How to Fix a Running Toilet
Updated February 20, 2026
Diagnose and fix a running toilet -- flapper replacement, fill valve swap, float adjustment, and overflow tube check -- with the dye test that tells you exactly what is wrong.
Overview
A running toilet can waste 200+ gallons of water per day. That is thousands of gallons a month and real money on your water bill. The good news: almost every running toilet comes down to one of three cheap parts inside the tank -- the flapper, the fill valve, or the float. No plumbing experience needed. Parts are $5-25, the repair takes 15-30 minutes, and a simple food coloring test tells you exactly which part is the problem.
What You'll Need
Safety First
- Turn off the supply valve (the oval handle on the wall behind the toilet) before pulling anything out of the tank. Clockwise to close.
- Flush after shutting off the supply to empty most of the water. Sponge out whatever is left before removing parts.
- The water in the tank is clean supply water, not bowl water. But old tanks can have mineral buildup and mold on the inside surfaces, so gloves are not a bad idea.
Step-by-Step Instructions
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Identify the Cause with a Simple Dye Test
Take the tank lid off (set it on a towel -- it will crack if you drop it). Drop a few drops of food coloring into the tank water and wait 15-20 minutes without flushing. Colored water seeping into the bowl? The flapper is leaking. Water level rising above the overflow tube and spilling into it? The fill valve or float needs adjustment. Toilet cycles on and off every few minutes? That is phantom flushing -- a slow flapper leak.
Tip: No food coloring? A dye tablet (free at most hardware stores) or a few drops of coffee work. You just need to see whether water is passing from the tank to the bowl without a flush. -
Replace the Flapper (Most Common Fix)
The flapper is the rubber seal at the bottom of the tank -- it lifts when you flush and seals when the tank refills. Over time, the rubber deteriorates, warps, or gets mineral buildup, and water seeps past. Shut off the water and flush to empty the tank. Unhook the old flapper from the pegs on the overflow tube (most just unclip) and disconnect the chain from the flush lever. Bring the old flapper to the hardware store to match the size -- 2-inch or 3-inch are standard. Clip the new one onto the same pegs, connect the chain, and adjust it so there is about 1/2 inch of slack.
Tip: Chain length matters. Too much slack and the flapper does not lift high enough for a full flush. Too little and it holds the flapper slightly open, causing it to leak. Half an inch of play when the flapper is seated is the target. -
Adjust the Float Level
Dye test showed water flowing over the overflow tube? The tank water level is too high. The float controls when the fill valve shuts off. Ball float (round ball on an arm): bend the arm slightly downward. Cup float (cylindrical float on the fill valve shaft): squeeze the side clip and slide the float down about 1/2 inch. Target is 1 inch below the top of the overflow tube. Turn the water back on and watch it fill -- the fill valve should shut off right at your target level.
Tip: Mark the overflow tube with a marker at 1 inch below the top. Gives you a clear target line and makes it easy to verify the adjustment without measuring every time. -
Replace the Fill Valve (If Adjustment Does Not Work)
Float adjustment did not fix it, or the fill valve is whining, humming, or hissing? The fill valve needs replacing. A universal Fluidmaster 400A fits most toilets and costs $8-15. Shut off the water, flush, sponge out what is left. Disconnect the supply line from the bottom of the tank (towel ready for drips). Unscrew the lock nut holding the old fill valve and pull it out. Drop the new one in, adjust the height so the top sits about 1 inch above the overflow tube, tighten the lock nut (hand-tight plus a quarter turn), reconnect the supply line, and hook the refill tube to the overflow tube. Turn the water on and check for leaks.
Warning: Do not crank on the lock nut. The tank is porcelain and it will crack. Hand-tight plus a quarter turn with pliers is all you need. If it drips, give it another quarter turn -- that is it. -
Check the Overflow Tube
Rare case: the overflow tube itself is cracked or too short, letting water drain continuously into the bowl even with a working float and fill valve. Check the tube for cracks, especially at the base where it connects to the flush valve. Cracked tube means replacing the entire flush valve assembly -- a bigger job that requires pulling the tank off the bowl. If the tube is fine but the water level will not adjust low enough (some older fill valves have limited range), swapping in a modern adjustable fill valve fixes it.
Tip: If the flush valve needs full replacement, consider whether a new toilet makes more sense. If it is 20+ years old and using 3.5-5 gallons per flush, a new 1.28 GPF toilet pays for itself in water savings within a year or two. -
Test and Verify the Repair
Turn the supply back on and let the tank fill completely. Listen -- the fill valve should shut off cleanly with no running water sound. Wait 30 minutes and check the water level -- it should not have dropped. If you replaced the flapper, do the food coloring test again: dye in the tank, wait 15 minutes, check the bowl. No color means the seal is good. Flush 2-3 times to confirm full flushes and proper refills. Check under the tank for drips from the supply line or lock nut.
Tip: Still runs after a new flapper? Check the flapper seat -- the ring the flapper seals against. If it has mineral buildup or pitting, clean it with fine steel wool or emery cloth. A rough seat prevents even a brand new flapper from sealing.
Pro Tips
- Buy a universal toilet repair kit ($15-20) -- flapper, fill valve, and supply line in one box. Even if you only need the flapper today, having the fill valve on hand saves a second trip when it eventually goes.
- Write the install date on the new flapper with a marker before you put it in. Flappers last 3-5 years. Knowing when it went in tells you when to start watching for the next failure.
- Hard water or chlorinated water? Standard black rubber flappers deteriorate faster. Look for red or blue flappers rated for hard water -- they last significantly longer.
- After the repair, note your water meter reading and do not use any water for 2 hours. If the meter moved, you have another leak somewhere in the house.
- Toilet parts are not fully universal. Toto, American Standard, and Kohler sometimes use proprietary flush systems. If a universal flapper will not seal, check whether the manufacturer offers a brand-specific replacement.
When to Call a Pro
This is one of the easiest DIY plumbing repairs. Call a plumber only if the tank is cracked and leaking onto the floor (replacement time), if the flush valve assembly needs replacing and you are not comfortable pulling the tank off the bowl, or if you have swapped the flapper and fill valve and it still runs (possible invisible crack in the porcelain).