How to Fix a Constantly Running Toilet
A practical guide to diagnosing and fixing a constantly running toilet, covering the four usual culprits inside the tank: the flapper, fill valve, float level, and overflow tube. Uses basic hand tools and inexpensive parts, with no work on the home's mains water.
Quick answer
A toilet that runs nonstop is leaking water out of the tank. Usually it's a worn flapper that won't seal, or a water level set so high it spills into the overflow tube. Replace the flapper or lower the float and the running almost always stops.
A running toilet wastes water around the clock, and the constant hiss or trickle gets old fast. Nearly every cause lives inside the tank. You won't touch the water main to the house, and most fixes take under an hour with parts that cost a few dollars. Here's how to find the part that's failing and swap it out.
What you’ll need
- Adjustable wrench or channel-lock pliers
- Sponge and a small bucket (or a few towels)
- Flashlight
- Food coloring or a dye tablet for the leak test
Materials
- Replacement flapper sized to your toilet (2-inch is standard; many newer toilets use 3-inch)
- Replacement fill valve (only if the fill valve turns out to be the problem)
- Replacement flush valve / overflow tube assembly (rarely needed)
Step by step
- 1
Take off the lid and watch the tank
Lift the tank lid straight up and lay it flat on the floor, since porcelain lids crack if they tip over. Flush once and watch the tank refill. The noise is coming from one of two things: the fill valve that never shuts off, or water draining out as fast as it comes in. Look closely. A steady trickle down into the bowl points to the flapper. Water rising to the top of the overflow tube and pouring in points to a float or water-level problem.
- 2
Run the dye test to confirm a leaking flapper
Put about 10 drops of food coloring (or a dye tablet) in the tank and don't flush. Wait 10 to 15 minutes. If color shows up in the bowl, the flapper at the bottom of the tank isn't sealing and water is sneaking past it. This is the most common cause by a wide margin.
- 3
Check and reset the water level
The water should sit about an inch below the top of the overflow tube, the open vertical pipe in the middle of the tank. If it's level with the rim or higher, water spills down the tube constantly and the fill valve keeps running to keep up. Lower the float. On a float-cup valve, pinch the clip on the adjustment rod and slide the float down, or turn the adjustment screw on top counterclockwise. On an older ball-and-arm float, turn the screw at the valve a little at a time, or gently bend the brass arm down. Flush and check the level again.
- 4
Shut off the supply and drain the tank
When the flapper or fill valve needs replacing, turn off the supply valve on the wall behind the toilet by rotating it clockwise until it stops. Flush to empty the tank, then sponge out the last inch of water into your bucket so you can work in a dry tank.
- 5
Replace the flapper
Unhook the old flapper's ears from the pegs on the sides of the overflow tube and unclip its chain from the flush lever. Take it to the store to match the size and shape, or fit a flapper you already know is compatible. Hook the new one onto the same pegs and reconnect the chain to the lever with just a little slack, about half an inch. Too tight and the flapper can't seal; too loose and it won't lift fully when you flush.
- 6
Replace the fill valve if that's the culprit
If the dye test came back clean and the water level is right but the valve still hisses or never shuts off, the valve itself is worn. With the supply off and the tank drained, disconnect the supply line underneath, unscrew the plastic locknut that holds the valve to the tank, and lift the old valve out. Set the new valve's height per its instructions, drop it in, hand-tighten the locknut, then snug it about a quarter-turn with a wrench. Reconnect the supply line and clip the refill tube onto the overflow tube (don't push it down inside the tube).
- 7
Turn the water back on and test
Open the supply valve counterclockwise and let the tank fill. It should shut off cleanly with no trickle, the water should settle about an inch below the overflow tube, and the bowl should go quiet. Flush a few times to confirm the flapper seals and the running has stopped. Re-run the dye test if you want to be sure.
Go easy on the plastic locknut and the supply-line fittings. Tank fittings and porcelain crack with surprisingly little force, and a cracked tank usually means a whole new toilet. Hand-tighten first, then add only a small snug with the wrench. Set the porcelain lid somewhere it can't get knocked onto the floor.
Common mistakes
- Grabbing a universal flapper that doesn't match your flush-valve seat, so it never seals. Match the size (2-inch vs 3-inch) and bring the old one to compare.
- Leaving the flapper chain too long or too short. Extra slack lets the flapper flop and leak; too little holds it propped open.
- Setting the water level too high so it spills into the overflow tube nonstop. Keep it about an inch below the rim.
- Forgetting to clip the small refill tube onto the overflow tube, or pushing it down inside the tube where it can siphon water and keep the valve cycling.
- Cranking fittings down with a wrench and cracking the tank or stripping the plastic threads.
Frequently asked
Why does my toilet run for a few seconds every so often when nobody flushed it?
That's phantom flushing. The flapper is slowly leaking tank water into the bowl, and once the level drops enough the fill valve kicks on to top it back up. Replace the flapper, and check the flapper seat for mineral buildup or grit that's keeping it from sealing.
I lowered the float and it still runs. Now what?
If the level is right and the dye test is clean, the fill valve is worn and not shutting off, so replace it. But if you hear water trickling into the bowl, it's still the flapper or the flapper seat, not the fill valve.
Do I need to shut off the water to the whole house?
No. There's a shutoff valve on the wall or floor right behind the toilet. Turn it clockwise and it isolates just this toilet while the rest of the house stays on.
The flapper seat feels rough or pitted. Will a new flapper fix it?
Maybe not by itself. If the seat the flapper presses against is corroded or scratched, even a brand-new flapper won't seal against it. Clean it with a scouring pad, fit a seat repair kit (a sealing ring that glues over the old seat), or replace the whole flush valve assembly.
Before you buy parts, jot down your toilet's brand and model. It's often stamped inside the tank or on the back. Many makers sell exact-match repair kits, which beats guessing with a universal part.
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