How to Fix a Running Toilet (5 Common Causes and Solutions)

A running toilet is one of those problems that starts as a minor annoyance and slowly drives you mad. That constant trickle of water into the bowl — sometimes barely audible, sometimes a full-on hiss — is also costing you money. A running toilet can waste 200 litres or more per day, which adds up fast on a water meter.

The good news? Most running toilet problems are simple to diagnose and cheap to fix. You don’t need a plumber, and you rarely need specialist tools. This guide covers the five most common causes and walks you through fixing each one.

How a Toilet Flush Mechanism Works (30-Second Explanation)

Before we dive into fixes, it helps to understand what’s going on inside the cistern. There are really only two mechanisms:

1. The fill valve — controls water coming into the cistern. When you flush, the water level drops, the float drops with it, and the fill valve opens to refill the cistern. Once the water reaches the right level, the float rises and shuts the valve off.
2. The flush valve/siphon — controls water going out of the cistern into the bowl. When you press the flush handle or button, the flush valve opens briefly, dumps the water, then closes again.

A running toilet means one of these mechanisms isn’t working properly — either water is leaking out through the flush valve, or the fill valve isn’t shutting off and water is overflowing into the overflow.

Important note for UK readers: UK toilets typically use a different mechanism from American ones. Most UK cisterns use a siphon-based flush (you’ll see a large plastic bell or diaphragm) rather than the flapper-based system common in the US. This guide covers both, as some modern UK toilets now use flapper-style valves.

Cause #1 — Worn Flapper/Siphon Diaphragm

This is the single most common cause of a running toilet. The rubber seal that keeps water in the cistern has degraded, letting water trickle constantly into the bowl.

How to Diagnose

Try the food colouring test:

1. Remove the cistern lid.
2. Add a few drops of food colouring to the cistern water.
3. Wait 15-20 minutes without flushing.
4. Check the bowl — if the coloured water has appeared in the bowl, your flush valve seal is leaking.

For siphon-based toilets (most UK): The diaphragm inside the siphon unit is a large rubber or plastic disc. Over time, it warps, tears, or develops mineral deposits. The result is a weak flush or water that continues trickling after flushing.

For flapper-based toilets: The rubber flapper at the bottom of the flush valve hardens, cracks, or collects limescale, preventing it from sealing properly.

How to Replace

Replacing a siphon diaphragm (UK toilets):

1. Turn off the water supply to the cistern — there’s usually an isolation valve on the pipe leading to the cistern. Turn it clockwise until it stops.
2. Flush the toilet to empty the cistern. Sponge out any remaining water.
3. Disconnect the flush linkage (the rod or cable connecting the handle/button to the siphon).
4. Undo the large nut on the underside of the cistern that holds the siphon in place. You may need to remove the cistern from the pan to access this — it’s held on by two bolts at the back.
5. Lift out the siphon unit and replace the rubber diaphragm. These are sold as universal diaphragms for a couple of pounds. Take the old one to the hardware shop if you’re unsure of the size.
6. Reassemble in reverse order. Turn the water back on and test.

Replacing a flapper (US-style toilets):

1. Turn off the water and flush to empty the cistern.
2. Unhook the old flapper from the mounting ears on the overflow tube. Note how the chain connects.
3. Hook the new flapper onto the mounting ears.
4. Reconnect the chain with about 12mm of slack (too tight and it won’t seal; too loose and it won’t lift enough).
5. Turn the water back on and test.

Total cost: £2-5 for a diaphragm or flapper. Total time: 20-45 minutes.

Cause #2 — Float Set Too High

If the float is set too high, the cistern overfills and water runs continuously into the overflow tube (and eventually into the bowl). You’ll usually hear a constant hiss of running water and see water flowing into the overflow.

How to fix it:

Ball float (older systems): The float is a ball on the end of an arm. Gently bend the brass arm downward by about 10-15mm. This lowers the point at which the valve shuts off, reducing the water level. If the arm is plastic, there’s usually an adjustment screw at the valve end — turn it to lower the float position.

Concentric float (modern systems): Modern fill valves have a float that slides up and down the valve body. There’s an adjustment screw or clip — turn the screw or move the clip down to lower the shut-off point. You want the water level to sit about 25mm below the top of the overflow tube.

How to check: With the cistern lid off, flush and watch the refill. The water should stop rising at least 25mm below the overflow. If it reaches the overflow and keeps going, the float needs lowering further — or the fill valve isn’t shutting off at all (see Cause #3).

Cause #3 — Fill Valve Failure

If adjusting the float doesn’t help, the fill valve itself may be worn out. Over time, the internal washer or diaphragm in the fill valve degrades, letting water through even when the float is in the shut-off position.

Symptoms: Water continues running into the cistern no matter where you set the float. The valve hisses or trickles constantly.

Fix: You can replace just the internal washer on some traditional brass ballcock valves (common in older UK toilets). For modern plastic fill valves, it’s usually easier and cheaper to replace the entire valve — they’re around £8-15 and take 20 minutes to swap.

1. Turn off the water supply and flush the cistern.
2. Disconnect the water supply pipe from the bottom of the fill valve (have a towel ready — there’ll be some water in the pipe).
3. Undo the back nut holding the fill valve to the cistern.
4. Remove the old valve and fit the new one, adjusting the height to match your cistern.
5. Reconnect the water supply, turn on, and adjust the float to set the correct water level.

Cause #4 — Overflow Tube Issues

The overflow tube is a safety feature — if the water level rises too high, it channels the excess into the toilet bowl rather than flooding your bathroom. But if the overflow tube is cracked, too short, or mispositioned, it can cause continuous drainage.

Check for:

• Cracks in the overflow tube — replace if damaged.
• The tube sitting too low — the top of the overflow should be at least 25mm above the normal water line.
• A refill tube that’s been pushed too far into the overflow — it should clip to the outside of the overflow, not extend down into it. If it’s pushed in too far, it can create a siphon effect that pulls water continuously from the cistern.

Cause #5 — Flapper Chain Too Long or Short

This applies only to flapper-based flush valves (some modern UK toilets, most US-style). The chain connecting the flush lever to the flapper needs a specific amount of slack:

• Too much slack: The chain can get caught under the flapper, preventing it from sealing. The toilet runs because the flapper can’t close fully.
• Too little slack: The chain holds the flapper slightly open at all times, allowing a constant trickle.

Fix: Adjust the chain so there’s about 12mm (half an inch) of slack when the flapper is closed. Most chains have a clip you can move to a different link. Trim any excess chain length so it can’t get tangled.

UK vs US Toilet Mechanisms — Key Differences

If you’ve been watching YouTube videos on fixing a running toilet, you’ve probably noticed most are American — and the mechanisms look different from what’s in your cistern. Here’s why:

The key practical difference: UK siphon mechanisms can’t leak water into the bowl by gravity alone (the siphon has to be activated), which is why they’re preferred under UK water regulations. If your UK siphon toilet is running, the problem is almost always the fill valve or overflow, not the flush mechanism itself.

However, many modern UK toilets now use drop-valve (flapper-style) mechanisms, especially dual-flush models. These can leak into the bowl, so the flapper/seal advice above applies.

When to Replace the Whole Flush Mechanism

Sometimes the mechanism is so old, corroded, or limescale-encrusted that patching individual components isn’t worth it. Signs it’s time for a full replacement:

• The siphon body is cracked or warped
• You’ve replaced the diaphragm and it still doesn’t flush properly
• The fill valve is brass and ancient — modern plastic valves work better and are easier to maintain
• You’re dealing with constant limescale problems (common in hard water areas)

A complete flush mechanism kit (fill valve + flush valve/siphon + fittings) costs £15-30 and replaces everything inside the cistern. It’s a bigger job (allow 1-2 hours) but gives you a completely fresh start.

Recommended Toilet Repair Kits

Keep these in the house and you’ll be able to fix a running toilet in minutes rather than waiting days for a plumber:

Universal siphon diaphragm pack:

A pack of assorted diaphragm sizes covers most UK toilets. Costs about £3-5 and saves multiple future repairs.

Universal fill valve:

Fluidmaster’s PRO45 or Dudley’s Hydroflo are both excellent universal fill valves that fit most UK cisterns. They’re adjustable for height and work with side or bottom entry.

Complete cistern repair kit:

For an older toilet that needs everything replaced, a kit with fill valve, flush mechanism, and all fittings is the best value. One job, done properly, and you won’t need to touch it again for years.

Tools you’ll need:

• Adjustable spanner (for the back nuts)
• Flat-head screwdriver (for float adjustment screws)
• Towel and small bucket (for the inevitable drips)
• Plumber’s PTFE tape (for any threaded connections)

Learning how to fix a running toilet is one of those skills that pays for itself immediately. The parts are cheap, the job is straightforward, and the satisfaction of silencing that constant trickle is genuinely sweet.