Water hardness refers to how much Calcium and Magnesium is in your water. Water rich in Calcium salts is considered "hard".
hard water. Compare with water softener. Hard water is water contaminated with compounds of calcium and magnesium. Dissolved iron, manganese, and strontium compounds can also contribute to the "total hardness" of the water, which is usually expressed as ppm CaCO3. Water with a hardness over 80 ppm CaCO3 is often treated with water softeners, since hard water produces scale in hot water pipes and boilers and lowers the effectiveness of detergents.
The U.S. Department of the Interior classifies hardness based on the grains per gallon (gpg) concentration of the hardness minerals. To put this in perspective, a typical aspirin equals about five grains of material. If the aspirin were dissolved in a gallon of water it would add 5 gpg of "aspirin" to the water. Where hardness is concerned, water containing 1-3.5 gpg of the hardness minerals calcium and/or magnesium is classified as slightly hard; water in the 3.5-7.0 gpg range is considered to be moderately hard; at 7.0-10.5 gpg water is considered to be hard; and very hard water is classified as water with concentrations greater than 10.5 gpg. (Conversely, Soft water has a hardness of less than 1 gpg.)