Toilet keeps running? How to find the cause and fix it yourself
A toilet that keeps running almost always comes down to one of two cheap tank parts. If it refills itself every few minutes, the flapper isn't sealing. If you hear a constant hiss and water spills into the overflow tube, the water level is too high or the fill valve is worn. Either swap takes 10–15 minutes.
Why does my toilet keep running?
Water is leaking from the tank into the bowl, or spilling over the top of the overflow tube, so the fill valve keeps topping the tank back up.
Two parts cause nearly every case:
- The flapper, the rubber seal at the bottom of the tank that lifts to flush and drops to seal. It is the number-one cause.
- The fill valve, the tower that refills the tank, and its float, which tells the valve when to stop.
It is worth fixing quickly. A running toilet can quietly waste up to 200 gallons a day (EPA), which adds up on your water bill.
How do I tell if it's the flapper or the fill valve?
Lift the tank lid (set it somewhere safe; it's heavy and cracks easily) and run two checks.
| What you notice | Most likely cause |
|---|---|
| Tank refills by itself every few minutes (a "phantom flush") | The flapper isn't sealing |
| Constant hiss, water running into the overflow tube | Water level set too high, or a worn fill valve |
| Weak or incomplete flush | Chain too long, or water level too low |
| Handle sticks down | Chain too short or tangled, holding the flapper open |
The dye test settles it. Put a few drops of food coloring in the tank and wait 10 to 15 minutes without flushing, then flush to clear the dye. Color in the bowl means the flapper is leaking. A clear bowl with water still running into the overflow tube points to the fill valve and float.
How do I replace a toilet flapper?
Shut off the water, drain the tank, and swap the flapper onto the same two pegs. It's a 10–15 minute job with nothing more than pliers and a sponge.
- Turn the shutoff valve on the wall below the tank clockwise, then flush and hold the handle to empty the tank. Sponge out the rest.
- Unclip the chain from the flush lever and slide the flapper's ears off the pegs at the base of the overflow tube.
- Take the old flapper to the store, or note your toilet brand. The wrong size is the top reason a new flapper still leaks.
- Fit the new flapper on the same pegs and reconnect the chain with a little slack. A chain pulled tight holds the flapper open.
- Turn the water back on, flush, and watch the flapper drop and seal. Re-run the dye test if you want to be sure.
How do I stop water running into the overflow tube?
Lower the float so the water shuts off about an inch below the top of the overflow tube.
On a modern cup-style fill valve, the adjustment is a clip or a screw on the side of the valve. On an older ball float, it's a screw where the arm meets the valve. A lower float means a lower water line.
Flush, let it refill, and confirm the water stops below the overflow tube with no trickling. If it still hisses or won't shut off after adjusting, the fill valve itself is worn and needs replacing. A universal fill valve swaps in with the supply line and one locknut under the tank.
Why is my toilet still running after I replaced the flapper?
Usually the new flapper is the wrong size, or the seat it closes against is rough.
- Match the flapper to your toilet. "Universal" flappers often don't seal older or odd-brand valves; a matched one does.
- Wipe the flush-valve seat clean, the ring the flapper closes against.
- Check the chain slack. A tight chain is a hidden cause of a flapper that never fully closes.
If the toilet is old, replace the flapper and fill valve together. The two run about $15–25 combined, and a single kit often fixes the problem for years.
Doing it yourself
A flapper is $5–10 and a fill valve $10–15
Calling a pro
$100–200 for a 15-minute job
Time: 10–15 minutes per swap.
When to stop and call a pro
- The tank or bowl is cracked, or water leaks from between the tank and bowl after you've tightened the bolts.
- Water pools at the base of the toilet on the floor. That's the wax ring or a bigger issue, not the tank.
- The bowl backs up or drains slowly. That's a clog, a different repair.
Want the full step-by-step?
How to Fix a Running Toilet (No. 10) has the labeled tank diagram, the dye test, all three fixes step by step including the full fill-valve swap, a tools-and-parts list, and a troubleshooting table. It's also in the $14 Bathroom Repair Kit with four other bathroom fixes.
Get “How to Fix a Running Toilet” — $6 →Or buy the guide inside the Bathroom Repair Kit (5 guides, $14) or the Core Library (all 24 home repair guides, $49). Instant PDF, no subscription.
Frequently asked questions
How much water does a running toilet waste?
A running toilet can quietly waste up to 200 gallons a day (EPA), which adds up on your water bill. Because the leak usually goes into the bowl rather than onto the floor, it's easy to ignore. Listening every few months for a hiss or a refill catches it early.
What is a phantom flush?
A phantom flush is when the tank refills on its own every few minutes even though nobody flushed. Water is seeping past the flapper into the bowl until the level drops enough to trigger the fill valve. Replacing the flapper, matched to your toilet's brand, is the fix.
Do in-tank bleach tablets damage a toilet?
They can. In-tank bleach tablets eat the rubber parts inside the tank, including the flapper, which is the part most likely to leak. Skipping them, and keeping a spare flapper on hand, prevents most running toilets from coming back.
Is a running toilet worth calling a plumber for?
Usually not. A plumber typically charges $100–200 for what is a 15-minute job, while a flapper costs $5–10 and a fill valve $10–15. Call a pro if the tank or bowl is cracked, water pools at the base of the toilet, or the bowl backs up.
Should I replace the flapper and fill valve at the same time?
If the toilet is old, yes. Both parts wear out, and replacing them together costs about $15–25 and often fixes the problem for years. On a newer toilet, run the dye test first so you only buy the part that's actually failing.
Sources
Safety guidance and key figures were checked against these outside sources on October 22, 2026. Dollar figures are typical ranges, not quotes.
- Fix a Leak Week: Family Fact Sheet (U.S. EPA WaterSense). A running toilet can waste 200 gallons or more per day; flapper decay is a common cause; food-coloring dye test.
- Fix a Leak Week (U.S. EPA WaterSense). Dye test: color in bowl after 10 minutes means a leak; a worn flapper can leak thousands of gallons a year.
- Fix a Leak (WaterSense) archived page (U.S. EPA WaterSense (archived 2017 snapshot)). Flapper is the most common culprit; replacing it is an inexpensive, easy project; dye test.
- Solving the Mystery of the Leaky Flapper (Pinellas County Government (FL)). Flappers deteriorate from in-tank cleaning products; dye test about 15 minutes; bad flapper can waste up to 200 gallons/day.
More home repair answers
For general homeowner reference only. When a repair involves gas, major electrical, or structural work, call a licensed professional.