Water hardness is calculated in grains per gallon.
Grains Per Gallon of Hardness Conversion (Used for Customers installing a Water Softener)
City Water has 350 mg/l of hardness
350 mg/l =350 parts per million
1 grain per gallon = 17.12 parts per million
1 part per million = 0.05841 grains per gallon
350 x 0.0584 = 20.44 grains per gallon in city water