For a standard 2-inch flush valve, the Fluidmaster 502P21 PerforMAX is the flapper to buy — adjustable, and built from a rubber that resists chlorine and mineral buildup. Korky’s 100BP is the safest universal if you do not know your toilet’s brand. Newer high-efficiency toilets with a 3-inch valve need the Fluidmaster 5403 instead.
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A running toilet isn’t just annoying — it can waste up to 200 gallons of water per day. That’s real money on your water bill. In 95% of cases, the fix is an inexpensive flapper that takes five minutes to replace. I’ve replaced thousands of flappers as a plumber. Here are the ones that actually seal properly and last.
| Product | Best For | Price tier |
|---|---|---|
| Fluidmaster 502P21 PerforMAX Flapper | Best Overall | Mid |
| Korky 100BP Universal Flapper | Best Universal Fit | Lowest |
| Fluidmaster 5403 3-Inch Flapper | Best for 3-Inch Flush Valves | Highest |
Best OverallEditor’s Choice
Fluidmaster 502P21 PerforMAX Adjustable Toilet Flapper
fits 2-inch flush valves · universal fit (1.28 / 1.6 / 3.5+ GPF) · adjustable dial to tune water per flush · adjustable solid-frame design · water-saving long-life rubber
Check the Fluidmaster 502P21 PerforMAX flapper on Amazon →
Fluidmaster is the brand I keep in my truck. The 502P21 is adjustable — meaning you can control how long it stays open after a flush, which directly affects how much water you use per flush. It fits most 2-inch flush valves and is compatible with American Standard, Kohler, Eljer, and most other major brands.
The rubber compound Fluidmaster uses resists chlorine and mineral buildup better than generic flappers. I’ve had these last 5+ years in hard water areas where cheaper flappers fail in 18 months.
- ✅ Adjustable flush volume
- ✅ Fits most 2-inch flush valves
- ✅ Chlorine-resistant rubber
- ✅ Works with most major brands
- ❌ Doesn’t fit 3-inch flush valves (get the 5403 for those)
- The adjustable dial lets you tune flush volume up or down without other parts.
- The long-life rubber compound holds its seal in chlorinated and hard water.
- The solid frame won’t bend, giving a secure, leak-free seal.
- It only fits 2-inch flush valves, not 3-inch.
- The adjustment dial can require some trial-and-error to fine-tune.
Verdict: Best for the typical standard toilet with a 2-inch flush valve that needs a reliable, tunable replacement.
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Best Universal Fit
Korky 100BP Universal Flapper
fits 2-inch flush valves · universal fit · Korky model 100BP · long-lasting rubber · Made in USA
Check the Korky 100BP Universal Flapper price on Amazon →
If you’re not sure what toilet brand you have or you just want the most universally compatible option, Korky’s 100BP is the standard. It’s been the go-to universal flapper for decades. The flexible design conforms to slightly different valve seat shapes, which is why it seals even on older or non-standard toilets.
- ✅ Fits virtually any 2-inch flush valve
- ✅ Seals well on older toilets
- ✅ Cheap and widely available
- ❌ Adjusted by a numbered band on the flapper’s underside, not a dial
- Korky sells it as a universal 2-inch flapper, so you do not have to identify your toilet brand first.
- The long-lasting rubber stays pliable instead of going hard and leaking at the seat.
- Made in the USA, and sold under the model number (100BP) so you can re-order the exact part.
- It is limited to 2-inch flush valves.
- Its adjustment settings are coarser than a dial-style flapper.
Verdict: Best for anyone who wants a no-guesswork flapper that fits the widest range of 2-inch toilet brands.
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Best for 3-Inch Flush Valves
Fluidmaster 5403 Flapper
fits 3-inch flush valves · for 1.28 / 1.6 GPF high-efficiency toilets · adjustable dial + adjustable float · long-life silicone seal · includes stainless chain · 10-year warranty
Check the Fluidmaster 5403 Flapper price on Amazon →
Many newer high-efficiency toilets use a 3-inch flush valve instead of the standard 2-inch. Go by the opening, not the shape of the valve: Fluidmaster’s own rule is to measure the drain hole the flapper covers — about 2.5 inches takes a 2-inch flapper, about 3.5 inches takes a 3-inch one. If instead you find a tall tower that lifts as one piece, that is a canister valve and no flapper fits it at all; Kohler’s Class Five toilets are built that way, and Kohler’s own line book says of them, “No flappers.” Those need a canister seal, not this. Where a 3-inch flapper seat is what you have, the Fluidmaster 5403 is made for it and won’t fail the way a universal flapper will if it’s the wrong size.
- ✅ Made for 3-inch flush valves
- ✅ Strong seal on high-volume flush systems
- ✅ Durable Fluidmaster quality
- ❌ Only for 3-inch valves — check before buying
- It is purpose-built for 3-inch high-efficiency toilet flush valves.
- Its silicone seal and 10-year warranty deliver long-lasting performance.
- The adjustable dial and float let you tune flush to original specs.
- It will not fit standard 2-inch flush valves.
- It is geared to newer HET toilets, not older high-volume models.
Verdict: Best for newer high-efficiency toilets that use a 3-inch flush valve.
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Worth knowing what the old flapper was costing you: the average household leak runs about 9,300 gallons a year, which works out to anywhere from roughly $66 to nearly $390 depending on your city’s water and sewer rates. I priced all 31 in what a water leak costs in 31 U.S. cities.
Korky or Fluidmaster? Both brands are in the picks above, and the question I get asked most is which one lasts longer. I broke them down part by part in Korky vs Fluidmaster — flappers, fill valves, and which one I actually carry on the truck.
What Is the Best Toilet Flapper Replacement?
For most toilets it is a 2-inch flapper, and the Fluidmaster 502P21 PerforMAX is the one worth buying: its dial tunes water per flush and its rubber holds a seal in chlorinated, hard water. Korky says most toilets built before 2000 take a 2-inch seal and some built after take a large 3-inch one, so check the valve opening first.
Replaced the Flapper and It’s Still Running?
This is the most common follow-up question we get: new flapper installed, toilet still runs — often in bursts every 10–20 minutes (“ghost flushing”). Before you buy another part, run this checklist:
- Do the food-coloring test. A few drops in the tank, wait 15 minutes without flushing. Color seeping into the bowl means water is still getting past the flapper seal.
- Inspect the flush valve seat. A new flapper can’t seal against a pitted, rough, or mineral-crusted seat. Wipe the rim it closes against — if it feels rough, the seat (not the flapper) is your problem.
- Verify size and fit. 2-inch vs 3-inch matters (see the FAQ below), and some toilets — Kohler and TOTO especially — seal best with brand- or model-matched flappers instead of universals.
- Check the chain. Too tight holds the flapper off the seat; too loose lets it get pinned underneath.
- Hard water? Mineral buildup defeats new flappers fast. Clean the seat — and skip drop-in chlorine tablets, which eat flappers prematurely.
Still running after all five? The fault is usually the fill valve or the flush valve seat rather than the flapper. Our flapper vs fill valve diagnosis guide shows how to tell them apart in two minutes. For the complete post-replacement walkthrough, see 7 things a plumber checks when a toilet still runs after a new flapper.
How to Replace a Toilet Flapper
- Shut off the water supply valve behind the toilet (turn clockwise).
- Flush to empty the tank.
- Unhook the flapper ears from the flush valve pegs and disconnect the chain from the flush lever.
- Hook the new flapper ears onto the valve pegs. The ears should snap securely into place.
- Connect the chain to the flush lever — leave about 1/2 inch of slack.
- Turn the water back on and let the tank fill.
- Flush once. The flapper should seal completely and the water should stop running within 30 seconds of filling.
Plumber’s tip: If the toilet is still running after replacing the flapper, the problem may be the fill valve, not the flapper. Check whether water is flowing into the overflow tube — if it is, your fill valve needs adjustment or replacement.
Before ordering, confirm which size you actually need. 2-inch vs 3-inch toilet flapper shows how to measure the valve seat — and how to spot a canister valve, which takes no flapper at all.
FAQ
How do I know if my toilet has a 2-inch or 3-inch flush valve?
Open the tank lid and look at the flush valve at the bottom. If the opening (the hole the flapper covers) is roughly the size of an orange or a baseball, it’s 2-inch. If it looks more like a grapefruit or a softball, it’s 3-inch. That is Korky’s own rule, and Korky make one of the flappers above. When in doubt, measure the opening, or bring the old flapper to the hardware store to match it.
How long should a toilet flapper last?
A quality flapper lasts 5–8 years in normal conditions. Chlorine tablets in the tank dramatically shorten flapper life — if you use those blue tablets, expect to replace the flapper every 1–2 years.
My toilet randomly runs for a few seconds. Is that the flapper?
Yes — this is called “phantom flushing” and it almost always means the flapper is leaking slowly. Water drains from the tank until the fill valve kicks on to refill it. Replace the flapper and it’ll stop.
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Bottom Line
Grab the Fluidmaster 502P21 PerforMAX. It’s the most reliable 2-inch flapper on the market, and it will pay for itself many times over on your water bill. If you have a 3-inch valve, get the 5403. Either way, this is a 5-minute fix you can do today.
If the toilet itself is 20-plus years old, or you’re back in the tank every year, a flapper is a band-aid on the real problem. Our toilet picks cover what’s worth replacing it with.
Lifted the lid and found no flapper? If a tall cylinder rises as one piece when you flush, you have a canister flush valve and no flapper will fit it — see Kohler canister flush valve seals.
If your toilet randomly refills on its own, a worn flapper is the usual culprit — see toilet phantom flushing to confirm before you swap parts.
Fix the 12 Most Common Plumbing Problems Yourself — Free Plumber’s Handbook — including the toilet fixes plumbers charge $150+ for
Step-by-step fixes for the 12 most common problems — and exactly when to call a pro. Free and instant, from a licensed plumber. Save hundreds on calls you didn’t need.