How to fix a running toilet by replacing the fill valve and adjusting float
A running toilet wastes water and can be fixed in one afternoon with basic tools. This guide walks you through replacing the fill valve and adjusting the float so the toilet stops running and refills to the correct level. You’ll need about 30–60 minutes and a few common supplies.
Step 1: Turn off the water supply
Locate the shutoff valve behind the toilet near the floor and turn it clockwise until it stops. Flush the toilet to drain most of the tank water; hold the handle to let as much water out as possible so you work with a low water level. This prevents spills and makes replacement easier.
[Illustration: hand turning shutoff valve behind toilet, tank open, water level low after flush]
Step 2: Remove remaining tank water
Use a small cup or sponge to soak up the last bits of water from the tank until it is nearly dry. Wring the sponge into a bucket or sink and leave a towel under the tank bolts to catch drips. A low water level keeps components dry and gives a clear view of the fill valve and float.
[Illustration: person sponging tank interior into a bucket with towel under bolts]
Step 3: Disconnect the water supply hose
Place a bucket under the supply hose nut and use an adjustable wrench to loosen the nut that connects the hose to the tank fill valve. Turn counterclockwise about 6–10 full turns until it comes free; expect a small residual drip. Removing the hose allows you to detach the old fill valve without water pressure.
[Illustration: wrench loosening supply line nut under toilet tank with small bucket below]
Step 4: Remove the old fill valve
Inside the tank, loosen the plastic locknut under the fill valve by hand or with pliers, turning it counterclockwise until the valve lifts out. Remove any clipped refill tube and set aside the old valve. Note how the old valve sat and the orientation of the refill tube for installing the new one.
[Illustration: hand unscrewing plastic locknut under fill valve from inside toilet tank]
Step 5: Install the new fill valve
Insert the new fill valve into the tank hole to the manufacturer’s recommended height (usually 1–2 inches above the overflow tube). Tighten the new locknut from under the tank finger-tight plus a quarter turn with pliers—do not over-tighten to avoid cracking the tank. Reattach the refill tube to the overflow pipe at the same angle as the original.
[Illustration: new fill valve inserted, person tightening locknut under tank with pliers, refill tube clipped to overflow tube]
Step 6: Reconnect supply and restore water
Reattach the supply hose to the new fill valve and tighten the nut by hand then a quarter turn with a wrench to ensure a snug fit. Slowly open the shutoff valve counterclockwise and let the tank fill; watch for leaks at the supply connection and under the locknut for 2–5 minutes. Tighten minor drips gently as needed.
[Illustration: person reconnecting supply hose to new fill valve and slowly opening shutoff valve to fill tank]
Step 7: Adjust float and test operation
Set the float height so the water stops about 1 inch below the top of the overflow tube or at the water level mark on the tank. For a cup-style or adjustable fill valve, turn the adjustment screw or slide the float to raise/lower by 1/4–1/2 inch increments until correct. Flush 3–5 times and observe 10 minutes to confirm the toilet no longer runs and refills properly.
[Illustration: hand adjusting float height on new fill valve while watching water level in tank]
- Buy a universal fill valve kit that fits most 2-piece toilets and includes a washer and refill tube. Expect to spend $15–30.
- Bring a small bucket and towels to catch water and protect the floor; a sponge or old rag speeds drying the tank.
- If the supply hose is old, replace it while you’re under the tank to avoid future leaks—new hoses cost about $5–15.
- Make small float adjustments (1/4 inch at a time) and wait 30–60 seconds after each change to see the new water level.
- Keep the old fill valve and parts until you’ve verified the fix for 24 hours in case you need to compare or reuse a fastener.
- If the flapper is worn or the seat is corroded, replace the flapper at the same time; a bad flapper can also cause running and costs $5–15.
- Do not overtighten the plastic locknut; excessive force can crack the porcelain tank and cause expensive damage. Tighten snugly by hand plus a small wrench turn only.
- Avoid working under the tank without support; if the tank is loose or cracked, do not proceed—call a plumber to prevent collapse or flooding.
- If you smell gas or detect a sewer odor while working, stop and ventilate the area immediately and contact a professional—this guide only covers water-side repairs.
- Turn the shutoff fully closed before disconnecting the supply; partial closure can still allow pressure and cause a spray when the hose is removed.
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