That constant hiss or trickle from the bathroom isn’t something you should just learn to live with. A running toilet wastes real water, and nine times out of ten it comes down to one of three parts inside the cistern, all of which are well within DIY reach.
Here’s why toilets keep running, how to work out which part is to blame, and how to fix the fill valve, outlet valve, and float.
Why Does My Toilet Keep Running?
A toilet tank refills after every flush, then it’s meant to go quiet once it reaches the right level. If it doesn’t, water is escaping somewhere it shouldn’t. Either it’s overflowing into the tube because the fill valve or float never shuts off, or it’s leaking past a worn seal into the bowl below.
It’s easy to underestimate how much this adds up to. A running toilet can waste hundreds of litres of water a day, and that shows up as a genuine jump on your next water bill. Sydney Water’s own water saving guidance is worth a look if you want to see just how much a small household leak can cost over a year.
The 3 Parts That Usually Cause It
| Part | What It Does | Tell-Tale Sign | Quick Test |
| Fill Valve | Controls water flowing into the tank after a flush | Constant hissing or refilling sound | Check if water is running into the overflow tube |
| Flapper (Outlet Valve) | Seals the tank so water only enters the bowl on flush | Quiet trickle into the bowl, no obvious overflow | Push down on the flapper, if the sound stops, it’s worn |
| Float | Tells the fill valve when to stop, based on water level | Tank overfills into the overflow tube | Check water level against the overflow tube height |
How to Diagnose the Cause
Lift the tank lid and have a look at what’s actually happening while the toilet is running.
- Water spilling into the overflow tube points to the float sitting too high, or a fill valve that isn’t shutting off properly.
- A quiet trickle with the tank at a normal level usually means the flapper isn’t sealing, letting water slip into the bowl.
- Still not sure which it is? Add a few drops of food colouring to the tank water and wait 10 to 15 minutes without flushing. If colour turns up in the bowl, the flapper’s your culprit.
Fixing the Fill Valve
- Turn off the water. The shut-off valve is usually on the wall or floor behind the toilet. Turn it clockwise.
- Flush and drain the tank, then dry it out with a sponge or towel.
- Inspect the valve for cracks, wear, or mineral build-up around the seal.
- Clean it first if it’s just grimy. Sometimes a wipe-down and reseat is all it needs.
- Replace it if worn. Unscrew it from the supply line and lift it out, then fit a new valve of the same size and reconnect.
- Turn the water back on and flush a few times to make sure it shuts off cleanly.
Fixing the Outlet Valve (Flapper)
- Turn off the water and drain the tank, same as before.
- Unhook the old flapper from the flush lever chain and lift it off the flush valve seat.
- Clean the valve seat. Mineral build-up here is a common reason a brand-new flapper still leaks.
- Fit the new flapper, matching the size and shape of the original, and reconnect the chain with a bit of slack.
- Turn the water back on and test the flush a few times. Our guide on fixing a leaking toilet cistern goes into more detail if the leak turns out to be a cracked or corroded flush valve seat rather than just the flapper.
Adjusting the Float
Most floats are either a ball on a metal arm or a small cup that slides along the fill valve.
- Ball float: Gently bend the metal arm downward to lower the water level.
- Cup float: Pinch the clip or turn the small adjustment screw to slide it lower on the fill valve.
Aim for the water level to sit about 2.5cm below the top of the overflow tube once the tank refills. Today’s Homeowner has a decent breakdown of adjusting a ball or cup float if a visual walkthrough of both types helps.
If water still creeps into the overflow tube after you’ve adjusted the float correctly, the fill valve itself is probably worn and due for replacement.
When to Call a Plumber
Most running toilets are a straightforward weekend fix, but it’s worth calling a licensed plumber if:
- You’ve replaced the flapper and fill valve and the toilet is still running
- The tank or bowl shows cracks, corrosion, or damage beyond the rubber parts
- Water is leaking from the base of the toilet rather than just inside the cistern
- You’d rather not deal with a full cistern strip-down yourself, in which case our toilet plumbing repairs service can sort it out properly the first time.
Key Takeaways
- A running toilet almost always comes down to the fill valve, the flapper, or the float
- Check the overflow tube first: water spilling into it points to the float or fill valve
- A quiet trickle with a normal water level usually means a worn flapper
- Replacement parts are cheap and the repair itself typically takes under an hour
If your toilet keeps running no matter what you try, get in touch with Rapid Service Plumbing and we’ll get it sorted properly.
