Degrees of general hardness (dGH or ðGH) 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 (Ca<sup>2+</sup>) and magnesium (Mg<sup>2+</sup>) per volume of water. Specifically, 1 dGH is defined as 10 milligrams (mg) of calcium oxide (CaO) per litre of water. Since CaO has a molar mass of 56.08 g/mol, 1 dGH is equivalent to 0.17832 mmol per litre of elemental calcium and/or magnesium ions.
In water testing hardness is often measured in parts per million (ppm), where one part per million is defined as one milligram of calcium carbonate (CaCO<sub>3</sub>) per litre of water. Consequently, 1 dGH corresponds to 10 ppm CaO but 17.848 ppm CaCO<sub>3</sub> which has a molar mass of 100.09 g/mol.