How to Fix a Toilet Running Constantly
Last updated: 30 September 2026
Adjust the float, replace the flapper, or clean the fill valve to stop the water from running.
Overview
A toilet that runs constantly wastes water and increases your bills. This guide covers diagnosing and fixing the most common causes of a continuously running toilet.
Step-by-Step Fix
Before beginning any repair, ensure you have isolated power or water supplies. Clear your workspace, gather standard hand tools, and wear appropriate personal protective equipment (PPE).
- Remove the cistern lid and set it aside carefully on a flat surface. Look inside the cistern to observe what is happening. Watch the water level and listen for the sound of water flowing even when the toilet has not been flushed recently.
- Check the water level in the cistern. The water should reach the marked water line usually about one inch below the top of the overflow pipe. If the water is spilling into the overflow pipe the fill valve is not shutting off correctly.
- Adjust the float mechanism to lower the water level. On traditional ball valve systems bend the float arm downwards slightly. On modern fill valves turn the adjustment screw clockwise to lower the water level. Turn it in small increments and check after each adjustment.
- Check the flapper valve at the bottom of the cistern. The flapper is a rubber seal that should drop down and seal the flush valve opening after a flush. If the flapper is warped hard or covered in mineral deposits it will not seal properly and water will leak into the bowl.
- Replace the flapper if it is damaged. Turn off the water supply flush the toilet to empty the cistern then disconnect the old flapper from the overflow tube. Install a new flapper of the correct size and reattach the chain with about half an inch of slack.
- Clean the fill valve if the water continues to run. Mineral deposits can prevent the valve from sealing properly. Turn off the water supply and disassemble the fill valve according to the manufacturer instructions. Soak the components in white vinegar to dissolve limescale then reassemble.
- Check the chain connecting the flush handle to the flapper. The chain should have a small amount of slack. If the chain is too tight it holds the flapper slightly open allowing water to leak continuously. Adjust the chain length so it has about half an inch of slack when the flapper is closed.
- Turn the water supply back on and observe the toilet. Flush it a few times and watch the water level in the cistern. The water should stop running once the cistern reaches the correct level. If the running continues the fill valve may need replacing.
- If you have tried all these steps and the toilet still runs constantly the fill valve or flush valve assembly may need replacing. These are inexpensive parts available at any hardware store. If you are unsure call a plumber.
Common Symptoms & Diagnostic Resolutions
Review the common error codes and operating symptoms below to quickly identify the root cause of the fault before ordering replacement parts.
| Symptom / Code | Likely Root Cause | Recommended Resolution |
|---|---|---|
| Water running constantly | Flapper not sealing or fill valve stuck | Replace the flapper or clean the fill valve diaphragm. Adjust the float to lower the water level. |
| Water spilling into overflow | Water level set too high | Adjust the float arm or fill valve screw to lower the water level to one inch below the overflow pipe. |
| Intermittent running | Flapper chain too tight | Adjust the chain to provide half an inch of slack when the flapper is closed. |
| Toilet refills randomly | Slow leak through flapper | Replace the flapper. Even a small warp or mineral buildup can cause a slow leak. |
Frequently Asked Questions
How much water does a running toilet waste?
A running toilet can waste up to two hundred gallons of water per day depending on the severity of the leak. This can add significantly to your water bill over a year.
Can a running toilet fix itself?
No a running toilet requires manual adjustment or part replacement. The flapper or fill valve is usually the cause and will not repair itself.
How do I know if my flapper is bad?
A bad flapper will be warped stiff or covered in mineral deposits. It may not drop fully to seal the flush valve. If the toilet runs continuously after a flush the flapper is the most likely cause.
How much does it cost to replace a toilet flapper?
A replacement flapper costs between three and ten pounds. The job takes about ten minutes and requires no special tools. This is one of the cheapest and most effective DIY repairs.