Most Likely Causes (Ranked)
The rubber flapper at the bottom of the tank lifts when you flush and seals the tank when it closes. After 3-7 years, the rubber warps, hardens, or develops mineral buildup that prevents a tight seal. Water slowly leaks from the tank into the bowl, the float drops, and the fill valve cycles on repeatedly. Chlorine tablets in the tank accelerate flapper deterioration dramatically — they can destroy a flapper in 6 months.
If the float is set too high, the water level rises above the overflow tube and continuously drains into the bowl. The fill valve runs non-stop trying to maintain level. This can happen after a fill valve replacement, when someone adjusts the float accidentally, or when the float develops a crack and fills with water (sinking lower, telling the valve the tank isn't full).
The fill valve controls water flow into the tank. Internal diaphragms and seals wear out, causing the valve to either run continuously, not shut off completely, or make a whining/humming noise. Older ballcock-style valves with a float ball on an arm are especially failure-prone. Modern Fluidmaster-style valves last 5-7 years on average.
The overflow tube (the large vertical tube in the center of the tank) can crack at its base where it connects to the flush valve. A crack below the water line causes a constant slow drain into the bowl. This requires replacing the entire flush valve assembly — not just the flapper.
If the chain connecting the flush handle to the flapper is too short, it holds the flapper slightly open. If a link catches on something, same effect. If it's too long, it can slip under the flapper and prevent sealing. This is the simplest fix but often overlooked.
How to Fix It
Replace the Flapper
$5-8- 1Turn off the supply valve behind the toilet. Flush to empty the tank.
- 2Unhook the old flapper from the overflow tube ears and disconnect the chain.
- 3Take the old flapper to the hardware store to match the size. Universal flappers (like Korky or Fluidmaster) fit 90% of toilets.
- 4Hook the new flapper onto the ears. Attach the chain to the flush lever with about 1/2 inch of slack.
- 5Turn on water, let the tank fill, and check for leaks with food coloring test.
Adjust or Replace Fill Valve
$8-12- 1To adjust: locate the adjustment screw or clip on the fill valve. Turn/slide to lower the water level to 1 inch below the top of the overflow tube.
- 2To replace: turn off supply, flush, sponge out remaining water. Unscrew the supply line and the lock nut under the tank. Lift out old valve.
- 3Install new valve (Fluidmaster 400A is the standard — $8). Set height to 1 inch below overflow tube. Hand-tighten lock nut + quarter turn.
- 4Reconnect supply, turn on water, and adjust the float to the water line mark on the tank.
Adjust the Chain
Free- 1Remove the tank lid. Flush and watch the chain.
- 2Adjust so there's 1/2 inch of slack when the flapper is closed.
- 3Cut excess chain length — extra links are the #1 cause of tangles.
- 4If the chain is corroded, replace it with a stainless steel chain ($2-3).
When to Call a Professional
A running toilet can waste 200 gallons/day — that's $50-100+ on your next water bill.
Don't use bleach tablets in the tank — they destroy flappers and gaskets within months.
If the toilet has been running for weeks and your water bill is very high, contact your water utility — many offer leak adjustments.
Shut off the supply valve as a temporary fix until you can repair it.
How to Prevent It
Replace the flapper every 4-5 years proactively ($5 and 5 minutes).
Do the food coloring test annually to catch slow leaks before they waste water.
Never use drop-in bleach/chlorine tablets — they destroy internal components.
Exercise the supply valve annually so it doesn't seize when you need it.
Frequently Asked Questions
A continuously running toilet wastes 4,000-8,000 gallons per month. Even a slow flapper leak wastes 200-1,000 gallons/month. At typical water rates, that's $20-100/month.
A slow flapper leak. The tank slowly loses water, the float drops, and the fill valve kicks on briefly. Replace the flapper — it's always the flapper.
Absolutely. Flapper replacement is one of the easiest home repairs. No tools needed. The entire job takes 5-10 minutes and costs $5-8.
Do the food coloring test. Color in bowl without flushing = flapper. Water flowing into overflow tube = float/fill valve. If neither, check the chain.