How to Fix a Running Toilet
This guide covers diagnosing and fixing a running toilet — flapper replacement, fill valve adjustment, float calibration, and flush valve seat repair for all tank types.
A running toilet wastes 200–1,000 gallons of water per day. The cause is almost always one of three components — the flapper, the fill valve, or the float. Identify which before buying parts.
The Three-Part Diagnosis
| Symptom | Cause | Fix |
|---|---|---|
| Food dye appears in bowl after 10 min | Flapper not sealing | Replace flapper ($7–$10) |
| Water at or above overflow tube rim | Float set too high / fill valve worn | Adjust float or replace fill valve ($12–$18) |
| Chain caught under flapper | Chain too long or slack | Adjust chain to ½ inch slack |
| New flapper still leaks | Flush valve seat damaged | Clean seat or install seat repair ring |
What You Will Need
- Adjustable pliers (channel-lock, 10-inch)
- Sponge and small bucket
- Food coloring for dye test
- Replacement flapper — Fluidmaster 502P or brand-matched equivalent
- Fill valve — Fluidmaster 400A if needed
- Complete rebuild kit — Fluidmaster 400CRP for toilets over 10 years old
The Repair Steps
Turn the oval valve on the wall behind the toilet clockwise until fully closed. Flush once to empty the tank. Sponge out the remaining 1–2 inches of water. If the shut-off valve is frozen or leaking, stop and address that first — do not attempt tank work with no way to stop the water.
Unhook the flapper ears from the flush valve pegs. Disconnect the chain. Take the old flapper to the hardware store for an exact match if the brand is unclear. Attach the new flapper, reconnect the chain with ½ inch of slack. Turn on water, flush twice, run the dye test again.
On float cup fill valves (the standard modern type), turn the adjustment screw clockwise to lower the water level. Target: 1 inch below the top of the overflow tube. Flush and allow to refill. Repeat until the valve shuts off cleanly at the correct level.
Disconnect the refill tube. Unscrew the lock nut at the tank bottom with channel-lock pliers — hand tight plus one quarter-turn only, the nut is plastic. Install Fluidmaster 400A per instructions. Reconnect refill tube and supply line. Turn on water and set the level.
Run a fingertip around the seat rim. Any nick, groove, or mineral scale allows water past a perfect flapper. Clean with a fine abrasive pad while the tank is dry. Significant damage requires a seat repair ring or full flush valve replacement.
Turn on the supply valve, allow the tank to fill, and run the dye test for 10 minutes. No color in the bowl confirms the repair. Inspect the floor around the toilet for any drips from disturbed supply connections.
Common Mistakes
- Buying a universal flapper for a TOTO or Kohler — these use proprietary seat geometry
- Overtightening the fill valve lock nut — hand tight plus ¼ turn is enough
- Setting the water level too low — weak flushes waste more water than the original leak
- Replacing the flapper without inspecting the seat for grooves or mineral scale