The toilet has been running for three days. Not constantly. Intermittently. It will be silent for an hour, then the water will start hissing for 30 seconds and stop. The cycle repeats. You have been jiggling the handle, which works about half the time, which is just enough reinforcement to keep jiggling it instead of actually fixing it. That intermittent hiss is the sound of the fill valve topping up the tank because water is escaping somewhere. The somewhere is almost always the flapper. When it is not the flapper, it is the fill valve. When it is neither, it is the overflow tube or the water level adjustment. Those are the only four things that can cause a toilet to run. All four are fixable in under an hour with parts that cost less than twenty dollars.
According to the EPA WaterSense program, a running toilet can waste up to 200 gallons of water per day. That is roughly 6,000 gallons a month. The water bill reflects this long before the sound becomes annoying enough to do something about it. This guide covers the diagnostic sequence that identifies the exact cause and the fix for each one.
The Diagnostic Sequence: Three Questions That Identify the Problem
Licensed plumber James Schuelke of Twin Home Experts, with over 32 years of plumbing experience, recommends starting with the simplest possible observation before touching any parts. The diagnostic sequence takes two minutes and requires zero tools.
Question 1: Is water overflowing into the overflow tube? Remove the tank lid. Look at the open vertical pipe in the center of the tank. This is the overflow tube. If water is trickling over the top of it, the water level is too high. The fix is adjusting the fill valve float down. Skip ahead to the fill valve section.
Question 2: Does the water stop running when you lift up on the fill valve float? The float is the plastic cylinder on the left side of the tank that rises and falls with the water level. Lift it gently with your hand. If the water stops, the fill valve is working but the float is set incorrectly or the fill valve needs minor adjustment. If lifting the float does nothing and water continues to flow, the fill valve has failed and needs replacement.
Question 3: Does the water level in the tank slowly drop when the toilet has not been flushed? Mark the water level with a pencil on the inside of the tank. Wait 15 minutes without flushing. If the level drops, water is escaping past the flapper into the bowl. Replace the flapper. If the level holds steady but the toilet runs anyway, the fill valve is refilling the tank in response to a leak that does not exist, which means the fill valve itself is faulty.
The food coloring test confirms a flapper leak. Drop a few drops of food coloring into the tank water. Do not flush. Wait 15 minutes. If colored water appears in the toilet bowl, the flapper is leaking. Clear water in the bowl means the flapper is sealing and the fill valve is the problem.
Fix 1: Replace or Clean the Flapper
The flapper is the round rubber disk at the bottom of the tank, connected to the flush handle by a chain. It is the single most common cause of a running toilet. The rubber hardens, warps, or develops mineral deposits over 4 to 7 years. It stops sealing.
Check the chain before replacing the flapper. A chain that is too short pulls the flapper slightly open. A chain that is too long can fall under the flapper as it drops, preventing a seal. The chain should have about half an inch of slack when the flapper is closed and the handle is at rest. Adjust the chain first. If the toilet continues to run, replace the flapper.
Replacing the flapper requires no tools and the water stays on. Unhook the old flapper’s ears from the pegs on the overflow tube. Disconnect the chain from the handle arm. Take the old flapper to the hardware store to match the size, which is either 2-inch, 3-inch, or 4-inch diameter. A universal flapper works in most standard toilets, but Kohler and Toto toilets often need brand-specific flappers. Install the new flapper by hooking the ears onto the pegs and clipping the chain to the handle arm with half an inch of slack. Flush and observe. The running should stop.
Fix 2: Adjust or Replace the Fill Valve
If the water level is overflowing into the overflow tube, the fill valve float is set too high. The float is attached to the fill valve on the left side of the tank. On most modern fill valves, an adjustment screw on the top of the fill valve raises or lowers the float. Turn the screw counterclockwise to lower the water level. The correct level is about one inch below the top of the overflow tube. Flush and observe. Adjust in small increments until the water stops just below the tube.
If adjusting the float does not stop the running, or if the fill valve makes a foghorn or whistling sound when refilling, the fill valve itself has failed. Replacing a fill valve requires turning off the water supply at the shutoff valve behind the toilet, draining the tank by flushing, and disconnecting the supply line from underneath the tank. Unscrew the plastic lock nut on the outside of the tank that holds the fill valve in place. Lift the old fill valve out. Install the new one in reverse order. Fill valves are universal and cost $12 to $20. The entire replacement takes 20 minutes.
Fix 3: Check the Overflow Tube and Refill Tube
The overflow tube prevents the tank from overflowing if the fill valve fails. It also has a small rubber refill tube that clips to its top edge. The refill tube sends a small stream of water down the overflow tube during the refill cycle to replenish the water level in the toilet bowl. If the refill tube has come loose and is spraying water into the tank instead of into the overflow tube, the fill valve runs longer because the bowl does not refill properly. Push the refill tube back into the overflow tube and secure the clip.
The refill tube should not extend below the water line in the overflow tube. If it does, it can create a siphon that continuously draws water from the tank into the overflow tube. Trim the tube so it ends above the standing water level in the overflow tube or clip it to the top edge as designed.
If the overflow tube itself is cracked, which is rare, water leaks through the crack and into the bowl continuously. The crack is usually at the base where the tube meets the flush valve. A cracked overflow tube means replacing the flush valve assembly, which requires removing the tank from the bowl.
Fix 4: Replace the Flush Valve (When Nothing Else Works)
If the flapper is new, the fill valve is new, the water level is correct, and the toilet still runs, the flush valve body has a crack or the flapper seat is damaged. The flush valve is the large plastic assembly in the center of the tank that includes the flapper seat and the overflow tube.
Replacing a flush valve is the most involved toilet repair. The tank must be removed from the bowl. The water supply is disconnected. The two tank-to-bowl bolts are removed. The tank is lifted off the bowl and set aside. The old flush valve is unscrewed from inside the tank. The new flush valve is threaded in its place with the included gasket. A new tank-to-bowl gasket is installed. The tank is lowered back onto the bowl and the bolts are tightened evenly, a few turns at a time on each side, to prevent cracking the porcelain.
The flush valve and gasket together cost $20 to $30. The repair takes about an hour. If the toilet is more than 20 years old, replace the fill valve at the same time while the tank is off. The two components share a service life, and the marginal cost of the fill valve is small compared to the labor of removing the tank twice.
Quick Reference: Symptom to Fix
| Symptom | Cause | Fix | Cost |
| Water runs intermittently, stops when handle is jiggled | Chain wrong length or tangled | Adjust chain slack to 1/2 inch | $0 |
| Water runs intermittently, jiggling does nothing | Flapper worn or dirty | Clean or replace flapper | $5-10 |
| Water runs constantly, overflows into tube | Fill valve float too high | Lower float with adjustment screw | $0 |
| Water runs constantly, float adjustment does nothing | Fill valve failed | Replace fill valve | $12-20 |
| Whistling or foghorn sound when filling | Fill valve diaphragm worn | Replace fill valve | $12-20 |
| All parts replaced, toilet still runs | Cracked flush valve or overflow tube | Replace flush valve assembly | $20-30 |
Frequently Asked Questions
Why does my toilet randomly flush itself in the middle of the night?
Ghost flushing occurs when the flapper leaks water slowly enough that the tank level drops over several hours. When the water level drops far enough, the fill valve activates and refills the tank. The sound is the same as a normal flush cycle except the flapper never opened. The fix is replacing the flapper. The leak is slow but consistent. The food coloring test will confirm it.
I replaced the flapper and the toilet still runs. What did I do wrong?
The most common cause is the wrong size flapper. A 2-inch flapper on a 3-inch flush valve will not seal. The second most common cause is a dirty flush valve seat. Run your finger around the seat edge. Mineral deposits prevent a new flapper from sealing. Clean the seat with fine steel wool. The third cause is the chain adjusted too tight, holding the flapper slightly open. The chain should have visible slack when the handle is at rest.
Should I turn off the water to the toilet while I am not home to stop the running?
Turning off the water is a temporary solution that prevents water waste until the repair can be made. Turn the shutoff valve behind the toilet clockwise until it stops. The toilet will not refill after the next flush, so flush once to empty the tank and leave the valve off. This is a stopgap. A running toilet left unrepaired wastes water every day. Fix it or call a plumber. The water bill from a running toilet over one billing cycle can exceed the cost of a plumber’s service call.
The Quiet Tank
A running toilet is not a plumbing emergency in the way a burst pipe is. It is a slow, persistent waste that punishes procrastination on a monthly billing cycle. The diagnostic sequence takes two minutes. The fix takes ten minutes for a flapper, twenty for a fill valve, and an hour for a flush valve. All three cost less than the water the toilet wastes in a month.
If the toilet runs and you are not sure which part to buy, start with the flapper. It is the cause in the majority of cases. If that does not fix it, you have ruled out the most likely problem and the fill valve is next. The parts are not expensive enough to justify guessing wrong. Replace both if the toilet is old. The marginal cost is fifteen dollars and the result is a toilet that does not make noise at night.
