Quick answer
A toilet that won't stop running is almost always a bad flapper (60% of cases), a faulty fill valve (25%), or a float set too high (10%). Diagnose by lifting the tank lid and watching the water level after a flush. The fix takes 15–30 minutes and costs $10–$30 in parts. Universal replacement parts fit most brands including Kohler, American Standard, and Toto.
Key facts
| Common causes | Worn or warped flapper; Faulty fill valve; Float set too high; Flapper chain too short or tangled |
|---|---|
| DIY difficulty | Easy |
| Time to fix | 20 min |
| Parts cost | $10–$30 parts |
| Tools needed | Adjustable wrench, sponge, towel |
| When to call a pro | If leak persists after replacing standard parts |
What's happening
After you flush, the tank refills to a preset water line. The flapper seals the flush valve at the bottom; the fill valve refills the tank and shuts off when the float hits its stop. If any of these three parts fails, water flows continuously — either running into the bowl (bad flapper) or being dumped down the overflow tube (fill valve or float issue). None of these fixes require shutting off the water main; there's a shutoff valve right behind the toilet.
Common causes (ranked by frequency)
- Worn or warped flapper 60% of cases
The rubber flapper hardens, warps, or loses its seal over 3–5 years. Water leaks past it into the bowl continuously. Dye test: put food coloring in the tank; if color appears in the bowl within 15 minutes without flushing, the flapper is the culprit.
- Faulty fill valve 25%
The fill valve (the tall tower on the left side) fails to shut off. Water continues into the tank, past the overflow tube, and down the drain. You'll hear constant water flow and see water spilling into the overflow tube.
- Float set too high 10%
The float triggers the fill valve to stop. If the water level is set above the overflow tube's top, water flows continuously down the tube. Fix by bending the float arm (older toilets) or turning the adjustment screw (modern fill valves) to lower the shutoff point.
- Flapper chain too short or tangled 3%
If the chain from the flush handle to the flapper is tight or tangled, the flapper can't seat fully. Slack should let the flapper rest naturally with about 1/2 inch of chain play.
- Cracked overflow tube or flush valve seat 2%
Rare — usually only on toilets older than 15 years. Requires replacing the entire flush valve assembly, which means removing the tank. Consider replacing the toilet at that point.
How to diagnose
- Lift the tank lid carefully — porcelain lids crack easily. Set it on a folded towel out of the way.
- Watch the water level after a flush. Does the water rise above the top of the overflow tube (the tall open pipe in the center)? That's a fill valve or float problem. Does the water stop at the correct line but you still hear water in the bowl? That's a flapper problem.
- Do the dye test. Put 5–10 drops of dark food coloring or a dye tablet in the tank. Don't flush. Wait 15 minutes. If color appears in the bowl, the flapper is leaking.
- Check the chain. Lift the flapper by pulling the chain. It should raise freely. When you release, it should drop and seat fully. If the chain is too short (flapper lifts on its own) or tangled (won't seat), that's your fix.
- Listen for the fill valve. A working fill valve fills the tank in 30–60 seconds and shuts off silently. A hissing sound after that means the fill valve isn't fully closing.
The fixes
Fix #1 — Replace the flapper Most common
The flapper is a $5–$15 part that swaps in under 10 minutes. Universal flappers (Fluidmaster 501, Korky Ultra) fit ~95% of toilets. For dual-flush toilets and specialty models, match the brand.
Tools
- Sponge or towel
- Bucket (optional)
Materials
- Replacement flapper (universal or brand-specific)
- Rubber gloves
- Shut off the water at the valve behind the toilet — turn clockwise until snug.
- Flush the toilet to empty the tank. Sponge out any remaining water.
- Unhook the old flapper from the two side pegs on the flush valve. Detach the chain from the flush handle arm.
- Install the new flapper by slipping the side ears over the pegs. Ensure it sits flat against the flush valve seat.
- Attach the chain to the flush handle arm. Leave about 1/2 inch of slack — too tight prevents sealing; too loose prevents a full flush.
- Turn the water back on and let the tank refill. Test a flush.
Fix #2 — Replace the fill valve Second most common
If the flapper checks out but the tank still runs, the fill valve is the culprit. A Fluidmaster 400A or Korky QuietFill runs $12–$20 and swaps in 20 minutes.
Tools
- Adjustable wrench or pliers
- Sponge and bucket
Materials
- Replacement fill valve
- Supply line (if old one is corroded)
- Shut off water and flush to drain the tank. Sponge out remaining water.
- Disconnect the supply line from the bottom of the tank using an adjustable wrench.
- Unscrew the plastic lock nut holding the fill valve to the tank (from underneath). Lift the old valve out.
- Adjust the new valve to your tank height. Most valves twist to extend. The top of the valve should sit about 1 inch above the overflow tube.
- Install the new valve with the rubber washer inside the tank and the lock nut outside. Hand-tighten plus a quarter-turn with the wrench.
- Reconnect the supply line and clip the refill tube into the overflow tube. Turn water on slowly and check for leaks under the tank.
Fix #3 — Adjust the float Free fix
If the water level sits above the overflow tube after refilling, the float is set too high. This is a 30-second adjustment.
Tools
- Screwdriver (Phillips) — for some models
Materials
- Identify your float type. Older toilets have a ball float on an arm. Modern toilets have a cup float that slides up and down the fill valve stem.
- For ball float: gently bend the metal arm downward to lower the shutoff point.
- For cup float: locate the adjustment screw or clip on top of the fill valve. Turn the screw counterclockwise (or slide the clip down) to lower the water level.
- Target water line: about 1 inch below the top of the overflow tube. Most tanks have a fill line marked inside.
- Flush and observe. The water should stop at your new level.
Prevention
Replace flappers proactively every 3–5 years. Avoid drop-in bleach tablets in the tank — they eat rubber parts fast. If your area has hard water, consider a water softener; mineral buildup shortens fill valve life. When you replace the flapper, replace the fill valve at the same time if it's more than 5 years old — you'll save a second trip.
When to call a pro
When to stop and call a pro
- You've replaced the flapper, fill valve, and adjusted the float, and the toilet still runs — this points to a cracked flush valve seat requiring tank removal.
- Water is visible on the floor around the base of the toilet, not just inside — that's a wax ring or supply line issue.
- The tank itself is cracked or leaking from a seam — replacement toilet is the fix.
- The shutoff valve behind the toilet won't fully close — a stuck angle stop needs replacing, and water main shutoff is required.
A licensed plumber typically charges $150–$350 for a running-toilet service call, including standard parts. For tank replacement (flush valve seat repair), expect $250–$500. Full toilet replacement runs $400–$800 including labor and disposal.
Cost estimate
Cost estimate
Frequently asked questions
No. All three fixes require the tank to be empty. There's a shutoff valve directly behind the toilet — turn it clockwise until snug. If it won't close (common on 15+ year old valves), you'll need to shut off the house main.
About 95% of standard toilets, yes. Exceptions: dual-flush toilets, older American Standard Champion models, and some Kohler models use proprietary flappers or canister flush systems. Match your brand if the toilet is less than 10 years old — the brand-specific flapper costs $2–$5 more and fits better.
Yes. A silent running toilet can waste 1–4 gallons per minute — 60–240 gallons per hour. At typical US water rates ($0.008 per gallon), a leaking toilet can add $50–$200 to your monthly water bill, plus wastewater charges.
That's a slow flapper leak. Enough water escapes the tank that the fill valve refills briefly. Do the dye test to confirm — if color appears in the bowl slowly, replace the flapper.
If the flush valve seat is cracked (requires tank removal), the tank itself is cracked, or the toilet is 20+ years old and uses more than 1.6 gallons per flush. Modern 1.28 gpf toilets save water and often pay for themselves within a few years.
Not recommended. Bricks degrade and can damage the flush valve. If you want to reduce flush volume, use a purpose-made tank displacement bag or install a dual-flush conversion kit. Or just replace the toilet with a 1.28 gpf model.