(Level of ) hardness of water

A value defined by the quantity of the alkaline earth metals calcium (Ca+2) and magnesium (Mg+2) in water, expressed as calcium oxide (CaO) or calcium carbonate (CaCO3). The former is known as German hardness, and shows the quantity of calcium and magnesium as milligrammes of calcium oxide per 100ml of water. Readings on the German scale (when expressed in Japanese) are not given as milligrammes, but in degrees (using the word 度 (do)). The latter is known as American hardness, and expresses the quantity of CaCO3 in parts per million (ppm, the number of milligrammes in 1000ml of water).

Formerly, the German scale was widely used, but now the American scale predominates. Values in the German hardness scale can be converted to the American by multiplying by 17.8.