Replacing the Flapper & Fill Valve
Updated February 20, 2026
A running toilet wastes around 200 gallons of water a day. That's real money on your water bill every month. Nine times out of ten, the fix is a flapper or fill valve -- both under $15, both replaceable in under 30 minutes with no special tools. These are the two parts that wear out most often, and they're the first things any plumber checks.
Overview
A running toilet wastes around 200 gallons of water a day. That's real money on your water bill every month. Nine times out of ten, the fix is a flapper or fill valve -- both under $15, both replaceable in under 30 minutes with no special tools. These are the two parts that wear out most often, and they're the first things any plumber checks.
What to Know
Tools & Materials
- Adjustable wrench or pliers
- Bucket and towels
- Replacement flapper (match size to your toilet)
- Replacement fill valve (Fluidmaster 400A fits most)
- Food coloring (for diagnosis)
Step by Step
-
Shut off the water
Turn the shut-off valve behind the toilet clockwise until it stops. Flush the toilet to empty the tank. Sponge or towel out any remaining water.
-
Replace the flapper
Unhook the old flapper from the overflow tube ears and disconnect the chain from the flush lever. Slip the new flapper over the ears. Connect the chain to the flush lever -- leave about 1/2 inch of slack. Too tight and it won't seal; too loose and it won't lift enough.
-
Replace the fill valve
Disconnect the water supply line from the bottom of the tank. Unscrew the mounting nut under the tank. Lift out the old fill valve. Insert the new one, adjusting the height so the top is about 1 inch above the overflow tube. Tighten the mounting nut hand-tight plus a quarter turn. Reconnect the supply line.
-
Set the water level
Turn the water back on. The tank should fill to about 1 inch below the top of the overflow tube. Adjust the float (usually a screw or clip on the float cup) until the level is correct.
-
Test everything
Flush a few times. Check for leaks at the supply connection and under the fill valve nut. Verify the flapper seals completely. Do the food coloring test again to confirm.
Pro Tips
- Buy both parts at once. If one is worn, the other usually isn't far behind.
- Flapper chains get tangled -- route the chain so it has a clear path and trim excess length.
- Don't overtighten the fill valve mounting nut. Plastic threads strip easily.
- If your shut-off valve behind the toilet is stuck, replace it while you're at it.
Mistakes to Avoid
- Buying the wrong flapper size -- 2-inch and 3-inch are not interchangeable.
- Setting the chain too tight, holding the flapper slightly open.
- Overtightening the fill valve nut and cracking the tank.
- Not checking the flush valve seat for mineral buildup -- even a new flapper won't seal on a rough seat.
When to Call a Pro
If the flush valve seat itself is cracked or heavily corroded, the entire flush valve needs replacement -- that requires pulling the tank off the bowl. If the toilet is old and multiple parts are failing, a full replacement is often more cost-effective.
Bottom Line
A running toilet is almost always a $10-$25 fix that takes 30 minutes. Replace the flapper first -- that's the culprit most of the time. If the fill valve is also acting up, replace both while you're in there.