See our map of water hardness (as calcium carbonate) in surface water of the United States. Hardness data (reflecting mostly calcium, plus a little magnesium) for individual drinking-water suppliers is at the EPA Office of Ground Water and Drinking Water website.
It is important to note that the U.S. Environmental Protection Agency (EPA) has not set a legal limit or standard for hardness in water. This is primarily because the constituents that contribute to hardness (generally calcium and magnesium ions) are not toxic; that is, they do not cause harmful health effects. Instead, there is a generally accepted division of water into categories of:
- Soft: 0-60 milligrams per liter (mg/L) as calcium carbonate
- Moderately hard: 61-120 mg/L as calcium carbonate
- Hard: 121-180 mg/L as calcium carbonate
- Very hard: more than 180 mg/L as calcium carbonate
Most water utilities try to provide water that is not in the very hard category because of unpleasant effects such as scaling in equipment and the need for more soap and synthetic detergents. Many homeowners in hard-water areas use water softeners to further reduce hardness by substituting sodium for calcium and magnesium.
Learn more: Hardness of Water