Grains per gallon
The standard U.S. unit for measuring water hardness.
Grains per gallon (gpg) is how water hardness is measured in the United States. The U.S. Geological Survey calls water soft below 3.5 gpg, moderately hard from 3.5 to 7, hard from 7 to 10.5, and very hard above 10.5 (one grain per gallon equals about 17.1 parts per million of calcium carbonate). You need your gpg number to set a water softener's hardness dial correctly — on well water, add a few grains to account for iron.