Short answer

A toilet that runs or refills on its own is usually leaking from the tank into the bowl through a worn flapper, which you can confirm with a few drops of food coloring in the tank. Water on the floor comes from the supply line, the tank bolts, condensation, or the wax seal under the toilet. Flappers and fill valves are simple DIY repairs; a leaking base needs the toilet lifted and reset.

Two kinds of toilet leaks

  • Internal leaks: water runs from the tank into the bowl and down the drain. No puddle, but the toilet runs, hisses or refills by itself, and your water bill climbs.
  • External leaks: water on the floor around the toilet.

Internal leaks: the dye test

A silent tank leak can go unnoticed for months.

Parts inside a toilet tankCutaway of a toilet tank and bowl: handle and chain lifting the flapper off the flush valve seat, overflow tube, fill valve with float, refill tube, supply line and shutoff valve at the wall, and the wax ring sealing the toilet to the floor flange.HandleFloatFill valveSupply shutoffChainRefill tubeOverflowFlapperFlush valveWax ring
The parts that cause most running and leaking toilets: the flapper, fill valve and float, and the wax ring at the base.
  1. Take the lid off the tank and add 10 or so drops of food coloring.
  2. Don't flush for 15 to 30 minutes.
  3. Look in the bowl. If color shows up, the tank is leaking into the bowl.

The usual culprits:

The flapper

The rubber flapper at the bottom of the tank seals the flush valve. Over time it warps, hardens or collects mineral buildup, and water seeps past.

  • Test: press down on the flapper with a stick or gloved hand. If the running stops, the flapper is the problem.
  • Fix: turn off the toilet's supply valve, flush to drain the tank, unhook the old flapper and chain, and fit a matching replacement. Take the old one to the store, since sizes and styles vary.

The fill valve and water level

If the water level is set too high, water spills into the overflow tube (the open pipe in the middle of the tank) and runs constantly.

  • Check: the water should stop about an inch below the top of the overflow tube. Most tanks have a line marked inside.
  • Fix: lower the float by turning the adjustment screw or sliding the clip on the fill valve. If the fill valve hisses, keeps running, or won't shut off, replace it. Fill valve kits are designed for homeowners.

The chain

A chain that's too tight holds the flapper open slightly. One that's too long can get caught under it. Leave a little slack.

External leaks: find the source

Dry everything thoroughly with a towel, then wait and watch. Run your hand (or a paper towel) along each part.

Supply line

The flexible line from the wall valve to the bottom of the tank.

  • Dripping at the connections: tighten the plastic nut on the tank a little by hand, and the valve end gently with a wrench.
  • A braided line that's bulging, frayed or old: replace it. Split supply lines cause serious flooding. They're inexpensive and easy to swap.

The shutoff valve

Old multi-turn valves can drip from the stem. Tightening the packing nut behind the handle slightly often stops it. If not, a plumber can replace it with a quarter-turn valve.

Tank bolts and tank-to-bowl gasket

Drips from under the tank where it meets the bowl point to the tank bolts or the gasket between tank and bowl. Tightening the bolts carefully can help. Porcelain cracks easily, so tighten evenly and gently. If the gasket has failed, the tank must come off to replace it.

Condensation

In humid weather, cold water in the tank causes condensation to drip down the outside. If the whole tank is beaded with water and there's no single drip point, it's likely condensation, not a leak.

Base of the toilet (wax ring)

Water seeping from under the toilet, especially when you flush, usually means the wax ring seal between the toilet and the floor drain has failed. Signs include a toilet that rocks, stains or soft flooring around the base, and sewer odor.

  • Don't just caulk around it. Caulk can hide a leak that's rotting the subfloor.
  • Fix: the toilet has to be removed, the old wax cleaned off, a new ring fitted and the toilet reset and secured. It's doable for a confident DIYer, but heavy and awkward, and a damaged flange or subfloor makes it a plumber's job.

A cracked tank or bowl

Hairline cracks in porcelain can leak. There's no reliable permanent repair. Replace the toilet.

What not to do

  • Don't ignore a running toilet. Even a slow internal leak wastes water every hour of the day.
  • Don't overtighten tank bolts or plastic nuts.
  • Don't use in-tank bleach tablets if your manufacturer advises against them; some can degrade flappers and seals.
  • Don't caulk over a base leak.

When to call a plumber

  • Water leaking from the base, especially with a rocking toilet or soft floor
  • A cracked tank or bowl
  • A shutoff valve that won't close or keeps leaking
  • Repeated flapper or fill valve failures despite replacing parts
  • You'd rather not lift a toilet

What to tell the plumber

  • Whether it's running (internal) or leaking on the floor (external)
  • Where the water appears, and whether it's worse when flushing
  • The toilet's age and brand if you know it
  • Whether the floor feels soft

See toilet repair for common repairs and when replacement makes more sense.

Prevention

  • Do the dye test once or twice a year.
  • Replace braided supply lines every several years, or at the first sign of wear.
  • Test the toilet's shutoff valve occasionally so it works when needed.
  • Don't flush wipes or paper towels, which can cause clogs that stress seals.

Frequently asked questions

Why does my toilet run by itself every few minutes?

That's usually a slow leak from the tank into the bowl through a worn flapper. As the tank level drops, the fill valve refills it. Replacing the flapper normally fixes it.

How do I know if my toilet's wax ring is leaking?

Look for water seeping from under the toilet, especially after flushing, plus a toilet that rocks, stains or soft flooring around the base, or a sewer smell. The wax ring can only be replaced by lifting the toilet.

Can a running toilet really raise my water bill?

Yes. A toilet running constantly can waste a large amount of water, and silent tank leaks add up over a billing period. The dye test takes minutes and tells you whether you have one.

Is it safe to use a toilet that's leaking at the base?

It's best to limit use until it's fixed. Each flush can push water, which may contain waste, into the floor and subfloor, causing damage and odors.

Should I repair or replace an old toilet?

Internal parts are cheap to replace, so repair usually makes sense. Replacement is worth considering if the tank or bowl is cracked, it clogs constantly, or it's an older high-volume model.