Nine times out of ten a running toilet is a worn flapper — the rubber seal at the bottom of the tank — and replacing it takes about ten minutes and costs under $15. Before you buy anything, do the dye test: put a few drops of food colouring in the tank, wait ten minutes without flushing, and look in the bowl. Colour in the bowl means the flapper is leaking. No colour, but the toilet still runs, and the problem is the fill valve or the float instead. That single test tells you which of the three repairs below you actually need.
It is worth doing today rather than at the weekend. The US EPA estimates a running toilet wastes 1,000 to 4,000 gallons per day, and a stuck fill valve can waste around 4,300 gallons a day — enough to add roughly $1,400 to an annual water bill. A toilet is the single largest water user in most homes even when it is working properly.
Diagnose it first: the dye test
- Take the lid off the tank and set it somewhere flat — porcelain lids chip easily and are expensive to replace.
- Add 5–10 drops of food colouring, or a dye tablet if your water utility gives them away (many do, free).
- Wait 10–15 minutes. Do not flush.
- Look in the bowl.
| What you see | What it means | Fix |
|---|---|---|
| Colour in the bowl | Flapper is not sealing | Replace the flapper |
| No colour, water trickling into the overflow tube | Water level set too high | Adjust the float |
| No colour, valve hisses on and off | Fill valve failing | Replace the fill valve |
| Colour only after you jiggle the handle | Chain length or handle arm | Adjust the chain |
The EPA’s other quick check needs no dye at all: a healthy toilet makes refill noise only after a flush. Any hissing, trickling, or periodic refilling at other times is a leak, even a silent one you have stopped noticing.
Fix 1: Replace the flapper (most common)
The flapper is a rubber or silicone flap covering the drain hole at the bottom of the tank. Minerals, chlorine, and age make it stiff and warped until it no longer seats properly, and water seeps past continuously.
- Turn off the supply valve behind the toilet, then flush to empty the tank.
- Unhook the old flapper from the two pegs on the overflow tube and unclip the chain from the flush lever.
- Take it to the shop with you. Flappers are not universal — 2-inch and 3-inch are both common, and a mismatched one will leak straight away.
- Clip the new one on, reattach the chain, turn the water back on.
Set the chain with about half an inch of slack. Too tight and the flapper is held fractionally open — a leak that is very hard to spot. Too loose and it tangles under the flapper, which causes the “runs until you jiggle the handle” symptom. Denver Water and most utility guides put flapper replacement on a roughly five-year cycle as ordinary maintenance, not as an emergency repair.
Fix 2: Adjust the water level
If water is running into the overflow tube — the open vertical pipe in the middle of the tank — the tank is simply filling too high and spilling continuously.
The water line should sit about an inch below the top of the overflow tube. Most toilets mark it inside the tank. On a modern float-cup valve, pinch the clip on the side and slide the cup down. On an older ballcock with a float on an arm, turn the adjustment screw, or gently bend the arm downward.
This one costs nothing and is the fix people most often overlook, because the toilet sounds like it is refilling normally rather than leaking.
Fix 3: Replace the fill valve
If the dye test is clean and the level is correct but the valve still cycles on and off, or hisses continuously, the fill valve has failed. A universal replacement runs about $12–20 and fits nearly every modern toilet.
- Shut off the water, flush, and sponge out the remaining inch in the bottom.
- Put a towel and a bowl under the tank — the supply line will drip.
- Unscrew the supply line, then the plastic locknut under the tank.
- Lift the old valve out, drop the new one in, and set its height so the top sits at least an inch above the overflow tube.
- Tighten the locknut by hand only. Over-tightening with a wrench cracks the tank, which turns a $15 job into a new toilet.
How much water is a running toilet actually wasting?
The numbers are large enough to be worth restating plainly:
| Leak type | Typical daily waste | Roughly equivalent to |
|---|---|---|
| Slow flapper seep (silent) | 30–100 gallons | Two to five extra showers a day |
| Visible constant run | 1,000–4,000 gallons | Filling a small swimming pool weekly |
| Stuck fill valve | up to 4,300 gallons | About $1,400 a year in water |
A silent leak is the expensive one, because it can run for months before anyone notices. That is the entire reason the dye test exists — it makes an invisible problem visible in ten minutes.
Why does my toilet run intermittently — “phantom flushing”?
If the toilet refills briefly every few minutes or hours without anyone touching it, the tank is slowly draining into the bowl past a worn flapper and the fill valve is topping it back up. It is the same flapper problem, just slow enough that you never hear a continuous run. The dye test catches it.
When it is not a DIY job
Call a plumber if water is appearing on the floor around the base (that is a wax ring or a cracked tank, not a running toilet), if the shut-off valve is seized and will not turn, or if you have replaced the flapper and fill valve and it still runs — which usually means the flush valve seat itself is pitted and needs resurfacing or replacing.
Worth doing at the same time
While the lid is off, check the tank’s fill level against the marked line and replace the flapper if it feels stiff or leaves black residue on your fingers, even if it is not leaking yet. It is a two-dollar part that fails predictably.
If you are working through water use more broadly, a running toilet is almost always the biggest single win available — larger than shorter showers, and far larger than most of the fixtures people replace first.