How to Fix a Toilet That Runs Constantly

A toilet that runs constantly is one of the most wasteful household plumbing problems — it can waste 200 gallons of water per day and add significantly to your water bill. The good news: it's almost always caused by one of two things, and both are easy to fix yourself.

Step 1: Diagnose the Problem

Remove the tank lid and look inside while the toilet is running. There are two possible causes:

Cause A: Worn Flapper

The flapper is the rubber seal at the bottom of the tank that covers the flush valve opening. When you flush, it lifts to let water into the bowl, then drops back down to seal the tank so it can refill.

How to tell: Put a few drops of food coloring in the tank. Wait 15 minutes without flushing. If color appears in the bowl, the flapper isn't sealing — water is leaking through.

Also check: push down on the flapper with your finger. If the running stops, the flapper is the problem.

Cause B: Faulty Fill Valve

The fill valve controls water flow into the tank after flushing. If it doesn't shut off properly, water keeps running into the tank and overflows into the bowl through the overflow tube.

How to tell: Look at the overflow tube (the tall tube in the center of the tank). If water is flowing into it, the fill valve isn't shutting off — the water level is too high.

Also check: bend the float arm down slightly. If the running stops, the float is set too high.

Fix A: Replace the Flapper

👉 Universal toilet flapper

Turn off the water supply valve (behind/beside the toilet, clockwise to close)
Flush to empty the tank
Unhook the old flapper from the two ears on the overflow tube
Disconnect the chain from the flush handle arm
Snap the new flapper onto the overflow tube ears
Connect the chain to the handle arm — leave about ½ inch of slack (too tight = flapper won't seal; too loose = flapper won't lift)
Turn water back on and test

Fix B: Replace the Fill Valve

👉 Fill valve and flapper repair kit — replace both at once for a complete fix

Turn off the water supply valve
Flush to empty the tank
Use a sponge to remove remaining water from the tank
Disconnect the supply line from the bottom of the tank
Unscrew the locknut holding the fill valve to the tank (counterclockwise from below)
Pull out the old fill valve
Install the new fill valve — adjust the height per the instructions
Reconnect the supply line and refill tube
Turn water back on, let tank fill, and adjust the water level if needed

Pro Tip: Replace Both at Once

If your toilet is more than 5 years old and running constantly, replace both the flapper and fill valve at the same time. The fill valve and flapper kit includes both — takes 30 minutes and eliminates the most common causes of a running toilet in one repair.

Bottom Line

A constantly running toilet is almost always a worn flapper or faulty fill valve. Do the food coloring test to confirm which one, then replace it. The repair takes under 30 minutes and costs under $20 — and stops wasting hundreds of gallons of water every day.

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