Degrees of general hardness (dGH) is a unit of water hardness, specifically of general hardness.
General hardness is a measure of the concentration of metal divalent ions such as calcium and magnesium (Ca<sup>2+</sup>, 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, which is equivalent to 0.17832 mmol per litre of elemental calcium and/or magnesium ions.
Note that 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<sub>3</sub>) per litre of water. Consequently, 1 dGH = 17.848 ppm.