How to Fix a Running Toilet:
Step-by-Step DIY Guide

A running toilet is one of the most common — and most annoying — plumbing problems in any home. It's also one of the most wasteful: a constantly running toilet can waste 200 gallons of water per day, which adds up to thousands of dollars annually on your water bill. The good news is that 95% of running toilets are caused by one of three components, all of which are cheap to replace and straightforward to fix.

Understanding How a Toilet Works

Before diagnosing anything, understand the three key components inside the tank:

• Flapper valve: The rubber disc at the bottom of the tank that seals water in the tank between flushes. When you push the handle, the flapper lifts, releasing water into the bowl.
• Fill valve: The tall mechanism on the left side of the tank that refills the tank after each flush. It controls water flow and shuts off when the tank reaches the correct level.
• Overflow tube: The vertical tube in the center of the tank. If the water level rises too high, excess water flows down this tube into the bowl — preventing the tank from overflowing.

A "running toilet" means water is continuously flowing from the tank into the bowl, causing the fill valve to keep running. The cause is always one of these three components.

Diagnosis: Finding the Cause

The Food Coloring Test

Remove the tank lid and add 5–10 drops of food coloring to the tank water. Wait 15–20 minutes without flushing. If colored water appears in the bowl, water is leaking past the flapper — the flapper is your problem.

Check the Water Level

Look inside the tank. Is the water level at or above the top of the overflow tube? If water is flowing into the overflow tube, the fill valve is allowing the tank to overfill. The fill valve needs adjustment or replacement.

Listen Carefully

If the toilet runs intermittently (cycles on for a few seconds, then stops, then runs again — sometimes called "phantom flushing"), the flapper is slowly leaking. If the toilet runs constantly and you can hear the fill valve hissing, the fill valve or float may be the problem.

Fix #1: Replace the Flapper Valve (Most Common — 70% of Cases)

The flapper is the single most common cause of a running toilet. Flappers are made of rubber and degrade over time — they become stiff, warped, or develop mineral deposits on the sealing surface. Chlorine in municipal water accelerates this degradation.

What You'll Need

• Replacement flapper ($3–$8 at any hardware store)
• That's it — no tools required

Step-by-Step

1. Turn off the water supply. Rotate the shut-off valve (usually located on the wall behind the toilet, near the floor) clockwise until it stops. Flush the toilet to drain the tank.
2. Remove the old flapper. Unhook the flapper ears from the pegs on either side of the overflow tube. Disconnect the chain from the flush handle lever.
3. Take the old flapper to the hardware store. Not all flappers are the same size. Bringing the old one ensures you get the right match. Or note your toilet brand and model (usually stamped inside the tank lid).
4. Install the new flapper. Hook the ears onto the overflow tube pegs. Connect the chain to the flush handle lever, leaving about ½ inch of slack in the chain (too tight and the flapper won't seal; too loose and the handle won't lift it enough).
5. Turn the water back on. Let the tank fill, then flush. Watch the flapper seat itself — it should create a solid seal. Do the food coloring test again to confirm.

Fix #2: Adjust or Replace the Fill Valve

If the water level in the tank is rising above the overflow tube, the fill valve isn't shutting off at the right level. This can be caused by a stuck float, a worn fill valve, or incorrect adjustment.

Adjusting the Float Level

The water level should be about ½ inch below the top of the overflow tube. Adjusting the float changes where the fill valve shuts off:

• Ball-cock float (older toilets): Bend the metal arm slightly downward to lower the water level, or adjust the screw at the top of the fill valve.
• Cup-style float (modern toilets): Squeeze the clip on the adjustment rod and slide the float down to lower the water level.

Replacing the Fill Valve

If adjustment doesn't work, the fill valve itself is worn and needs replacement. Universal fill valves (like the Fluidmaster 400A) fit most toilets and cost $8–$15.

1. Turn off water and flush to drain the tank. Use a sponge or towel to remove remaining water.
2. Place a towel under the tank. Disconnect the water supply line from the bottom of the fill valve (below the tank).
3. Remove the locknut securing the old fill valve to the tank bottom. Lift out the old valve.
4. Adjust the new fill valve to the correct height (follow the product instructions — it should be set so the critical level mark is at least 1 inch above the overflow tube).
5. Insert the new fill valve through the tank hole, hand-tighten the locknut underneath, and reconnect the water supply.
6. Turn on the water and adjust the float to set the correct water level (½ inch below overflow tube top).
7. Connect the refill tube to the overflow tube per the instructions.

Fix #3: Replace the Overflow Tube / Flush Valve Assembly

If the overflow tube itself is cracked or damaged, water will continuously drain from the tank. This is the least common cause but does happen, especially in older toilets. Replacing the overflow tube requires removing the tank from the bowl — a more involved repair but still doable for a confident DIYer.

A complete flush valve assembly (includes the overflow tube, flapper seat, and mounting hardware) costs $15–$30. If you're uncomfortable removing the tank, this is a good job for a handyman or plumber.

When to Call a Professional

Most running toilet repairs are straightforward DIY projects. But consider calling a plumber or handyman when:

• The toilet is very old and replacement parts aren't readily available
• You need to replace the flush valve assembly (tank removal required)
• The issue persists after replacing the flapper and fill valve
• There's a leak at the base of the toilet or between the tank and bowl
• You're not comfortable working with water supply connections

Find a local handyman or plumber through our directory of verified handyman professionals. Running toilet repair is a standard service call — most can fix it in 30 minutes or less.

Cost Summary

• DIY flapper replacement: $3–$8 (parts only)
• DIY fill valve replacement: $8–$15 (parts only)
• Professional service call: $75–$200 (includes parts and labor)
• Water wasted by running toilet: Up to 200 gallons/day = $50–$100+/month

Whether you do it yourself or hire a pro, fixing a running toilet quickly pays for itself in water savings. If you're a handyman or plumber looking for local service calls, list your business with National Handyman Connect.