What is a toilet flapper?
Updated August 2026
A toilet flapper is the hinged rubber seal at the bottom of the tank that lifts when you push the handle and drops back to hold the water in. It is a $5–15 part about the size of a coffee mug lid, tied to the flush lever by a light chain. Nearly every complaint about a toilet that will not stop running traces back to this one piece of rubber.
Where it is
Lift the tank lid and look straight down at the drain opening in the tank floor. The flapper is the rubber disc covering that hole, hooked onto two ears on the sides of the overflow tube or onto a collar around it. Most are black or red rubber; newer ones tend to be blue or translucent silicone. Openings come in two sizes, 2 inch on older and standard toilets and 3 inch on many 1.28 and 1.6 gallon models built since about 2005, and matching that size matters more than matching the brand.
What it does
The flapper seals under nothing but the weight of the water above it, so a slight warp or a mineral crust on the seat is enough to open a hairline gap. Water then trickles into the bowl, the tank level falls, and the fill valve wakes up to top it off, again and again. The EPA puts a silently running toilet at up to 200 gallons a day, which at typical municipal rates adds $20–70 to a monthly bill and considerably more on a well with a pump cycling all night. On a septic system the stakes rise, because that extra volume floods the tank and pushes solids toward the drain field. Warped rubber, a chain caught under the flapper, or a chain pulled so tight the flapper never fully seats all produce the identical symptom.
Maintenance
Run a dye test twice a year on every toilet in the house, detailed in the leak check schedule, and plan on a new flapper every 4 to 6 years. The swap takes 10 minutes and no tools: close the supply stop, flush to empty the tank, unhook the old flapper and chain, snap the new one on, and leave about 1/2 inch of slack in the chain. If the tank still refills on its own afterward, the culprit is upstream at the fill valve. Everything else the tank needs sits on the toilet service list.
What it costs to replace
A replacement toilet flapper runs $5–15 for a universal flapper, or $125–250 for a plumber to rebuild the tank.
OnOtto's photo scan identifies parts like this in your home and builds the maintenance schedule for you. See how the scan works.
Frequently asked questions
How do I test whether my flapper is leaking?
Drop 10 drops of food coloring or a dye tablet into the tank, then leave the toilet completely alone for 15 to 20 minutes. Color appearing in the bowl means water is slipping past the flapper. The test catches leaks long before you can hear one, and it is especially worth running on a guest bathroom toilet nobody uses, since those run unnoticed for months.
What size flapper do I need?
Measure the opening in the tank floor, not the old flapper. A hole about 2 inches across takes the common 2 inch flapper; a wider one, roughly 3 inches, needs a 3 inch model and sometimes a matching seat. Photograph the tank interior along with the model number stamped under the lid or behind the seat, then buy the brand-specific part if the toilet is a Kohler, Toto, or American Standard with a proprietary seat.
Why does my toilet refill by itself every 20 minutes?
That is a phantom flush, the classic signature of a slow flapper leak. Water seeps past the seal until the level drops far enough to trip the fill valve, which runs for 15 to 30 seconds and shuts off. A dye test confirms it. If the flapper is brand new and the phantom flush continues, scrub the seat with a plastic scouring pad, because a pitted or rough plastic seat leaks past even a perfect flapper.
Do in-tank cleaning tablets damage the flapper?
Chlorine tablets and drop-in blue cleaners chew through rubber and can cut flapper life from 5 years to under 1, and many toilet makers void tank part warranties if they find one in the tank. The same chemistry attacks fill valve seals and the tank-to-bowl gasket, which is part of why some toilets demand constant attention instead of quietly reaching the 25 to 30 years they should. Clean the bowl directly instead.