DGH
# DGH

Degrees of general hardness (dGH or °GH) 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 $(Ca^{2+})$ and magnesium $(Mg^{2+})$ 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, paper strips often measure hardness in parts per million (ppm), where one part per million is defined as one milligram of calcium carbonate $(CaCO_3)$ per litre of water. Consequently, 1 dGH corresponds to 10 ppm CaO but 17.848 ppm $CaCO_3$ which has a molar mass of 100.09 g/mol.

