What it is
Magnesium is a mineral dissolved from rocks such as dolomite and from some clays. With calcium it determines how hard the water is.
Where it comes from
Groundwater passing through magnesium-bearing limestone carries the most. Levels are generally lower than calcium in UK supplies.
Why it matters
Magnesium is an essential nutrient and there is no drinking-water limit. Very high levels, well above anything usual in the UK, can give a bitter taste and a mild laxative effect, especially alongside high sulphate.
What can be done about it
Magnesium is removed with calcium by a water softener if scale is a problem. On its own it rarely needs treatment.
Information last reviewed 5 October 2026 by Pozzani Pure Water.