Degrees of general hardness (
dGH) is a
unit of
water hardness, specifically of general hardness. General hardness is a measure of the concentration of
divalent metal ions such as
calcium and
magnesium (Ca
2+, Mg
2+) per volume of water. Specifically, 1 dGH is defined as 10 milligrams (mg) of
calcium oxide (CaO) per litre of water, which is equivalent to 0.17832
mmol per litre of elemental calcium and/or magnesium ions, since CaO has a
molar mass of 56.0778 g/mol.