How to Fix a Running Toilet: Find the Cause in 5 Minutes

A toilet that runs on its own is almost never a mystery. There are only three usual suspects, and one simple test (a few drops of food coloring) tells you which one it is before you spend a cent. Here is the 5-minute diagnosis plumbers use, and the fix for each result.

Why a running toilet is worth fixing today

That faint hiss from the bathroom is money leaving the house. The US Environmental Protection Agency's WaterSense program lists worn toilet flappers among the most common household leaks and estimates that household leaks waste nearly 10,000 gallons per home each year on average. A toilet that refills every few minutes can be a large share of that.

The good news: the parts that fail are cheap, standardized and designed to be replaced without tools. Most repairs take less than 30 minutes, and you do not need to remove the toilet.

Before you buy anything, though, you need to know which part has failed. Guessing is how people end up with three new parts and the same noise.

How a toilet tank works (in 60 seconds)

Lift the tank lid and set it somewhere soft. You will see three things that matter:

• The flapper: a rubber seal at the bottom of the tank. Pressing the handle lifts it, water rushes into the bowl, and it drops back to seal.
• The fill valve: the tall assembly on the left that refills the tank after a flush.
• The float: a cup or ball that rises with the water and tells the fill valve to shut off.
• The overflow tube: the open pipe in the middle. If the water level rises above its top, water spills down it into the bowl, which is a safety feature and a clue.

The 5-minute diagnosis

Test 1: Look at the water level

With the tank full and quiet, look at the overflow tube. If water is trickling into the top of it, the tank is overfilling. That is a float or fill valve problem. Skip to the fill valve section.

Test 2: The food coloring test

If the water is below the top of the overflow tube, add 5 to 10 drops of food coloring to the tank. Do not flush. Wait 15 to 20 minutes. If color appears in the bowl, water is seeping past the flapper.

Test 3: Watch the chain

Flush and watch. If the flapper does not drop fully closed, or the chain catches underneath it, the problem is simply chain length.

Use this checklist as you go (your ticks are saved on this device):

• Water level checked against overflow tube
• Food coloring test done (15–20 minutes)
• Chain slack checked (about 1/2 inch of slack when closed)
• Flapper inspected for warping, mineral build-up or slime
• Water supply valve located and working

Fix 1: Adjust the chain

The chain should have roughly half an inch of slack when the flapper is closed. Too tight and the flapper cannot seal; too loose and it can get trapped under the flapper. Move the clip on the handle arm one or two links and test again. This costs nothing and fixes a surprising number of "running" toilets.

Fix 2: Replace the flapper

Flappers are made of rubber, and chlorine and minerals in water slowly make them stiff and warped. If the dye test showed a leak, replace it.

1. Turn off the water at the shut-off valve behind the toilet (turn clockwise).
2. Flush and hold the handle down to drain the tank.
3. Unhook the old flapper's ears from the pegs on the overflow tube (or slide the ring off) and unclip the chain.
4. Take the old flapper to the store, or check the size: most are 2-inch; many newer high-efficiency toilets use 3-inch.
5. Wipe the flush valve seat (the ring it seals against) with a non-scratch pad to remove grit.
6. Install the new flapper, attach the chain with about 1/2 inch slack, turn the water back on and repeat the dye test.

Fix 3: Lower the float

If water is spilling into the overflow tube, the float is set too high. The water line should usually sit about an inch below the top of the overflow tube, and many tanks have a marked fill line.

• Cup-style float (slides up and down the fill valve): squeeze the clip or turn the adjustment screw on top to lower it.
• Ball float on an arm (older toilets): turn the screw where the arm meets the valve, or gently bend the brass arm down.

Flush and let the tank refill. If it now stops at the right height, you are done.

Fix 4: Replace the fill valve

If the float is set correctly and water still trickles into the overflow, or the valve hisses, squeals, or refills on its own every so often, the fill valve is worn. Universal replacement fill valves fit most toilets and include instructions.

1. Shut off the water, flush, and sponge the remaining water out of the tank.
2. Put a towel under the supply line. Unscrew the supply line from the bottom of the tank.
3. Unscrew the large plastic locknut under the tank and lift the old valve out.
4. Set the new valve's height per the instructions (the critical level mark usually sits at least an inch above the overflow tube).
5. Insert, hand-tighten the locknut plus a quarter turn, reconnect the supply line and clip the refill tube to the overflow tube.
6. Turn the water on slowly and check for drips under the tank.

The "phantom flush": a toilet that refills on its own

If the toilet briefly refills every 20 minutes or so with nobody using it, that is almost always a slow flapper leak (the tank level drops until the fill valve kicks in). The dye test will confirm it. Replace the flapper first.

When it is not a DIY job

Call a plumber if the shut-off valve will not turn or leaks when you touch it, if you see a crack in the tank, or if water is pooling around the base of the toilet (that points to the wax ring, which means removing the toilet).

What it costs

A flapper is usually a few dollars, a universal fill valve a little more, and a complete tank repair kit (flapper, fill valve and washers) is often under the cost of a single plumber's service call. Doing it yourself is one of the best-value home repairs there is.