A toilet that won't stop running is the most common no-emergency call we get. The good news: 80% of the time, it's a $5–$15 fix you can do yourself in under 15 minutes. Here's the diagnostic, in the order we run it.
First: confirm what "running" actually means
Pop the lid off the tank and watch. You'll see one of three things:
- Water trickling from the tank into the bowl — flapper issue
- Water dribbling down through a tube into the overflow — fill valve set too high, or fill valve failing
- Tank refilling repeatedly without flush — silent leak past the flapper, refilling automatically
The most common cause: bad flapper
The flapper is the rubber disk at the bottom of the tank. If it's not sealing, water leaks from tank to bowl, the fill valve refills, and the cycle repeats — forever.
Test: drop a few drops of food coloring into the tank. Wait 15 minutes without flushing. If color appears in the bowl, your flapper is leaking.
Fix: Shut off the supply (knob behind the toilet), flush to drain, unhook the chain, pop the old flapper off the pegs, snap a new one on, reattach chain with about 1/2" of slack. $4 part. Total time: 5 minutes.
Second-most common: float / fill valve issue
If the water level in the tank is hitting the top of the overflow tube and dribbling down, the fill valve is set too high (or sticking).
Quick fix: bend the float arm down (old-style ball-and-rod), or pinch the clip and slide the float down (modern Fluidmaster). Aim for the water line to be about 1/2"–1" below the top of the overflow tube.
If that doesn't hold, the fill valve itself is failing. New Fluidmaster 400A is $11, takes 10 minutes to swap.
Less common but serious: fill tube wrong
Some installs have the small refill tube (the thin one that runs from the fill valve into the overflow) pushed too far down. This creates a siphon that pulls water from the tank long after the flush is done. Bend it back so the end is above the overflow rim, with about 1" clearance.
When it's not the toilet
Sometimes a "running toilet" isn't the toilet:
- High household water pressure (over 80 psi) makes fill valves chatter and leak. Test pressure with a gauge from any hardware store. If you're over 80 psi, you need a PRV — that's a plumber call.
- Bad shutoff dripping into the tank (rare).
When to call a plumber
- You replaced flapper and fill valve and it's still running
- The tank cracks or you find standing water at the floor
- Water hammer / banging when the toilet finishes filling (suggests a pressure issue)
- More than one toilet in the house has the same problem
Call us — we'll usually have it sorted in under an hour.
