How to Fix a Running Toilet in 15 Minutes

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Diagnose the Problem First (30 Seconds)

A toilet runs because water is leaking from the tank into the bowl. There are three common causes — each has a different fix.

To find which one: take the lid off the tank and observe.

• Water level is above the overflow tube (the tall tube in the center of the tank) → Flapper or fill valve issue. Water is overfilling and draining down the overflow.
• Flapper is visibly not seating properly (you can see it's slightly open, or there's mineral buildup around the flapper seat) → Replace the flapper.
• Water level is normal but toilet still runs intermittently (phantom flush) → Almost always a worn flapper that's leaking slowly.

The flapper is the cause in 80% of running toilet cases. Start there.

Fix 1: Replace the Flapper (Most Common, $5–$8)

The flapper is the rubber seal at the bottom of the tank that controls water flow into the bowl. It degrades over 2–5 years, warps, or accumulates mineral buildup that prevents a clean seal. See our full faucet repair guide for the other most common plumbing fix — the tools and steps are nearly identical.

What You Need

• Replacement flapper (match to your toilet brand — Korky Universal works for most, or bring the old one to the hardware store)
• Gloves (optional but recommended)
• That's it

Steps

1. Turn off the water supply. The shutoff valve is behind the toilet near the floor. Turn it clockwise until it stops.
2. Flush the toilet to empty the tank.
3. Disconnect the old flapper. Unhook the ears from the overflow tube ears, and detach the chain from the flush handle arm. Note which hole the chain is attached to — you'll use the same one.
4. Install the new flapper. Hook the ears onto the overflow tube, attach the chain to the handle arm. Leave a little slack in the chain — too much tension keeps the flapper from sealing; too much slack means it doesn't open fully.
5. Turn water back on. Let the tank fill and confirm the running has stopped.

Total time: 5–10 minutes. If the toilet still runs, check whether the chain length is right before moving to the next step.

Fix 2: Adjust or Replace the Fill Valve (If Water Is Above Overflow)

If the water level in the tank is too high — at or above the overflow tube — water is constantly draining into the bowl. This means the fill valve isn't shutting off at the right level.

Try Adjusting the Float First

On older ball-float mechanisms: bend the float arm down slightly so the ball sits lower. This tells the valve to stop filling at a lower level.

On modern fill valves (a single vertical valve): look for an adjustment screw or turn the top of the fill valve counterclockwise while pressing down, then turn clockwise to lock it at a lower setting. The water level should be about 1 inch below the top of the overflow tube.

If Adjusting Doesn't Work: Replace the Fill Valve ($10–$15)

A Fluidmaster 400A is the standard replacement — compatible with most toilets, about $10, and includes instructions.

1. Shut off water, flush, and use a sponge to remove remaining water from the tank.
2. Disconnect the water supply line from the bottom of the tank.
3. Unscrew the lock nut holding the old fill valve from below the tank (pliers or wrench).
4. Insert the new fill valve, adjust its height per the instructions, and tighten the lock nut.
5. Reconnect the supply line, turn water on slowly, adjust the water level.

Fix 3: The Dye Test (Confirm the Flapper Is Leaking)

If you're not sure whether the flapper is the problem, add a few drops of food coloring to the tank. Don't flush for 15 minutes. If color appears in the bowl, the flapper is leaking — replace it. No color means the flapper is sealing properly and the issue is elsewhere.

When to Call a Plumber

None of these fixes require a plumber. The one exception: if the toilet is running because the tank itself is cracked (water is visible on the outside of the tank). Cracked porcelain = replacement, not repair.

Also worth calling a plumber: if the shutoff valve behind the toilet won't fully close (it drips even fully clockwise). That valve needs replacing before you can do any tank work.

Total Cost Breakdown

• Flapper only: $5–$8
• Fill valve only: $10–$15
• Flapper + fill valve together (full rebuild kit): $15–$20
• Plumber for same repair: $125–$250

A $15 rebuild kit handles 95% of running toilet problems. This is one of the best ROI home repairs you can do. While you're at it, check the caulk around the toilet base — cracked caulk lets water seep underneath and damage the subfloor.