Where the water level should sit
Inside the tank, look at the central plastic tube (the overflow tube). On modern toilets, there\'s a "WL" or "Water Line" marking on the inside tank wall — water should sit exactly at this line. If unmarked, water should sit 1 inch below the top of the overflow tube.
Too low: tank refill stops before the proper line. Flush feels weak, doesn\'t fully clear the bowl.
Too high: water rises above the overflow tube and constantly drains into the bowl. You hear continuous trickling between flushes; water bill creeps up.
Identify your fill-valve type
Cup/cylinder float fill valve (Fluidmaster 400A, Korky 528 — the modern standard): the float is a black plastic cup that slides up and down a central column.
Ball-float fill valve (Mansfield, older Kohler, older American Standard): a brass arm with a ball float at the end of a long horizontal arm.
Float-cup with screw (some Kohler, all Geberit, premium): a vertical screw adjusts the float position.
Adjusting a cup-float fill valve
Look at the float cup — there\'s typically a metal clip or plastic clip attached to a thin metal rod. Squeeze the clip and slide it down the rod (lowers water level) or up the rod (raises water level). Each 1/4-inch of movement equals roughly 1/4 inch of tank water level. After adjusting, flush and let refill; verify against the WL line. Iterate as needed.
Adjusting a ball-float arm
The arm is brass with a screw at the fulcrum. Turn the screw clockwise to lower water level, counterclockwise to raise. On older arms without a screw, you bend the arm gently — bend down to lower water, bend up to raise. Bend carefully — over-flexing snaps the brass.
Adjusting a vertical-screw float
Turn the screw on top of the fill valve clockwise to lower water level, counterclockwise to raise. 1 full turn ≈ 1/4 inch of water level change.
If adjustment doesn\'t work
Two possibilities: (1) the fill valve itself is shot — silt buildup or worn seal — replace the entire valve ($12, 25 minutes); (2) the overflow tube is too short for your float setting — increase by replacing the flush valve with a taller overflow.
The flush-power test
After adjustment, flush and observe: the tank should empty in 4-7 seconds with strong bowl-clearing action. If it dribbles weakly, water level is still too low or the flapper isn\'t fully opening. If the bowl rim sprays water out the front, water level is too high — back off the float by 1/2 inch.