Quick answer: A toilet that keeps running is almost always leaking water from the tank into the bowl through a worn flapper, or overfilling into the overflow tube because the fill valve or float is set too high. EPA says a running toilet can waste up to 200 gallons of water a day. Find out which it is with EPA's food coloring test, then replace the flapper, adjust the chain or float, or replace the fill valve. On a septic system, fix it quickly, because that extra water goes straight to your drain field.

How a toilet tank works

When you flush, the flapper at the bottom of the tank lifts and the tank empties into the bowl. The flapper drops back and seals, and the fill valve refills the tank until the float rises to the shut-off point. Any water above the top of the overflow tube (the open pipe in the middle of the tank) drains into the bowl. A toilet runs when either the flapper does not seal or the water never reaches the shut-off point before spilling into the overflow tube.

Find the cause

Take the lid off and look. If water is pouring into the overflow tube, the water level is too high: a fill valve or float problem.
If the water level is below the overflow tube, do EPA's dye test. Put a few drops of food coloring in the tank and wait 10 minutes without flushing. If color shows up in the bowl, the flapper is leaking. Flush afterward so the dye does not stain.
Listen. A toilet that refills on its own every so often, with no one using it, usually has a slow flapper leak.

Common fixes

Flapper. Flappers wear out, warp or collect mineral buildup and stop sealing. EPA recommends replacing them at least every five years. Shut off the water at the valve behind the toilet, flush to empty the tank, unclip the old flapper and take it to the store so you get a match.
Chain. A chain that is too short holds the flapper open; one that is too long can get caught under it. Leave a little slack when the flapper is closed.
Float. If the water rises into the overflow tube, lower the float so the tank stops filling below the top of the tube. Many tanks have a water line marked inside.
Fill valve. If the valve keeps hissing or will not shut off after the float is adjusted, cleaning or replacing it usually solves it.
Flush valve seat. If a new flapper still leaks, the seat it closes on may be cracked or rough, and the flush valve may need replacing.

Why it matters, especially on septic

A running toilet wastes water whether you are on a water district or a private well. On septic, EPA lists a running toilet as one of the leaks to fix to protect the system, because the extra water flows through the tank to the drain field. Too much water can slow treatment and flood the field. See our guides to septic system maintenance and drain field warning signs.

A toilet that leaks silently is also one of the most common hidden household leaks. Our hidden water leak guide covers the water meter test.

Frequently asked questions

How much water does a running toilet waste?

EPA says a running toilet can waste up to 200 gallons of water a day.

How can I stop a running toilet for now?

Turn the shut-off valve behind the toilet clockwise until it stops. Turn it back on to flush, or until the repair is done.

How often should I replace the flapper?

EPA recommends at least every five years, or sooner if the dye test shows a leak.

Sources

U.S. EPA, How to Care for Your Septic System
U.S. EPA WaterSense, Fix a Leak Week

Need help with a running toilet?

Wade's Plumbing & Septic provides toilet repair across Santa Cruz County and nearby Santa Clara County communities. Quotes are free and pricing is flat rate, quoted on site before work begins. Request service or call during office hours, Monday to Friday, 9:00 AM to 5:00 PM.