A water hardness chart displays the amount of dissolved minerals in a single gallon of water. Usually, the chart lists the amount of mineral solids in grains, milligrams or parts per million.
Water "hardness" occurs when a given volume of water contains an excessive level of dissolved mineral solids. This makes the water more difficult to use for cleaning, and requires more detergent or soap to get the same scrubbing power as is present in the same volume of pure or "soft" water. Hard water also damages heaters and industrial equipment. Although various minerals contribute to water hardness, the most important are metallic compounds, especially compounds containing calcium and magnesium.
Water hardness charts list the amount of mineral solids present in a gallon of a given supply of water in units called grains. Often, they also provide the figures in milligrams per liter and parts per million The last component of a water hardness chart is a hardness rating in simple English. Water that has more than 7.0 grains per gallon is rated Hard, while water that has more than 3.5 grains per gallon is rated Moderately Hard.