Hard water is probably the most common water problem found in the home. Hard water spots your glasses and dishes, makes laundry dull, and causes soap scum and scale to build-up making cleaning a chore. The most common hardness causing minerals are Calcium and Magnesium that is dissolved in a water supply.
According to the Water Quality Association, hard water is water that contains dissolved hardness minerals above 1 GPG (grains per gallon). Relative levels of hardness have been established:
Soft Water - less than 1 gpg
Slightly hard - 1 to 3.5 gpg
Moderately hard - 3.5 to 7 gpg
Hard - 7 to 10.5 gpg
Very Hard - 10.5 and higher gpg
Using an ion exchange water softener is the most common form of treatment.