How to Fix a Running Toilet

Remove the toilet tank lid and set it on a towel (porcelain lids are heavy and crack easily on hard floors). Listen and watch: if water is trickling into the bowl, the flapper is leaking. If the fill valve keeps cycling on and off every few minutes (called "phantom flush"), the flapper is the culprit. If water is flowing into the overflow tube continuously, the float or fill valve needs adjustment. Understanding which part is failing tells you exactly what to fix.

Locate the shut-off valve on the wall or floor behind the toilet — it's a small oval handle or knob on the supply line. Turn it clockwise until it stops. Flush the toilet once to empty most of the tank water. Use a sponge to soak up the remaining water in the bottom of the tank. This gives you a dry workspace and prevents spills when you remove parts.

The flapper is the rubber disc at the bottom of the tank that seals the flush valve opening. Unhook the flapper's ears from the pegs on either side of the overflow tube and disconnect the chain from the flush lever. Examine the flapper: if it's warped, cracked, or feels slimy and deteriorated, it needs replacing. Take the old flapper to the hardware store to match the size (most toilets use a 2-inch or 3-inch flapper). Snap the new flapper onto the same pegs and reattach the chain.

The chain connecting the flapper to the flush lever needs exactly the right amount of slack — about 1/2 inch. If the chain is too short, it holds the flapper slightly open, causing a constant leak. If it's too long, it can get caught under the flapper and prevent a full seal. Hook the chain to the flush lever hole that gives you 1/2 inch of slack when the flapper is closed. Trim excess chain with wire cutters or bend the last link to keep it from tangling.

The water level in the tank should sit about 1 inch below the top of the overflow tube — a line is usually marked on the inside of the tank or on the overflow tube itself. If water is flowing over the overflow tube, the float is set too high. For a ball float (round ball on a metal arm), bend the arm gently downward to lower the water level. For a cylinder float (plastic cylinder that slides on the fill valve), squeeze the spring clip and slide the float down the rod about 1/2 inch at a time. Test by turning the water back on and checking where the level settles.

If adjusting the float doesn't stop the running and the fill valve hisses, vibrates, or won't shut off, replace the entire fill valve. Disconnect the water supply line from the bottom of the tank using adjustable pliers. Unscrew the lock nut on the outside of the tank that holds the fill valve in place. Lift out the old valve. Insert the new fill valve (Fluidmaster 400A is the industry standard at $8-12), adjust its height so the overflow tube sits 1 inch below the critical level mark, and tighten the lock nut. Reconnect the supply line and the refill tube to the overflow tube.

Open the shut-off valve by turning it counterclockwise. Let the tank fill completely — this takes 1-2 minutes. Watch the water level: it should stop about 1 inch below the overflow tube. Listen for any hissing or trickling sounds, which indicate a leak. Flush the toilet 3-4 times and confirm that it fills, stops, and holds water silently between flushes. Check the supply line connection and the bottom of the tank for drips.