What running means
Water moving from tank to bowl when nobody flushed. Usually you hear it refill every few minutes. It is almost always the flapper, the fill valve, or the chain length, and all three are inexpensive parts.
What it is costing you
A continuously running toilet can waste enough water to show up clearly on a bill. It is one of the few plumbing faults where the repair genuinely pays for itself quickly.
The flapper test
Put a few drops of food coloring in the tank and do not flush. If color appears in the bowl within fifteen minutes, the flapper is leaking. That is a five minute job and a cheap part.
When it is not the flapper
A fill valve that will not shut off, a float set too high so water runs into the overflow, or a cracked overflow tube. All still parts, not a new toilet.
When it IS a new toilet
Water at the base rather than inside the tank is a different problem. That is the wax seal or the flange, and a long-running one means the subfloor is wet. Worth looking at properly rather than mopping.
Questions about any of this on your own property? (940) 231-5706 reaches Norman Alambar directly, Texas Master Plumber M-44741, working Krugerville and seventy-five towns across North Texas.
