Note 42 · DIY

How to Fix a Running Toilet

Oct 4, 2026 · 4 min read

The notes

A toilet that keeps running, hissing or refilling on its own is easy to ignore until the water bill arrives or the noise keeps someone awake. Most of the time the fix is inside the tank and needs no special tools. The trick is to look before you buy anything, because four different parts can cause the same symptom.

Safety and preparation

Turn off the water supply first: the shut-off valve is usually on the wall behind or beside the toilet. Turn it clockwise until it stops, then flush to empty most of the tank. Have a towel and a small bucket nearby. If the valve is stiff or leaks when you turn it, stop there; forcing an old valve can make things worse, and a plumber may be the better route.

Lift the tank lid and set it somewhere flat and soft. Lids are heavy and break easily.

How a toilet tank works, briefly

When you flush, a lever lifts a rubber flapper at the bottom of the tank, and water rushes into the bowl. The flapper drops back and seals. A fill valve then refills the tank until a float rises high enough to tell it to stop. Any extra water spills into an overflow tube so the tank never floods. A running toilet means one of these steps is not finishing properly.

Find the cause

What you notice	Likely cause
Water trickles into the bowl constantly, tank refills every so often	Flapper not sealing
Water level reaches the top of the overflow tube and keeps going	Float set too high or fill valve faulty
Toilet runs after flushing until you jiggle the handle	Chain too long, too short or tangled
Fill valve hisses or never fully shuts off	Worn fill valve

A simple test for the flapper: put a few drops of food coloring in the tank and wait without flushing. If color appears in the bowl, the flapper is leaking.

Fix 1: Adjust the chain

The chain between the handle lever and the flapper should have a little slack, roughly enough that it hangs in a gentle curve when the flapper is closed. Too tight, and the flapper cannot seal. Too loose, and it can tangle or get caught under the flapper. Move the clip one or two links and test.

Fix 2: Clean or replace the flapper

With the water off and tank drained, unhook the flapper from the overflow tube pegs and the chain.
Look at it. If it is warped, cracked, stiff or slimy, it needs replacing.
Wipe the valve seat it rests on with a soft cloth. Mineral build-up there can stop a good flapper sealing.
Take the old flapper to the hardware store, or check the brand and model printed inside the tank, so you buy a match.
Fit the new one, reattach the chain with a little slack, turn the water on and test.

Fix 3: Lower the float

The water should stop about an inch below the top of the overflow tube; many tanks have a line marked inside. If it goes higher:

Float cup (a cylinder that slides on the fill valve): squeeze the clip or turn the adjustment screw to lower it.
Float ball (a ball on an arm): turn the screw at the top of the fill valve, or bend the arm gently downwards on older metal designs.

Flush and watch the tank fill. Adjust in small steps.

Fix 4: Replace the fill valve

If the float is set correctly and the valve still hisses or never shuts off, the fill valve is worn. Replacement kits are made to fit most standard toilets and come with instructions. The job involves disconnecting the supply line under the tank and unscrewing a locknut, so keep the bucket and towel handy for the water left in the line. Hand-tighten plastic nuts plus a small extra turn; overtightening cracks them.

When to call a plumber

The shut-off valve will not turn or leaks.
Water seeps from the base of the toilet or between tank and bowl.
The tank or bowl is cracked.
You have replaced the parts and it still runs.

There is no shame in handing a job over once it stops being simple. Our pieces on how plumbers keep homes reliable and local plumbers describe what that help looks like.

Checklist

Water off, tank drained, lid set down safely
Food coloring test for the flapper
Chain slack checked
Flapper and valve seat cleaned or replaced
Water level about an inch below the overflow tube
Fill valve replaced if it still hisses

Work through the parts in that order, test after each change and you will usually find the culprit long before you need a new toilet.

Every guide

What you can expect on each how-to page

The layout is deliberately predictable.

Preparation and safety notes come before the first numbered step.
Steps are numbered in the order you will carry them out.
At least one table compares options, causes or methods.
A section says when the job belongs with a professional.
A closing checklist sums up the whole task in a few lines.
No prices, ratings or figures we have not checked ourselves.