A grain is equivalent to 1/700th of a pound and is equal to 17.118 parts per million (ppm) or milligrams per liter (mg/l). In the context of water hardness, it is a measurement of the presence of calcium and magnesium in the water supply, usually termed grains per gallon or gpg. Note that each 1ppm of iron present in the water is equivalent to 3 grains of water hardness.