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 (Ca2+, Mg2+) 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.
In water testing, paper strips often measure hardness in parts per million (ppm), where one part per million is defined as one milligram of calcium carbonate (CaCO3) per litre of water. Consequently, 1 dGH corresponds to 17.848 ppm, since CaCO3 has a molar mass of 100.0875 g/mol.
References
DGH Wikipedia(Text) CC BY-SA