problem has stopped. If it continues, revisit fit and diagnosis instead of progressively tightening or replacing unrelated parts.
If the toilet has a canister, identify that seal instead. The 2-inch vs 3-inch flapper guide helps distinguish flapper sizes; the canister seal guide addresses a different mechanism.
How These Picks Were Compared
This guide compares maker applications, installation instructions and current exact-product offers. It does not claim that Isaac tested or personally approved these parts. No fixed replacement time or service-life estimate is promised.
Make a Fit Record Before Shopping
Write the tank model and the replacement maker’s matching part on the same note. A toilet’s product name, the number on a tank and the number on a replacement package can be different identifiers. Keep them separate so a search for one does not silently substitute another part.
Compare the mounting points, opening and original flush requirements with the chosen part’s instructions. A broad brand compatibility claim is less useful than a match for the actual tank. If the part finder and the listing disagree, resolve the disagreement before opening a replacement package or discarding the old part.
Also record the symptom that led you to shop. Was water entering the overflow, was the chain holding the seal open, or did the problem persist after a previous replacement? That observation helps you avoid ordering the same incorrect part again. A shopping list that says only “running toilet” leaves out the information needed to distinguish these repairs.
Finally, compare the exact item offered. Korky lists 100BP as an alternate package number for model 100; that does not make a similarly named 3-inch part equivalent. With Fluidmaster, keep 502P21 and 5403 separate. Confirm the current New seller and return terms on the offer itself rather than assuming every marketplace variation is the selected product.
Frequently Asked Questions
Which universal replacement flapper should I buy for a running toilet?
Diagnose the leak and confirm the valve first. Compare Fluidmaster 502P21 for its documented 2-inch application, Korky 100BP against its tank compatibility list, and Fluidmaster 5403 for a compatible 3-inch valve. None is universal across all three mechanisms.
Are toilet flappers really universal?
They fit many models within a stated application, not every toilet. Valve size, mounting, flush requirements and exclusions still apply. Fluidmaster’s 502 instructions specifically direct owners of older brass flush valves to a different model.
How do I know which size flapper I need?
Identify the tank model and flush-valve opening, then compare the maker’s compatibility information. Do not infer size from the toilet’s age. A tower or canister uses a different seal and is not a reason to buy a larger flapper.
Is Korky or Fluidmaster more durable?
We have not run a durability test, so there is no measured winner here. Both publish product-specific material and warranty claims. Choose correct fit first and follow cleaner restrictions; warranty length is not a service-life forecast.
Will a new flapper always stop random refilling?
No. Refilling can continue when the part is wrong, its chain holds it open, the seat is damaged or another part is responsible. Observe the leak path and follow the toilet’s troubleshooting instructions before buying another part.
Sources and Current Offers
Fluidmaster 502 installation instructions and 5403 installation instructions (fluidmaster.com); Korky model 100/100BP product, compatibility and installation page (korky.com).
October 17, 2026: all three linked flappers had New buy-box offers sold by Amazon.com. Offers can change. Confirm tank compatibility before ordering, and check the current seller and offer terms.