grains per gallon (GPG)
A common basis of reporting water analysis in the United States and Canada. One grain per U.S. gallon equals 17.1 milligrams per liter or 17.1 ppm. One grain per Imperial gallon equals 14.3 milligrams per liter or 14.3 ppm. One grain is 1/7000 pounds or 0.0647 grams.