Total Hardness (TH) is a measure of the total amount of calcium and magnesium that has naturally leached into the water supply during its journey through the watershed. It is hard to produce soap suds in water with high levels of calcium and magnesium ions, hence the term "hardness". In addition to reducing the effectiveness of soaps and detergents, hard water may cause an insoluble scale to form on fixtures and on the inside of pipes. Scale formation depends on several factors, including temperature and pH.

The EPA does not regulate the levels of hardness in the water supply. Although the perception of hardness varies from place to place. the hardness level can be loosely classified according to the ranges below: