What is a grains per gallon (gpg)?
Definition of grain per gallon (gpg)
A grain is a historic unit of weight, originally defined in England as the weight of a barleycorn. One grain is equivalent to 64.799 milligrams. A grain per gallon is the weight of a substance in one gallon of water, similar to a milligram per liter. Converting grains per gallon to milligrams per liter:1 grain = 64.799 milligrams 1 gallon = 3.785 liters 64.799 divided by 3.785 liters = 17.1 1 gpg = 17.1 mg/L.