How do I adjust the water level in a toilet cistern?

Cistern level is the setting that quietly decides both whether your flush works and whether you are running water into the pan continuously. It takes two minutes to check and is worth doing whenever a toilet is behaving oddly.

What you'll need

• A screwdriver
• A ruler or your eye
• Possibly an adjustable spanner for old ballvalves

Step by step

1. Take the lid off and find the marked line.Most cisterns have a moulded water-line mark on the inside wall, or the fill valve carries one. That is the target.
2. Locate the internal overflow.On modern cisterns it is part of the flush valve — an open tube in the middle. The water should sit roughly 20mm below its top edge.
3. Identify your float type.A cup float sliding on a vertical stem, a small float on a short arm, or an old plastic or copper ball on a long brass arm.
4. Adjust a modern cup float.Squeeze the clip on the stem and slide the float down to lower the level, up to raise it. Some use a thumbwheel or a screw at the top of the stem instead.
5. Adjust a short-arm valve.There is usually an adjusting screw where the arm meets the valve. Turning it in lowers the level. Small movements — a turn is a lot.
6. Adjust an old ballvalve.Bend the brass arm gently downwards to lower the level, upwards to raise it. Support the valve body while you do it so you are not levering on the pipework.
7. Flush and refill to test.Adjust, flush, and let it refill fully before judging. Guessing from a half-full cistern is useless.
8. Check the flush is still strong.Too low a level gives a weak, ineffective flush that leaves the pan dirty. If in doubt, err slightly high but stay below the overflow.
9. Confirm nothing runs into the pan.Leave it ten minutes and look at the back of the pan. Any trickle means either the level is above the overflow or the flush valve seal is leaking.

Tips

• Adjust in small steps and always retest with a full refill. It is easy to overshoot.
• If the level creeps up over the hours after filling, the valve is not shutting off — that is a diaphragm problem, not a level one.
• Some dual-flush cisterns have a separate adjustment for the flush volume as well as the fill level. Check both.

Common mistakes

• Setting the level above the internal overflow — Water runs continuously into the pan through the overflow. It is silent, invisible on a metered supply until the bill arrives, and can waste hundreds of litres a day.
• Bending a brittle old plastic float arm — Old plastic arms snap rather than bend. Those types use an adjusting screw instead — find it rather than forcing the arm.

If it doesn't work

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