Calcium and magnesium levels combine to determine the mineral water’s “hardness”. Bottled water is naturally soft, thanks to low levels of calcium and magnesium. Higher levels are often found in municipal water, which is often “softened”—particularly in the United States—to be used at home. The taste of water is impacted heavily by softening.

Hard Water

Hard tap water makes cleaning more difficult and more dependent on soaps and synthetic detergents. Scaling in boilers and teakettles comes from hard water. But hard water does not pose any danger to your health: According to the U.S. National Research Council, the magupnesium and calcium in hard water can actually contribute to your daily dietary requirements.

Soft Water

Water is softened with an ion-exchange water softener, which adds sodium (salt) to the water. About 8 mg/l of sodium are added for every grain of hardness (17.1 mg/l) taken out. Water softening accustoms most Americans to slightly salty water. In the United States, water hardness is not regulated by the Environmental Protection Agency. Instead the Department of the Interior and the international Water Quality Association offer the following classifications:

Hardness can be calculated with this formula (calcium and magnesium should be measured in milligrams per liter): HARDNESS = (calcium x 2.5) + (magnesium x 4)