Total Water Hardness (TH)  is often measured in parts per million (ppm), grains (64.8 g of calcium carbonate per gallon, gpg), and millimoles of calcium per liter (mmol/L). The most common unit is ppm, which represents one milligram of calcium carbonate CaCO3 in one liter of solution (mg/L). To indicate total hardness, concentrations of dissolved salts of calcium and magnesium are usually combined and expressed as ppm of calcium carbonate.

The advantages of removal of hard ions from industrial and domestic water are many. Hardness salts in water are the primary cause of tube and pipe scaling, which frequently causes failures and loss of process efficiency due to clogging or loss of heat transfer.

"Hard ions" are removed by replacing them with so called "soft ions", such as sodium and potassium, which is achieved in water softening and conditioning systems.

Water treatment to decrease its hardness has significant economical effect. In industrial plants, boilers, and coolers, hard water causes scaling. Hard water negatively affects application of soaps and detergents. For every grain of water hardness, detergent use increases by 2 to 4 % per 1,000 gallons of water used.