Korky vs Fluidmaster: Which Toilet Parts Should You Buy? (A Plumber’s Verdict)

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Short answer from a licensed plumber: For most homeowners, Korky flappers and fill valves last longer in chlorinated or hard water, while Fluidmaster is the easiest to find and the better-known universal standard. If your flapper keeps failing within a year, switch to the Korky 100BP Ultra; if you want the most widely-stocked part, the Fluidmaster 502P21 PerforMAX is a safe pick.

Korky vs Fluidmaster at a Glance

Flappers: Which Lasts Longer?

The most common cause of a running toilet is a worn flapper. In water with a lot of chlorine or chloramine, cheap rubber hardens within a year. Korky’s red chlorine-resistant rubber holds up noticeably better — it’s what I reach for on repeat-failure toilets. Fluidmaster’s basic flappers are fine in average water — and their chlorine-resistant lines, the 502 PerforMAX included, carry the same chlorine and hard-water rating Korky claim, so on that one spec the two brands are level. Where Fluidmaster still win is being the easiest to match, because nearly every store carries them.

Fill Valves

Both make a reliable universal fill valve. Fluidmaster’s 400A is the most recognized fill valve in the world. Korky’s QuietFILL runs quieter and resists hard-water debris. For a rental, grab whichever is on the shelf; for your own home in hard-water country, I lean Korky.

Whichever brand you land on, the size has to be right first. See 2-inch vs 3-inch toilet flapper for how to measure the valve seat rather than the warped old part.

The Plumber’s Verdict

No wrong choice — both beat a generic flapper. Pick Korky for hard/chlorinated water or repeat failures; pick Fluidmaster for the universal standard any store stocks. Either way the fix is a cheap part and ten minutes versus a $150 service call.

FAQ

Are Korky and Fluidmaster interchangeable?

Mostly yes for flappers — both make universal 2-inch and 3-inch sizes. Match flapper size to your flush valve opening, not the brand.

Why does my toilet still run after a new flapper?

Either the flapper doesn’t seat (check chain slack), or the real problem is the fill valve or a float set too high.

How do I know if I have a 2-inch or 3-inch flapper?

Look at the flush valve opening: about orange- or baseball-sized = 2-inch; grapefruit- or softball-sized = 3-inch (common on newer high-efficiency toilets).

Neither brand makes a flapper that fits a canister flush valve. If the whole cylinder in your tank lifts as one piece when you flush, the part you need is a canister seal, not a flapper.

The Parts You’ll Wish You’d Ordered at the Same Time

Swapping a flapper or a fill valve is a ten-minute job. What turns it into two trips is the part you had to disturb on the way in and had no replacement for. Three of them account for nearly every callback I get on a tank repair.

• A supply line, if you’re touching the fill valve. You have to break the water connection under the tank to get the old valve out, and a supply line that has sat untouched for fifteen years often weeps once it’s been moved. Fit a new one while it’s off: the Fluidmaster B1T12 braided stainless toilet connector is 3/8-inch compression at the shutoff, 7/8-inch ballcock nut at the tank, 12 inches long, with a 5-year warranty. Measure from your shutoff to the tank first — the same connector is sold in other lengths, and a line that’s too short is worse than the old one.
• Tank-to-bowl bolts and gasket, if the tank comes off. Any repair that needs the tank lifted — a leak at the seam, a new flush valve, a cracked spud washer — means those bolts come out, and the rubber washers on twenty-year-old bolts are done. The Fluidmaster 6102 tank-to-bowl bolt and gasket kit uses a sponge rubber gasket that fits most close-coupled toilets with either round or hex flush valve nuts, and the bolts are stainless steel, so they resist corrosion and still come apart next time. Fluidmaster do not rate this kit for Gerber or Kohler tanks. Skip it if you’re fitting the 400AKRP10 rebuild kit below, which brings its own tank-to-bowl hardware.
• A flush lever, only if yours is the actual problem. A cracked handle, a bent arm, or a lever that has to be held down is a real failure and it’s the cheapest part in the tank — but don’t replace one that works. The Fluidmaster 640 Perfect Fit tank lever in white has a plastic arm that adjusts for both length and angle, and Fluidmaster list it as a direct replacement for the toilet-specific levers on Toto, Kohler, Mansfield, Eljer and American Standard tanks.

None of this changes which brand of flapper you buy. It’s just the difference between finishing on Saturday morning and going back to the store.

Where to Buy: the exact parts, verified

Both brands sell the same repairs as single parts or as a kit. These are the specific models I would put in a customer’s tank, and what each one actually fixes. Prices checked 24 July 2026 and will drift.

Check your flapper size first. Two-inch is standard on most toilets built before the mid-2000s. A lot of newer Kohler and American Standard bowls use a three-inch flush valve, and a 2″ flapper will never seal a 3″ seat. Lift the tank lid and measure the opening the flapper covers before you order.

A note on the Fluidmaster 400A

The 400A is the valve every plumber of my generation learned on, and it is still made — but online it now turns up mostly in two-packs and rebuild kits rather than as a lone valve. The single valve Fluidmaster actually pushes today is the 400H PerforMAX: same shank, same lock-nut mounting, same height adjustment, faster refill. If the only 400A you can find as a single is priced near the two-pack, buy the 400H and don’t think twice about it.

What I’d buy in each situation

• Flapper failed inside a year, and you’re on chlorinated city water or hard well water — Korky 100BP, for the red chlorine-resistant rubber. Do not read that as Fluidmaster being the wrong call: Fluidmaster rate the 502 PerforMAX chlorine and hard water resistant too, so whichever of the two your store stocks is rated for the job.
• Tank is loud enough to hear from another room — Korky 528MP QuietFILL.
• You’re already in the tank and want to do both parts at once — check the flush volume before you order, not the age. It is stamped inside the tank or behind the seat. Fluidmaster rate the 400A fill valve + flapper kit for toilets of 3.5 gpf or more made before 1994; every gravity-flush toilet built after 1 January 1994 is capped at 1.6 gpf by federal law, so on a newer tank use the 400H PerforMAX (rated 1.28–3.5 gpf) or the 400AKRP10 kit, which Fluidmaster rate down to 1.6 gpf. Either way, replacing one part on a tired tank just books you a second trip.
• You want the part your local hardware store definitely stocks — Fluidmaster, every time.

Prices and availability change. If a model shows as unavailable, the equivalent from the same brand line does the same job.

Not sure which part is actually failing? Run through our flapper vs fill valve diagnosis first — it takes two minutes and saves buying the wrong part.