Quick answer
Most running toilets are fixed by either replacing a worn flapper or adjusting/replacing the fill valve. Start with a dye test to confirm if water is leaking past the flapper into the bowl. If not, check whether water is spilling into the overflow tube and adjust or replace the fill valve. These two steps solve the issue in the vast majority of cases.
How a running toilet wastes water
A slow, constant flow from the tank to the bowl can waste hundreds of gallons per day. Causes include:
- A deteriorated flapper not sealing the flush valve
- A misadjusted or failing fill valve allowing water to rise and spill into the overflow
- A chain that’s too tight or tangled, preventing the flapper from fully closing
- Less common: a cracked overflow/flush valve seat or specialty mechanisms (dual-flush, pressure-assist)
Tools and materials
- Adjustable wrench or 8" tongue-and-groove pliers
- Screwdriver (if replacing handle or tank bolts)
- Sponge and small bucket/towel
- New flapper (2" or 3"; match your toilet)
- Universal fill valve (e.g., Korky QuietFILL or Fluidmaster 400-series)
- Refill tube clip
- Optional: replacement handle/lever, tank-to-bowl gasket and brass bolts if tackling a flush valve
- Food coloring for dye test
Estimated time and cost:
- Flapper replacement: 10–20 minutes; $5–$15
- Fill valve adjust/replace: 15–40 minutes; $15–$30
- Flush valve (requires removing tank): 60–120 minutes; $15–$35
Safety and prep
- Turn off the toilet’s shutoff valve clockwise. If it’s stuck, don’t force it—gently work it back and forth. Place a towel under the supply line.
- Don’t overtighten plastic locknuts; tanks can crack.
- If your supply line is corroded, consider replacing it with a braided stainless connector during the repair.
Step-by-step: Diagnose and fix
1) Do a quick inspection
- Remove the tank lid. Is water trickling into the overflow tube? If yes, your water level is too high or the fill valve is failing.
- Do you hear hissing even when the tank is full? That points to the fill valve.
- Is the chain too tight or caught under the flapper? You want a little slack, not a droop that tangles.
2) Dye test for flapper leaks
- Put 5–10 drops of food coloring into the tank. Wait 10–15 minutes without flushing.
- If the bowl water changes color, the flapper is leaking. Replace it:
Flapper replacement steps:
1. Turn off water. Hold the flush lever to drain the tank; sponge out remaining water.
2. Unhook the old flapper from the posts and detach the chain.
3. Clean the flush valve seat (the round rim the flapper seals against) with a non-scratch pad.
4. Install the new flapper. Ensure it fits your valve size: most older toilets use 2"; many newer high-efficiency models use 3".
5. Adjust chain so there’s 1/4"–1/2" slack with the flapper closed.
6. Turn water on, test several flushes, and recheck the dye test if needed.
3) Set the correct water level or replace the fill valve
If water is entering the overflow tube or you hear hissing:
- Adjust the float:
- For a float-cup valve: turn the adjustment screw (or squeeze the clip) to lower the float until the water stops about 1" below the top of the overflow tube.
- For a float-ball: bend the arm down slightly to lower the shutoff level.
- If adjustment doesn’t hold or the valve keeps hissing, replace the fill valve:
Fill valve replacement steps:
1. Turn off water and flush. Sponge out remaining water so the tank is mostly dry.
2. Disconnect the supply line at the tank with an adjustable wrench.
3. Unscrew the fill valve locknut under the tank and lift out the old valve.
4. Set the new valve height so the CL (critical level) mark sits at least 1" above the top of the overflow tube.
5. Insert the valve, hand-tighten the locknut, then snug it 1/4 turn with pliers—do not overtighten.
6. Reconnect the supply line. If it’s a compression connector, no tape is needed on the threads.
7. Attach the refill tube using the clip so it discharges into the overflow, not down inside it (preventing siphoning).
8. Turn water on, check for leaks with a dry tissue, and fine-tune the water level.
Key specs:
- Water level: ~1 inch below top of overflow tube
- Chain slack: 1/4–1/2 inch
- Fill valve: CL mark at least 1 inch above overflow tube
4) Check the handle and lever
- Ensure the lever returns freely and the chain doesn’t snag on anything.
- If the handle is corroded or sticking, replace it (simple nut inside the tank). Cost: ~$8–$15.
5) Less common: Replace the flush valve (tank off)
If the overflow tube is cracked or the flush valve seat is pitted, you’ll need a new flush valve:
1. Turn off water, drain tank, and disconnect the supply.
2. Remove the two or three tank-to-bowl bolts (nut under the bowl). Use a screwdriver inside and pliers below. Penetrating oil helps on stuck fasteners.
3. Lift the tank onto a towel. Remove the large locknut holding the flush valve and pull the assembly.
4. Install the new flush valve with a fresh tank-to-bowl gasket and brass bolts. Reassemble, fill, and test.
Tips for best results
- Match parts to your toilet: 2" vs 3" flapper and the correct style (rigid vs flexible). Bring the old part to the store or check the model number inside the tank.
- Avoid drop-in tank cleaners; they degrade rubber parts and cause early flapper failure.
- Replace old, crusty supply lines while you’re there. Braided stainless lines are affordable and reliable.
- After any repair, let the toilet sit for 30–60 minutes and recheck for slow seepage.
Common mistakes
- Pushing the refill tube down inside the overflow tube (can siphon water and make it run continuously). Use the clip so it hovers at the top.
- Too little chain slack, which holds the flapper open after a flush.
- Overtightening plastic nuts, leading to cracked porcelain.
- Setting the water level above the overflow—water will never stop filling.
When to call a pro
- The shutoff valve won’t close or is corroded and leaking.
- Hairline cracks in the tank or persistent leaks at tank-to-bowl connections.
- Specialty toilets (dual-flush carriers, pressure-assist units) that require brand-specific parts.
- Severely rusted bolts that may need cutting and careful re-sealing.
With a basic DIY toolkit and a $5–$30 part, most running toilets are fixed in under an hour and stay that way with proper adjustment.