A constantly running toilet is the most expensive sound a bathroom makes — a bad flapper can silently pass hundreds of gallons a day, and the first notice many homeowners get is a water bill that doubled. The good news: the diagnosis takes three minutes, no tools, and the fix is usually one inexpensive part.
The 3-minute diagnosis
Lift the tank lid. First test: put a few drops of food coloring in the tank, wait ten minutes without flushing. Color in the bowl means water is escaping past the flapper — the rubber seal at the tank's bottom. Second check: look at the thin tube (overflow tube) in the tank's center. If water is trickling into its top, the fill valve is set too high or failing. Third: jiggle the handle — if the running stops, the chain or handle linkage is hanging the flapper open.
Fix 1: the flapper (most common)
Flappers are rubber; chlorinated water hardens and warps them in 3–5 years. Replacement is a few-dollar part: shut the tank's supply valve, flush to empty, unhook the old flapper, clip on the new one, reattach the chain with just a little slack. If the seat under the flapper is rough or pitted with mineral deposits, clean it — a good flapper on a bad seat still leaks.
Fix 2: the fill valve
If water spills into the overflow tube, adjust the fill level first (a screw or clip on the valve lowers it — the waterline should sit about an inch below the tube's top). If the valve won't hold its setting, hisses constantly, or cycles on and off through the day ('ghost flushing' is the flapper; steady hissing is the valve), replace it. Modern fill valves cost little and install in fifteen minutes for a confident DIYer.
When it's worth a service call
Corroded tank bolts, a cracked overflow tube, a flush valve seat that no flapper will seal against, or shutoff valves seized open — the moment the simple parts don't solve it, or the shutoff won't shut, the job changes character. Also worth a call: toilets that run and rock, or run and leak at the base, which point beyond the tank. Lumin Drain NJ repairs and replaces toilets across New Jersey with flat upfront pricing — 929-662-1816 — and yes, we'll happily tell you over the phone if it sounds like a DIY flapper.