Why a toilet runs when nothing is flushing
A toilet tank is a closed loop with one job: hold a fixed volume of water, then release it on demand. Three parts enforce that contract — the inlet valve fills it, the float tells the valve when to stop, and the flapper seals the drain so the water stays put.
Break any one of them and the loop never closes. The level drops, the float drops with it, the valve opens to compensate, and the water escapes down the drain at exactly the rate it arrives. That's the sound you hear at 2 a.m.
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The three ways this loop breaks
- Worn flapper (most common). The rubber hardens, warps, or picks up mineral scale. It hangs slightly open, so water seeps past it continuously.
- Fill level set too high. The valve keeps filling past the overflow tube rim, and the surplus drains straight into the bowl.
- Float stuck or waterlogged. The valve never gets the signal to close, so the tank fills until it overflows — every cycle.
The cheap test: shut the supply valve for eight hours. If the running stops, the fault is in the tank. If it continues, the problem is downstream in the supply line or the bowl itself.
What this is actually costing you
A running toilet wastes up to 200 gallons a day. Adjust to what you measured above.
Water wasted per year—
Water bill per year—
Fix it yourself—
Call a pro instead—
You save by DIY—
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Before you start. This walkthrough is a simulated demonstration, not a photograph of your fixture. Tank designs vary; always shut off the supply and confirm the drain size before buying parts. We do not cover gas, electrical, or jobs requiring the bowl to be lifted — those need a licensed plumber.