Grains per Gallon ↔ ppm Hardness

The grain is a survivor from the old apothecaries' system (1/7000 of a pound, originally the weight of a single barleycorn) and North American water treatment never let it go. One grain of CaCO₃ dissolved in one US gallon is 64.79891 mg in 3.785412 L, which is 17.118 mg/L. That is the whole conversion: 10 gpg is 171 mg/L, and a residential softener rated "30,000 grains" will remove 30,000 grains of hardness before it needs salt. Softener control valves, resin capacity tables and salt-dose settings are all published in grains, while every laboratory report and every regulatory limit is in mg/L, so the technician lives in both worlds at once.

Watch out for the imperial gallon. A British or Canadian "grain per gallon" historically meant grains per imperial gallon (4.546 L), which works out to 14.25 mg/L: a 20% difference that will undersize a softener if you take a UK spec sheet at face value. And remember the informal hardness bands are quoted in grains: under 1 gpg is soft, 1–3.5 slightly hard, 3.5–7 moderately hard, 7–10.5 hard, and anything over 10.5 gpg (180 mg/L) is very hard and will scale a water heater within a few years untreated.