A constantly running toilet isn't a major plumbing issue, but it can be a massive annoyance for any homeowner. Between the incessant noise that echoes throughout the house to the added time it takes to actually flush the toilet after use, this phenomenon is as irritating to live with as it can be to diagnose. Though this problem may start out as just a nuisance, it could cost you big time in the long-run when it comes to utility bills if it is left unfixed.
There are generally three reasons why your toilet will start to run. The first thing you need to do is check your toilet's overflow tube, which is a floating device that catches water when it is backing up into the tank instead of filling up the bowl after being flushed. If this piece is filled, then you can usually assume that the fill valve isn't working and needs to be replaced.
Should the overflow tube and the fill valve be in fine working condition, then take a look at the flapper chain. This piece will control the flapper, which is a device that seals water from the tank from entering the bowl. If the chain is too long or short, it will either prevent the flapper from opening, or keep it ajar when it shouldn't be.
You may actually find that the flapper itself is damaged. Over time, these valves can get warped or crack depending on the age of your toilet, and you may need to replace one outright. A great way to detect this is to put a few drops of food coloring into the toilet and watch to see if the flow of water finds its way toward a specific damaged area of the flap.
While smaller problems like a running toilet may be easy to diagnose, be sure to contact an experienced Washington DC plumber like John C. Flood for larger issues.