, or general hardness, is a combination of Calcium plus Magnesium ions. But it would be very rare to have a water supply high in magnesium but low in calcium; usually the opposite is true, and calcium makes up a much greater percentage of the hardness in GH
. So effectively, if you have high GH
you can safely assume you have a pretty high level of calcium.
is carbonate hardness. It may have some accidental relationship to calcium (and often does) but there is no guarantee in that at all. So KH
, of itself, is not of any particular value in determining the calcium content of the water. GH
is a much better test of that.