Water is considered “hard” when it contains dissolved minerals, primarily calcium and magnesium. These are not harmful to your health, but they can create a white “scale” coating on fixtures and on the insides of appliances like your dishwasher or water heater. Some people find the effects of hard water on hair and skin annoying as well as stains in their bathroom.
The level of hardness is measured in grains per gallon (GPG).