Definition of Grains Per Gallon

Hardness is a term to describe the presence of calcium and magnesium minerals in water. A chemical analysis accurately measures the amount of minerals in grain weight. To relate it to something familiar, one aspirin tablet usually weighs five (5) grains.


Example: One gallon of water with 5 grains per gallon, (gpg) hardness has dissolved minerals, calcium and magnesium, that if solidified would equal the size of 1 ordinary aspirin tablet. One gallon of water that is 25 gpg hard, has the mineral content of 5 aspirin tablets. A softener rated at 23,000 gpg (grains per gallon) simply stated, means that the softener will remove 23,000 gpg before it regenerates. A 31,000 gpg unit removes 31,000 before it regenerates and so on.


Sometimes hardness is provided in ppm (parts per million). Use the following information to convert gpg to ppm: 1 grain per U.S. gallon (gpg) = 17.1 milligrams per liter (mg/L) = 17.1 ppm (parts per million).