From the U.S. Environmental Protection Agency (EPA)
If you can hear the water in your toilet making noise, even when no one flushed recently, you have a running toilet that could be wasting 200 gallons of water or more every day. Sometimes you just need to jiggle the handle to fix it, but sometimes a part needs to be replaced.
Many toilet leaks are caused by an old or faulty toilet flapper. Over time, this inexpensive rubber part decays, or minerals build up on it. It’s usually best to replace the whole rubber flapper—a relatively easy, inexpensive do-it-yourself project that pays for itself in no time.