Water hardness is a function of the amount of dissolved calcium salts, magnesium salts, iron, and aluminum. These salts occur in a variety of forms, but are usually calcium and magnesium bicarbonates (referred to as "temporary hardness"), and sulphates and chlorides (referred to as "permanent hardness").

The best known effect of hard water is the prevention of soap from lathering. Most people cannot tolerate drinking water that exceeds 300ppm carbonate, or 1500ppm chloride, or 2000ppm sulphate. More than 500ppm sulphate can have a laxative effect on the body.

This formula is used to calculate total hardness:

Total Hardness in ppm Carbonate = (ppm Calcium x 2.497)
+ (ppm Magnesium x 4.115) + (ppm Iron x 1.792) + (ppm Manganese x 1.822)

Hardness is also measured in "grains per gallon" and "degrees". These are the equivalents:

• 1 ppm = 0.058 grains/US gallon
• 1 ppm = 0.07 Clark degrees
• 1 ppm = 0.10 French degrees
• 1 ppm = 0.056 German degrees
• 1 French degree = 1 hydrotimetric degree
• 1 Clark degree = 1 grain/Imperial gallon as calcium carbonate
• 1 French degree = 1 part/100,000 calcium carbonate
• 1 German degree = 1 part/100,000 calcium oxide
• 1 grain/US gallon = 17.1 ppm
• 1 grain/US gallon = 1.20 Clark degrees
• 1 grain/US gallon = 1.71 French degrees
• 1grain/US gallon = 0.958 German degrees