Part of The Complete Guide to Home Plumbing
The flapper isn't sealing
This is the most common cause by a wide margin. The flapper is the rubber piece at the bottom of the tank that lifts when you flush and should drop back down to seal the tank. Over time it warps, cracks, or collects mineral buildup and stops sealing completely, letting water slowly leak into the bowl and triggering the tank to keep refilling. Flappers are inexpensive and take about ten minutes to swap — take the old one to a hardware store to match it exactly, since sizes vary.
The fill valve is misadjusted
If the water level in the tank is set too high, it flows continuously into the overflow tube instead of stopping where it should. Most fill valves have a simple adjustment screw or clip to lower the shutoff level — the water should stop about an inch below the top of the overflow tube.
The flush handle or chain is off
If the chain connecting the handle to the flapper is too short, it can hold the flapper slightly open all the time. Too long, and it can get caught underneath the flapper and prevent a full seal. There should be a small amount of slack, but not so much that it tangles.
Mineral buildup or a warped flapper seat
Hard water can leave mineral deposits on the seat the flapper sits on, preventing a clean seal even with a brand new flapper. A quick scrub with a soft brush and some vinegar often resolves this.
How to actually confirm it's leaking
Add a few drops of food coloring to the tank water and wait about ten minutes without flushing. If color shows up in the bowl, water is leaking past the flapper seal, confirming the diagnosis before you buy parts.
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