What Is a Toilet Fill Valve?

The vertical valve inside a toilet tank that refills it after a flush and shuts off when the float reaches the set water level. A fill valve that won't hold a consistent level has a failing internal seal; replacement valves cost $12–$18 and install hand-tight.

Two failure modes matter. If the water level sits too high and spills into the overflow tube, the float is just set wrong. Most modern valves adjust with a screw or a sliding clip, a 5-minute fix costing nothing. If the level wanders no matter where you set the float, or the valve hisses and cycles on its own, the internal diaphragm is worn out and adjustment will never hold.

Replacing one is easier than it looks: shut the supply valve, empty the tank, unscrew the supply line and the plastic locknut under the tank, drop the new valve in, and set its height so the top sits about an inch above the overflow tube. Everything is hand-tight. Porcelain cracks long before plastic threads strip, so no wrench-force anywhere. Fluidmaster's 400-series is the de facto standard and fits nearly every two-piece toilet in the US.