How to know it\'s the flapper (not the fill valve)
If you hear water trickling into the bowl between flushes, lift the tank lid and look at the water level. If water is flowing from the flush valve at the bottom of the tank down into the bowl (you can see the ripples around the flapper), it's a flapper problem. If water is overflowing into the central plastic tube (the overflow tube), it's a fill-valve problem and a different repair.
The 10-minute procedure
1. Shut off the water. Close the angle stop (silver oval handle) under the toilet by turning clockwise until snug. Flush once to drain the tank.
2. Sponge remaining water. A small amount of water always remains under the flapper. Sponge it into a bucket so you can work dry.
3. Unhook the chain. The flapper chain connects to the flush handle lever via a small clip. Unhook it.
4. Unhook the flapper ears. The two flexible "ears" of the flapper hook over the flush valve pins on either side. Pull each ear off — gently, the old rubber may be brittle and tear. Lift the flapper out and discard.
5. Choose the replacement. Take the old flapper to the hardware store and match by size and shape. The two main sizes are 2-inch (standard) and 3-inch (larger, used by Kohler, American Standard Champion, and most post-2005 high-efficiency models). Universal flappers (Fluidmaster 502, Korky 2001) cover most installations.
6. Install the new flapper. Hook the ears over the flush valve pins. Make sure the flapper sits flat on the valve seat — no twist, no gap.
7. Adjust the chain. Hook the chain to the flush handle lever, leaving 1/2 inch of slack. Too tight: flapper opens slightly between flushes. Too loose: flapper doesn\'t fully open during flush, causing weak flush.
8. Turn water back on and test. Open the angle stop, let tank refill, flush. Listen for silence after refill (no trickling). If still running, the flapper isn\'t seating — re-check positioning, or check that the flush valve seat isn\'t cracked or mineral-scaled.
If a new flapper doesn\'t fix it
Three remaining possibilities: (1) the flush valve seat itself is cracked or pitted — clean with a Scotch-Brite pad; if scoring is deep, replace the entire flush valve (45-minute repair, $20-30 in parts), (2) the chain is binding the flapper open, (3) the fill valve is overflowing into the overflow tube — adjust fill-valve float level down by 1/2 inch.
Flapper lifespan
Rubber flappers last 3-5 years before chlorine, mineral buildup, and rubber aging degrade the seal. Treated municipal water with high chloramine accelerates aging — flappers in those systems may last only 2 years. Replace proactively at year 4 to avoid the slow water-bill creep of a marginal flapper.