How to Fix a Running Toilet
Published February 18, 2025 · Updated March 1, 2026
A constantly running toilet wastes 200 gallons of water per day — enough to add $50-100 to your monthly water bill. Over 80% of running toilets are caused by a worn flapper, a $5 part you can replace in 10 minutes.
What You'll Need
🛠 Tools
📦 Materials
Step-by-Step Instructions
Diagnose the Problem
Drop 10-15 drops of food coloring into the toilet tank. Wait 15 minutes without flushing. If color bleeds into the bowl, the flapper is leaking. Listen for sounds: a hissing or sloshing sound usually means a faulty fill valve; quiet color bleeding into the bowl is a faulty flapper.
Most running toilets (over 80%) have a worn flapper as the cause. Start there before considering other repairs.
Turn Off Water and Empty the Tank
Locate the shut-off valve on the wall behind the toilet at floor level and turn it clockwise until it stops. Flush the toilet to empty the tank. Remove the tank lid and set it somewhere safe — they're surprisingly fragile.
Inspect and Replace the Flapper
The flapper is the rubber seal at the bottom of the tank that covers the flush valve opening. Lift it — if it feels stiff, crumbly, warped, or shows mineral deposits, replace it. Unhook the chain from the flush handle arm, slide the flapper ears off the overflow tube, and take it to the hardware store for an exact or universal match.
When installing a new flapper, the chain should have just enough slack that the flapper seats fully, but no more than 1/2 inch of extra chain length. Too much slack prevents a complete seal; too little keeps the flapper from opening fully when you flush.
Adjust the Fill Valve Water Level
Look at the water level in the tank. It should sit 1 inch below the top of the overflow tube (the tall center pipe). If water is flowing over or nearly over the overflow tube, the fill valve needs adjustment. Find the adjustment screw or pinch clip on the fill valve body and lower the float setting until the level is correct.
If the fill valve still runs after adjustment, or if it's visibly cracked or corroded, replace the entire fill valve. Replacement valves cost $10-15 and take 20 minutes to install. A faulty fill valve that runs continuously wastes even more water than a bad flapper.
Check the Overflow Tube Height
Water draining directly into the overflow tube causes a quiet, continuous trickle sound. The top of the overflow tube should be at least 1 inch below the fill valve's critical level mark (usually marked "CL" on the valve). If needed, cut the overflow tube shorter with a hacksaw.
Test with Food Coloring Again
Turn the water supply back on and let the tank fill completely. Add food coloring to the tank and wait 15 minutes. If no color appears in the bowl, the repair was successful. Flush and listen — the tank should refill within 30-60 seconds and stop completely.
If color still bleeds into the bowl, the flapper seat (the ring the flapper presses against) may be pitted or corroded. Try cleaning the seat with a wet cloth. If still leaking, a new flapper seat insert can be pressed over the old one.
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