A toilet that keeps running is almost always caused by one of four things: a worn flapper, a float set too high, a faulty fill valve, or a tangled lift chain. The good news — every one of these is a beginner-level fix. You likely won't need a plumber.
Likelihood: Most common — 60% of cases
The rubber flapper at the bottom of the tank deteriorates over time and no longer seals properly, letting water trickle into the bowl continuously.
Fix: Replace the flapper ($5–10 at any hardware store). No tools required.
Likelihood: Common — 25% of cases
The float ball or cup controls when the fill valve shuts off. If set too high, water spills into the overflow tube constantly.
Fix: Bend the float arm down or turn the adjustment screw to lower the water level 1 inch below the overflow tube.
Likelihood: Less common — 10% of cases
The fill valve refills the tank after each flush. A worn or dirty fill valve may not shut off completely, causing a hissing sound.
Fix: Replace the fill valve ($10–20). Universal valves fit most toilets.
Likelihood: Less common — 5% of cases
If the chain between the flush handle and flapper is caught or has too little slack, the flapper can't seal fully.
Fix: Untangle the chain and adjust so there's about 1/2 inch of slack when the flapper is closed.
Remove the tank lid and set it safely aside. Flush the toilet and watch: if you can stop the running by pressing down on the flapper, replace it. If pressing down on the float arm stops it, adjust the float. If water is overflowing the tube, lower the float.
Turn off the shutoff valve behind the toilet. Flush to drain the tank. Unhook the old flapper from the overflow tube ears and disconnect the chain. Snap the new flapper onto the ears and reconnect the chain with 1/2 inch of slack.
Tip: Match the flapper to your toilet brand for best results. Fluidmaster 501 or Korky work on most brands.
For ball float: bend the arm gently downward so water shuts off 1 inch below the overflow tube. For cup float: pinch the adjustment clip and slide it down the fill valve shaft, or turn the adjustment screw counterclockwise.
Shut off water and flush the tank empty. Disconnect the supply line. Unscrew the locknut under the tank and pull the old valve out. Insert the new valve, set to 1 inch below overflow, tighten the locknut, reconnect supply line, turn water back on.
Tip: Have a towel handy — a cup or two of water will remain in the tank lines.
Turn the water back on and let the tank fill completely. Flush twice and confirm the running stops within 30 seconds each time. Put food coloring in the tank — if it seeps into the bowl without flushing, the flapper still needs replacing.
This is called phantom flushing or ghost flushing. It's almost always a leaky flapper allowing water to slowly seep into the bowl. When the tank level drops enough, the fill valve kicks on to refill it. Replace the flapper — typically a $7 fix.
Check whether water is flowing over the top of the overflow tube. If so, your float is set too high. Also listen for a hissing sound from the fill valve — a worn diaphragm inside the valve can cause continuous flow even when the float is correctly positioned.
A continuously running toilet typically wastes 200–400 gallons per day — adding $50–150 to your monthly water bill. This makes it worth fixing promptly even if the sound doesn't bother you.
Sometimes. If the chain is too short (flapper can't fully close) or tangled, adjusting it may solve the problem. But if the flapper rubber itself is cracked or mineralized, you'll need to replace it — chain adjustments won't help.
No — a running toilet is not an emergency and won't cause water damage. But it is wasting water continuously, so fix it within a day or two. The total repair cost is almost always under $25 and takes under 30 minutes.
Call a licensed plumber if: (1) the toilet runs after replacing all tank parts, (2) you notice cracks in the porcelain tank, or (3) the shutoff valve behind the toilet is stuck or leaking.
ChatDIY can adapt this diagnosis to your exact fixture, tools, photos, and skill level.