Adjust the fill valve inside your toilet cistern so it shuts off at the right water level, stopping waste or weak flushes.
WT-01Find your water supply valveLook for a small isolation valve on the pipe leading into the cistern, often behind or below the toilet. It usually has a slotted screw head you turn with a flat screwdriver or coin. If there is no isolation valve, find your main stopcock, usually under the kitchen sink, and check it turns.
WT-02Turn off the water and drain the cisternTurn the isolation valve fully clockwise to shut it. Flush the toilet and hold the handle down to drain out as much water as you can. This gives you a dry, clear view of the fittings inside.
WT-03Mop out the last of the waterUse a sponge or old cloth to soak up any water left in the base of the cistern. This lets you see the float, the fill valve and the overflow pipe clearly, and check the current water mark before you change anything.
WT-04Identify your fill valve typeOlder cisterns have a 'ballcock': a plastic or copper float on a metal arm connected sideways to the valve. Newer cisterns have a 'float cup' or diaphragm valve, with a float that slides up and down a vertical rod, usually with a plastic clip or screw. Take a photo on your phone before you touch anything, so you can see how it was set.
WT-05Adjust an old-style ballcock (float arm)To lower the water level, gently bend the metal float arm downward a small amount. To raise it, bend it upward slightly. Some plastic arms have a small screw at the pivot instead of a bendable arm — turn this rather than forcing the plastic, as it can snap.
WT-06Adjust a modern float cup valveLook for an adjustment screw on top of the valve, or a plastic clip on the float rod. Turning the screw clockwise, or sliding the clip down the rod, usually lowers the water level. Check any arrows or markings on the valve body, as some models work the opposite way.
WT-07Turn the water back on and let it refillOpen the isolation valve slowly. Watch the cistern fill and confirm the valve shuts off by itself once it reaches the new level. Do not walk away yet, in case it overfills or fails to stop.
WT-08Check the level against the overflow markMost cisterns have a 'water line' marked on the inside wall, or the correct level is about 25mm below the top of the overflow pipe (the wider tube standing up inside the cistern). Compare the filled level to this mark.
WT-09Fine-tune and re-testIf the level is still wrong, turn the water off again, make a small further adjustment, then refill and check. Small tweaks are easier to control than big ones, so go gradually.
WT-10Check for leaks, then flush and tidy upFlush once fully and watch the refill again to be sure it stops cleanly. Feel around the valve and isolation valve for any drips, and check the outside overflow pipe on the wall of the house isn't dripping. Dry the cistern rim and replace the lid.
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