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 Korky; if you want the most widely-stocked part, Fluidmaster is a safe pick.
Korky vs Fluidmaster at a Glance
| Korky | Fluidmaster | |
|---|---|---|
| Best for | Hard / chlorinated water, longevity | Availability, easiest install |
| Flapper material | Chlorine-resistant red rubber | Standard rubber / PerforMAX line |
| Fill valve noise | Generally quieter | Slightly louder on some models |
| Availability | Common | Everywhere (default stocked brand) |
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 standard flappers are fine in average water and 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.
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 $5–$25 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 baseball-sized = 2-inch; orange/softball-sized = 3-inch (common on newer high-efficiency toilets).