Why your toilet keeps running (and what actually fixes it)

A running toilet is the most expensive small problem in a house. A worn flapper can push a thousand gallons a week past the tank and straight down the drain, and you pay for every one of them.

Published 2026-08-30 by Floriano Plumbing Service · Serving all of Connecticut · (888) 809-6756

The three parts that fail, in order of likelihood

Nearly every running toilet comes down to one of three components inside the tank.

1. The flapper. A rubber seal at the bottom of the tank. It hardens, warps or gets a mineral crust and stops sealing. This is the cause in most cases, and it is the cheapest to fix.
2. The fill valve. The tall assembly that refills the tank. When it fails to shut off cleanly you get a constant trickle or a cycle that restarts every few minutes.
3. The flush valve or overflow tube height. If the water level is set above the overflow tube, water pours down it continuously. Nothing is broken - it is set wrong.

The two-minute dye test

This tells you whether the flapper is leaking without taking anything apart.

1. Take the tank lid off and set it somewhere flat.
2. Add a few drops of food colouring to the tank water. Do not flush.
3. Wait fifteen minutes and look in the bowl.
4. Colour in the bowl means water is passing the flapper. That is your culprit.

If the bowl stays clear but the tank still refills on its own, the fill valve is the more likely suspect.

What you can do yourself

Flapper replacement is genuinely a homeowner job on most toilets. Shut the supply at the angle stop behind the bowl, flush to empty the tank, unclip the old flapper, clip the new one on and reconnect the chain with about half an inch of slack. Chain too tight and it holds the flapper open - which is the same symptom you started with.

Adjusting the water level is easier still: most modern fill valves have a clip or a screw on the float. Aim for about an inch below the top of the overflow tube.

When to stop and call a plumber

• The tank has been rebuilt once already and it is running again
• Water is appearing on the floor around the base, not just inside the tank
• The bowl rocks when you sit on it - that is usually the wax ring or the flange
• The toilet needs two flushes every time, which points at the flush valve rather than the flapper
• There is a hairline crack in the tank or bowl
• You have hard water and the whole tank hardware is furred with scale

At that point a rebuild kit is often false economy. A full toilet replacement gets you a working fixture and, on an older bowl, a meaningful drop in water use.

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Frequently asked questions

How much water does a running toilet waste?

It varies with the size of the leak, but a badly seated flapper commonly passes hundreds to a couple of thousand gallons a week. It shows up on the bill long before you notice the sound.

Can a running toilet raise my water bill a lot?

Yes, and it is one of the most common reasons for a bill that jumps with no change in habits. If your bill has climbed and nothing else has changed, check every toilet in the house with the dye test before assuming a hidden leak.

Is it worth repairing an old toilet or replacing it?

If the fixture is sound and only the tank hardware has failed, repair. If the bowl is cracked, the flange is broken, or you have already rebuilt it once, replacement usually costs less over the next few years.