Grains per gallon (GPG),

Definition of Grains per gallon (GPG):

1. One unit of water hardness is equal to 1 grain (64.8 mg) of calcium carbonate dissolved in 1 gallon of water. This is equivalent to 1 part in 70,000 parts of water, or 14.3 parts per million (ppm). Also known as the Clark Class. See also hard water.