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

A running toilet is one of the most common plumbing problems in any home, and it is also one of the easiest to fix yourself. That constant sound of water trickling into the bowl is more than annoying — it can waste 200 gallons of water per day and add $50 or more to your monthly water bill. The good news is that the repair almost always involves one of three inexpensive parts, and you can handle it in under 30 minutes with no special tools.

This guide walks you through diagnosing which part is causing the problem and replacing it step by step.

Always shut off the water supply before working on toilet components. If you are unsure about any step, consult a licensed plumber.

Quick Overview

Detail	Info
Difficulty	Beginner
Time	15–30 minutes
Cost	$5–$25
Tools needed	Adjustable pliers, towel, bucket
Parts you may need	Flapper, fill valve, or flush valve seal

How a Toilet Works (60-Second Primer)

Understanding the three key components inside your tank makes diagnosis straightforward:

Flapper — A rubber seal at the bottom of the tank. When you flush, the flapper lifts to let water rush into the bowl, then drops back down to seal the tank so it can refill.
Fill valve — The tall assembly on the left side of the tank. It refills the tank after a flush and shuts off when the water reaches the correct level.
Overflow tube — The vertical tube in the center of the tank. If the water level rises too high, water spills into this tube and drains into the bowl as a safety measure.

A running toilet means water is continuously flowing from the tank into the bowl (or down the overflow tube). The cause is almost always one of these three parts.

Step 1: Diagnose the Problem

Remove the tank lid and set it on a towel on the floor (porcelain lids are heavy and crack easily).

Test A: The Flapper Test

Add 5–10 drops of food coloring to the tank water. Wait 15 minutes without flushing. If the colored water appears in the bowl, your flapper is leaking and needs to be replaced.

Test B: The Water Level Test

Look at the water level in the tank relative to the overflow tube. If water is flowing over the top of the overflow tube, the fill valve is not shutting off at the correct level.

Test C: The Fill Valve Test

Flush the toilet and watch the fill valve as the tank refills. If it hisses, vibrates, or never fully shuts off even when the water level is below the overflow tube, the fill valve needs to be replaced.

Step 2: Fix a Leaking Flapper

This is the most common cause — about 80 percent of running toilets are caused by a worn flapper.

Tools and Materials

Replacement flapper (universal flappers fit most toilets — bring the old one to the hardware store to match the size)
Towel

Instructions

Shut off the water supply by turning the valve behind the toilet clockwise until it stops.
Flush the toilet to empty the tank.
Remove the old flapper. Unhook the flapper ears from the pegs on the overflow tube. Disconnect the chain from the flush lever arm.
Inspect the flush valve seat (the ring the flapper sits on). Run your finger around it. If it feels rough or corroded, clean it with an emery cloth or fine sandpaper. A smooth seat is essential for a good seal.
Install the new flapper. Hook the ears over the pegs on the overflow tube. Connect the chain to the flush lever arm, leaving about half an inch of slack.
Turn the water back on and let the tank fill.
Test. Repeat the food coloring test to confirm the leak is fixed.

Pro tip: If you have replaced the flapper and it still leaks, the flush valve seat may be too corroded to seal. In that case, install a flush valve seat repair kit (about $10) that fits over the existing seat, or replace the entire flush valve.

Step 3: Adjust the Water Level

If the water level is too high, water continuously spills into the overflow tube. The fix depends on your fill valve type.

Float Ball (Older Toilets)

The float ball is a rubber or plastic ball on a metal arm. To lower the water level:

Gently bend the float arm downward about half an inch.
Flush and check. The water should stop about one inch below the top of the overflow tube.

Float Cup (Modern Toilets)

The float cup slides up and down on the fill valve shaft. To adjust:

Find the adjustment screw or clip on the fill valve.
Turn the screw counterclockwise (or squeeze the clip and slide the cup down) to lower the water level.
Flush and check. Target one inch below the overflow tube.

Step 4: Replace the Fill Valve

If the fill valve is worn out and will not shut off regardless of the water level, replace the entire unit.

Tools and Materials

Universal fill valve kit ($8–$15)
Adjustable pliers
Towel and bucket

Instructions

Shut off the water supply and flush to empty the tank.
Disconnect the water supply line from the bottom of the fill valve. Place a bucket and towel underneath to catch residual water.
Remove the old fill valve. Unscrew the mounting nut on the outside bottom of the tank. Lift the old valve out.
Adjust the new valve height. Most universal fill valves are adjustable — set it so the critical level mark (CL) on the valve sits one inch above the overflow tube.
Insert the new valve through the hole in the tank bottom. Hand-tighten the mounting nut, then snug it a quarter turn with pliers. Do not overtighten.
Reconnect the water supply line.
Attach the refill tube. Clip the small refill tube to the top of the overflow tube. The tube should not extend below the top of the overflow tube.
Turn the water on and check for leaks at the supply connection and mounting nut.
Adjust the water level using the adjustment screw on the new valve until the water stops one inch below the overflow tube.

Troubleshooting

Symptom	Likely Cause	Fix
Toilet runs intermittently (phantom flush)	Slow flapper leak	Replace flapper
Toilet runs constantly	Flapper, fill valve, or water level too high	Run diagnostic tests above
Hissing sound from tank	Fill valve debris or wear	Clean or replace fill valve
Water on floor around base	Wax ring seal, not a running toilet issue	See home plumbing guide

When to Call a Professional

Most running toilet repairs are straightforward, but call a licensed plumber if:

The toilet continues running after you have replaced both the flapper and fill valve
You notice cracks in the porcelain tank
The shutoff valve behind the toilet is stuck or leaking
You are uncomfortable working with plumbing connections

For help finding a qualified plumber in your area, see our guide on how to find a reliable handyman. You can also compare costs in our home repair cost guide.

Frequently Asked Questions

How much water does a running toilet waste? A running toilet can waste 200 gallons or more per day — that is over 6,000 gallons per month. Fixing it promptly saves both water and money.

Can I use a universal flapper on any toilet? Universal flappers fit most standard toilets, but some brands (especially Kohler and American Standard) use proprietary sizes. Bring your old flapper to the store or note the toilet model number to get the right fit.

Is a running toilet an emergency? It is not a plumbing emergency, but it wastes a significant amount of water and money. Fix it as soon as practical.

This article is for informational purposes only. Always verify local building codes and consult a licensed professional if you are unsure about any repair.