Short answer
A running toilet usually means a worn flapper, a misadjusted fill valve, or a chain that's too short or tangled. Lift the tank lid, check that the flapper seals fully, adjust the float to the fill line, and untangle the chain. These are safe, simple fixes — but if tank parts are corroded or the toilet runs, call a plumber.
How a toilet tank works (60-second primer)
The tank has four working parts: the fill valve (brings water in), the float (tells the fill valve when to stop), the flapper (the rubber seal at the tank's bottom that releases water to the bowl when you flush), and the chain (connects the flush handle to the flapper). The overflow tube in the middle is the safety drain.
A running toilet means water is continuously escaping the tank — almost always past a bad flapper seal into the bowl, or through the overflow tube because the fill valve never shuts off. Both are visible with the lid off, and both are homeowner-fixable in most cases.
Fix 1: the flapper
The flapper is the number-one cause of running toilets. Lift the tank lid, flush, and watch: the flapper should lift, then settle flat over the drain opening and seal completely. If water keeps trickling into the bowl, the flapper is warped, mineral-caked, or not seating — run a finger along its underside and the drain rim to feel for buildup.
Flappers are inexpensive universal parts. Turn off the toilet's supply valve, flush to empty the tank, unhook the old flapper, and clip on the new one — then adjust the chain so it has about half an inch of slack. Too tight and it holds the flapper open; too loose and it tangles under the flapper.
Fix 2: the fill valve and float
If the tank fills past the overflow tube and water pours down it continuously, the fill valve is not shutting off. First check the water level: it should sit about an inch below the top of the overflow tube, at the marked fill line inside the tank. Adjust the float down — a screw, clip, or sliding adjustment depending on the valve type — and see if the running stops.
A fill valve that hisses constantly, fills painfully slowly, or will not shut off even with the float held up is worn out. Replacement fill valves are universal, install with the water off, and take about fifteen minutes with basic hand tools. If the valve body or supply connection is corroded, stop and call a plumber rather than forcing it.
Fix 3: the chain
The chain causes more running toilets than it gets credit for. A chain that is too short holds the flapper slightly open — water seeps continuously and the fill valve runs forever to compensate. A chain that is too long tangles or gets caught under the flapper, with the same result.
With the tank full and settled, the chain should have slight slack — roughly half an inch — when the flapper is closed. Re-hook it at a different link to adjust, and trim excess links if the chain is long enough to tangle. While you are in there, check that the chain and handle arm are not corroded; a crumbling handle assembly is a cheap full replacement.
When it is not a DIY fix
Stop and call a plumber if the tank itself is cracked, if supply valves or tank bolts crumble with corrosion when touched, or if water leaks at the toilet's base — that points to a failed wax ring or flange, which means pulling the toilet. Also stop if you have replaced the flapper and fill valve and the toilet still runs: the problem may be a hairline tank crack or a mis-sized part.
The same goes for any toilet that rocks on the floor or has recurring base leaks — those are structural mounting issues, not tank adjustments. Tank internals are safe homeowner territory; anything involving the toilet's seal to the floor is pro work.
