A running toilet is usually caused by either a leaking flapper (80% of cases) or a faulty fill valve. Do the food coloring test: add dye to the tank—if color appears in the bowl without flushing, replace the flapper. If water runs into the overflow tube, adjust or replace the fill valve.
That constant running sound isn’t just annoying—it’s wasting water and money. The good news is that this is one of the easiest plumbing repairs, usually taking less than 30 minutes with parts costing under $20.
Understanding Your Toilet’s Mechanism
Before diagnosing, know the key components:
Flapper: The rubber seal at the bottom of the tank that lifts when you flush and drops to seal the tank for refilling.
Fill valve: The tall mechanism that refills the tank after flushing. Older toilets have a ball float; newer ones have a cup float on the valve shaft.
Overflow tube: The large tube in the center of the tank. Excess water drains into it to prevent tank overflow.
Flush valve: The assembly the flapper seals against. Includes the overflow tube.
The Diagnostic Test
This simple test tells you exactly which component is failing.
Food Coloring Test
- Remove the tank lid and set it safely aside
- Add 5-10 drops of food coloring to the tank water
- Wait 15-20 minutes without flushing
- Check the toilet bowl water
If color appears in the bowl: Your flapper is leaking—it’s not sealing properly, allowing water to seep from tank to bowl.
If color stays in tank: The flapper seal is fine. Your problem is the fill valve.
Visual Inspection While Running
With the tank lid off, watch what’s happening:
Water flowing into overflow tube: Fill valve issue—water level is too high, constantly draining excess.
Water level stable but you hear running: Flapper leak—water is slowly escaping to the bowl.
Fill valve cycles on periodically: Also a flapper leak—tank slowly empties, triggering refill.
Take a photo or video of the inside of your tank before touching anything. This helps you remember what goes where during reassembly.
Fixing a Leaking Flapper
The flapper is the culprit about 80% of the time. Here’s how to replace it.
Checking the Flapper
- Flush the toilet and watch the flapper
- It should drop quickly and seal completely
- Feel the flapper surface—it should be smooth and pliable
- Check for warping, cracks, or mineral buildup
- Inspect the flush valve seat (what the flapper seals against) for rough spots
Replacing the Flapper
- Turn off water at the shut-off valve behind the toilet
- Flush to empty the tank
- Unhook the old flapper from the flush valve pegs or ring
- Disconnect the chain from the flush lever
- Take the old flapper to the hardware store for matching
- Install the new flapper onto the flush valve pegs or ring
- Connect the chain to the flush lever—leave about 1/2 inch slack
- Turn water back on and let the tank fill
- Test by flushing several times
Chain Adjustment
The chain connecting the flapper to the flush lever matters:
- Too much slack: Flapper won’t lift enough for a complete flush
- Too little slack: Flapper can’t seat properly, causing leaks
- Correct: About 1/2 inch of slack when the flapper is closed
If your new flapper still leaks, the flush valve seat may be corroded or rough. You can buy a flush valve seat repair kit that installs over the old seat, or replace the entire flush valve.
Fixing Fill Valve Problems
If your flapper passed the dye test, the fill valve needs attention.
Adjusting Water Level
The water level should be about 1 inch below the top of the overflow tube. If it’s too high, water constantly drains into the overflow.
For cup-float fill valves (most modern toilets):
- Find the adjustment screw or clip on the valve
- Turn the screw clockwise or slide the clip down to lower water level
- Flush and observe the new water level
- Repeat until level is correct
For ball-float fill valves:
- Bend the float arm down slightly to lower water level
- Or turn the adjustment screw on the arm
- Flush and check the new level
Cleaning the Fill Valve
Debris can prevent the fill valve from shutting off completely:
- Turn off water supply
- Remove the fill valve cap (usually twists off)
- Cover the valve with your hand and briefly turn water on to flush debris
- Clean any visible debris from inside the cap
- Reassemble and test
Replacing the Fill Valve
If cleaning and adjusting don’t work, replace the fill valve:
- Turn off water and flush to empty tank
- Sponge out remaining water from the tank bottom
- Disconnect supply line (have a towel ready for drips)
- Remove locknut under the tank that holds the fill valve
- Pull out old fill valve from inside the tank
- Set new fill valve height per instructions (top should be above overflow tube)
- Insert new valve and tighten locknut underneath
- Connect supply line to the new valve
- Attach refill tube to overflow tube
- Turn on water and adjust water level
Most universal fill valves fit any toilet and cost $15-25. Brands like Fluidmaster and Korky are reliable choices.
Other Causes of Running Toilets
Faulty Flush Valve
If the flush valve seat is damaged, even a new flapper won’t seal properly. Signs include:
- New flapper still leaks
- Visible cracks or rough spots on the valve seat
- Mineral buildup that won’t scrub off
Solutions:
- Flush valve seat repair kit (covers damaged seat)
- Complete flush valve replacement (more involved)
Lift Chain or Handle Issues
- Chain tangled or caught under flapper
- Handle sticking in down position
- Handle mechanism damaged
Check that the handle moves freely and returns to position after flushing.
Cracked Overflow Tube
A crack in the overflow tube allows water to drain continuously. This requires flush valve replacement.
When to Call a Plumber
Most running toilet repairs are DIY-friendly, but call a pro if:
- Tank has visible cracks
- Flush valve needs replacement and you’re uncomfortable
- Multiple repairs haven’t solved the problem
- Toilet is very old and parts aren’t available
Prevention Tips
- Replace flappers every 4-5 years proactively—they degrade from chemicals and minerals
- Avoid drop-in tank tablets—the chlorine damages rubber components
- Don’t use chemical toilet cleaners in the tank
- Check your toilet monthly—listen for running, look for bowl movement
Cost Comparison
| Fix | DIY Cost | Plumber Cost |
|---|---|---|
| Flapper replacement | $5-15 | $75-150 |
| Fill valve replacement | $15-25 | $100-200 |
| Flush valve replacement | $20-40 | $150-250 |
The DIY savings are significant for these straightforward repairs.
Next Steps
If your toilet flushes poorly even after fixing the running issue, see Toilet Won’t Flush Well. For more toilet and general plumbing repairs, check our comprehensive Plumbing Fixes Guide.