Water hardness relates to the specific amount of calcium ions in a given volume of water measured in parts per million (ppm).
Measurements include 'General Hardness'. One degree of General Hardness is defined as 10 milligrams of calcium oxide per litre of water, which is the same as one German degree (17.848 ppm). Or 'degrees Clark' where one degree Clark is defined as one grain (64.8 mg) of calcium carbonate per Imperial gallon (4.55 litres) of water, equivalent to 14.254 ppm.
