How to Fix a Running Toilet: Flapper, Fill Valve, and Float Repairs

A running toilet is one of the most wasteful household problems — a toilet that runs continuously can waste 200 gallons of water per day, adding significantly to your water bill. The good news is that a running toilet is almost always caused by one of three components: the flapper, the float, or the fill valve. All three are inexpensive and easy to replace without a plumber. This guide covers how to diagnose which component is causing the problem and how to fix it.

Understanding How a Toilet Tank Works

Before diagnosing the problem, it helps to understand the three key components in the tank:

• Flapper — A rubber seal at the bottom of the tank that covers the flush valve opening. When you flush, the flapper lifts to allow water to flow into the bowl. After flushing, it drops back down to seal the tank so it can refill. A worn or warped flapper doesn't seal properly, allowing water to continuously leak from the tank into the bowl.
• Float — A device that rises with the water level in the tank and signals the fill valve to shut off when the tank is full. If the float is set too high, the water level rises above the overflow tube and drains continuously. If the float is damaged, it may not rise properly and the fill valve never shuts off.
• Fill valve (ballcock) — The valve that refills the tank after flushing. If the fill valve is worn or damaged, it may not shut off completely even when the float signals it to, causing a continuous trickle of water.

Diagnosing the Problem: Three Simple Tests

Test 1: The food coloring test (for flapper leaks)
Add a few drops of food coloring to the tank water. Wait 15 minutes without flushing. If colored water appears in the bowl, the flapper is leaking — water is seeping from the tank into the bowl through a faulty seal.

Test 2: Check the overflow tube (for float issues)
Remove the tank lid and look at the overflow tube — the tall tube in the center of the tank. If water is flowing into the overflow tube (you'll see water running down into it), the float is set too high or is damaged, causing the tank to overfill.

Test 3: Listen and watch after flushing (for fill valve issues)
Flush the toilet and watch the fill valve. The tank should fill and the fill valve should shut off completely when the water reaches the correct level. If the fill valve continues to run after the tank is full, or makes a hissing sound, the fill valve needs adjustment or replacement.

What You'll Need

• Universal flapper replacement — Fluidmaster 502P21 Universal Flapper (2-pack) — fits most toilet brands and models. The most common running toilet fix. Replaces in under 10 minutes with no tools required.
• Fill valve replacement kit — Fluidmaster 400A Fill Valve — the most widely used toilet fill valve replacement. Adjustable for any tank height, fits virtually all toilets, and includes all hardware needed for installation.
• Complete toilet repair kit — Fluidmaster 400CRP14 Complete Toilet Repair Kit — includes fill valve, flapper, and supply line. The best value if multiple components need replacement or if the toilet is old and all components are worn.
• Adjustable wrench — For disconnecting the water supply line during fill valve replacement.
• Sponge and bucket — For removing remaining water from the tank during fill valve replacement.

Fix 1: Replace the Flapper (Most Common Fix)

Flapper replacement is the most common running toilet repair — flappers wear out every 3–5 years and are the cause of most toilet leaks. The repair takes under 10 minutes and requires no tools.

Step 1: Turn off the water supply. Turn the shut-off valve behind the toilet clockwise until it stops. Flush the toilet to empty the tank.

Step 2: Remove the old flapper. The flapper is attached to the overflow tube by two ears (tabs) that hook onto pegs on either side of the flush valve. Unhook the ears from the pegs and disconnect the chain from the flush handle arm. Remove the old flapper.

Step 3: Inspect the flush valve seat. Run your finger around the flush valve seat — the ring the flapper seals against. If it feels rough, pitted, or corroded, the seat may need to be cleaned with fine steel wool or replaced. A damaged seat will cause even a new flapper to leak.

Step 4: Install the new flapper. Hook the ears of the new Fluidmaster flapper onto the pegs on either side of the flush valve. Connect the chain to the flush handle arm, leaving about 1/2 inch of slack. Too much slack and the flapper won't lift fully when flushed; too little and the flapper won't seal completely after flushing.

Step 5: Turn on the water and test. Turn the water supply back on and allow the tank to fill. Flush several times and check that the flapper seals completely after each flush. Repeat the food coloring test to confirm the leak is fixed.

Fix 2: Adjust the Float (For Overflow Tube Running)

If water is running into the overflow tube, the float is set too high. The water level should be about 1 inch below the top of the overflow tube.

For ball float (older toilets): The float is a ball on the end of a metal or plastic arm. Bend the arm slightly downward to lower the float position, which lowers the water level at which the fill valve shuts off. Alternatively, turn the adjustment screw on the fill valve if one is present.

For cup float (modern fill valves): The float is a cup or cylinder that slides up and down the fill valve shaft. Pinch the clip on the float and slide it down the shaft to lower the water level. The adjustment is usually marked on the fill valve shaft.

Target water level: The water level should be 1 inch below the top of the overflow tube, or at the water line marked on the inside of the tank. Adjust and flush several times to confirm the level is correct.

Fix 3: Replace the Fill Valve (For Persistent Running)

If adjusting the float doesn't stop the running, or if the fill valve makes a hissing or whistling sound after the tank fills, the fill valve itself needs replacement. Fill valve replacement takes about 30 minutes.

Step 1: Turn off the water supply and flush. Turn the shut-off valve clockwise to close. Flush to empty the tank. Use a sponge and bucket to remove any remaining water from the tank.

Step 2: Disconnect the supply line. Use an adjustable wrench to disconnect the water supply line from the bottom of the fill valve (the fitting on the outside bottom of the tank). Have a towel ready — some water will drip.

Step 3: Remove the old fill valve. Inside the tank, hold the fill valve body steady while unscrewing the locknut on the outside bottom of the tank counterclockwise. Once the locknut is removed, lift the old fill valve out of the tank.

Step 4: Install the new fill valve. Insert the new Fluidmaster 400A fill valve into the tank opening. Adjust the height according to the instructions — the critical fill line mark on the valve should be at least 1 inch above the overflow tube. Hand-tighten the locknut on the outside bottom of the tank, then tighten an additional 1/2 turn with the wrench. Don't overtighten — the tank can crack.

Step 5: Connect the refill tube. Connect the small refill tube from the fill valve to the overflow tube using the clip provided. This tube refills the bowl after flushing.

Step 6: Reconnect the supply line and turn on water. Reconnect the supply line and turn the water supply back on. Allow the tank to fill and check for leaks at the supply line connection and the fill valve locknut.

Step 7: Adjust the water level. Adjust the float to set the water level 1 inch below the top of the overflow tube. Flush several times and confirm the fill valve shuts off completely after each fill.

When to Replace Everything: The Complete Kit

If your toilet is more than 10 years old and running, all three components — flapper, fill valve, and supply line — are likely near the end of their lifespan. Replacing all three at once with the Fluidmaster Complete Toilet Repair Kit is more cost-effective than replacing components one at a time as they fail, and ensures the toilet runs reliably for another decade.

Pro Tips

Don't overtighten the fill valve locknut. The toilet tank is porcelain and can crack from overtightening. Hand-tight plus 1/2 turn with a wrench is sufficient.

Check the chain length after flapper replacement. The chain between the flapper and the flush handle arm should have about 1/2 inch of slack. Too much slack and the flapper won't lift fully; too little and it won't seal after flushing.

Replace the supply line while you're in there. Toilet supply lines are inexpensive and fail without warning, causing significant water damage. If the supply line is more than 5–7 years old, replace it during any toilet repair.

Test with food coloring after any repair. The food coloring test confirms the flapper is sealing completely. A toilet can appear to have stopped running while still leaking slowly — the food coloring test catches slow leaks that aren't audible.

Frequently Asked Questions

How much water does a running toilet waste?
A toilet with a leaking flapper can waste 200–400 gallons of water per day. A toilet running into the overflow tube wastes even more. Fixing a running toilet typically saves $50–$200 per month on water bills depending on local water rates.

How do I know if my flapper needs replacing?
The food coloring test is the most reliable method. If colored water appears in the bowl within 15 minutes without flushing, the flapper is leaking. Flappers typically need replacement every 3–5 years.

Can I fix a running toilet without turning off the water?
For flapper replacement, you can work quickly without turning off the water, but it's messier and harder to control. Always turn off the water supply for fill valve replacement.

My toilet runs for a few seconds after flushing, then stops — is that normal?
Yes — the fill valve runs until the tank refills, which takes 1–2 minutes. If the toilet runs continuously without stopping, or runs intermittently throughout the day without being flushed, that's the problem this guide addresses.

I replaced the flapper but the toilet is still running — what next?
Check the flush valve seat for damage (rough or pitted surface that prevents the new flapper from sealing). If the seat is damaged, it needs to be replaced or the entire flush valve assembly needs replacement. If the seat is smooth, check the float and fill valve as described above.

Final Thoughts

A running toilet is almost always a DIY repair — the three components that cause 95% of running toilet problems (flapper, float, fill valve) are all inexpensive and straightforward to replace. Start with the flapper (the most common cause), confirm with the food coloring test, and work through the fill valve and float adjustments if needed. For older toilets, the Fluidmaster Complete Repair Kit replaces everything at once for lasting reliability. The water savings from fixing a running toilet typically pay for the repair kit within the first week.