Short answer: Lift the tank lid and look. If water is spilling into the overflow tube, the fill valve or float is set too high. If not, the flapper is leaking water into the bowl. The EPA recommends a simple dye test: a few drops of food coloring in the tank, wait 10 minutes, and color in the bowl means a leak.
Quick diagnosis
| What you notice | Most likely cause | What to check | Call a pro? |
|---|---|---|---|
| Water spills into the overflow tube | Float set too high or fill valve failing | Lower the float; see if it stops | If the valve keeps running |
| Refills by itself every so often (phantom flush) | Leaking flapper | Dye test | Rarely |
| Runs after you changed the flapper | Wrong size flapper, chain too tight | Check the chain slack and flapper size | Sometimes |
| Runs even with the float up | Fill valve seal worn | Replace the fill valve | If not comfortable |
| Hissing and never fills fully | Fill valve or low supply | Check the supply valve is fully open | Sometimes |
Do the dye test
The U.S. EPA’s WaterSense program says to put a few drops of food coloring in the tank, wait 10 minutes without flushing, and check the bowl. If color appears, the toilet is leaking. Flush right away so the tank does not stain.
Fix a leaking flapper
The EPA notes that a worn flapper can leak thousands of gallons a year without obvious signs, and recommends checking it periodically and replacing it at least every five years. Take the old flapper (or a photo) to the store to match the size, and leave a little slack in the chain.
Adjust or replace the fill valve
The water level should sit about an inch below the top of the overflow tube. Adjust the float (a screw on a float arm, or a clip on a cup-style float). If water still runs into the overflow with the float lowered, the fill valve is worn and should be replaced.
When to call a professional
- The tank or bowl is cracked, or water leaks at the floor.
- The shutoff valve under the tank will not close or leaks.
- You replaced the parts and it still runs.
What it usually costs
A flapper or fill valve costs a few dollars to about $25 at a hardware store. HomeGuide (2026) lists toilet repair by a plumber at about $100 to $300.
Frequently asked questions
How much water does a running toilet waste?
It can waste thousands of gallons a year. The EPA counts toilet leaks among the most common household leaks.
Why does my toilet run on and off by itself?
A leaking flapper lets the tank slowly drain into the bowl until the fill valve turns on to refill it.
Related problems
Sources
- U.S. EPA WaterSense: Fix a Leak Week
- Minnesota Pollution Control Agency: Toilet leak dye test card (PDF)
- HomeGuide: Plumber cost per hour (2026)
This guide is general information, not a substitute for an inspection by a licensed professional. See our disclaimer.