What Is a Toilet Flapper?

The hinged rubber seal at the bottom of a toilet tank that lifts when you flush and seals the tank shut while it refills. Because it sits in treated water around the clock, it hardens and warps on a schedule, and a worn flapper is the single most common cause of a running toilet. Flappers come in 2-inch and 3-inch sizes, and the wrong size physically cannot seal.

How to tell yours is failing: the toilet refills briefly every hour or so with nobody using it (the "phantom flush"), or food coloring dropped in the tank shows up in the bowl within 15 minutes without flushing. Both mean water is seeping past the sealing edge. A flapper typically lasts 3–5 years, less if drop-in chlorine tank tablets are used, which chew through the rubber in months.

Buying one: check the size first. A drain opening the size of a baseball or orange takes a 2-inch flapper ($5–$8); softball or grapefruit means 3-inch ($8–$15). Solid-frame universal flappers from Korky or Fluidmaster fit most toilets, but TOTO's 3-inch valves and some Kohler and American Standard models seal best with the brand part. Replacement is a 10-minute, no-tools job: supply off, tank flushed empty, old flapper unhooked from the overflow-tube pegs, new one on, chain set with half an inch of slack.