grain per US gallon (as CaCO₃)
The grain per US gallon is the North American water-hardness unit, defined as one grain of calcium carbonate equivalent in one US gallon of water. One grain is exactly 64.79891 mg and one US gallon exactly 3.785411784 L, which works out to 17.118061 mg/L; this catalog rounds that to 17.118 ppm, so the stored factor is a rounded value rather than an exact one. Softener capacities, salt dosing and regeneration schedules are all specified in these units.
Watch out: Grains per gallon and ppm are both in daily use and differ by a factor of 17.1. 10 gpg is 171 ppm, not 10 ppm. The gallon is the US gallon of 231 cubic inches; an imperial gallon would give 14.25 mg/L instead, and British sources historically used a different grain-per-gallon unit for exactly that reason. Finally, the hardness is expressed as CaCO3 equivalent, not as actual calcium.
| 1 gpg | 0.0017118 % |
The grain is the oldest English mass unit, originally the weight of a barleycorn and now fixed at exactly 64.79891 mg, with 7000 grains to the pound. Its survival in water treatment is entirely practical: raw water hardness in North America mostly falls between 1 and 30 gpg, so the unit puts everyday values in single or double digits.
About the grain per us gallon (as caco₃)
Grains per gallon is the unit a softener is sized in, and the arithmetic behind that sizing is the reason it is worth understanding. One grain per gallon means one grain of calcium carbonate equivalent hardness in each US gallon of water, and the conversion is \(1 \ ext{ gpg} = 17.1 \ ext{ ppm as CaCO}_3\). The exact value is 17.118 mg/L, from a grain of exactly 64.79891 mg divided by a US gallon of exactly 3.785411784 L. Most field work rounds to 17.1.
The common scale: below 1 gpg (17 ppm) is soft, 1 to 3.5 gpg is slightly hard, 3.5 to 7 gpg is moderately hard, 7 to 10.5 gpg is hard, and above 10.5 gpg (180 ppm) is very hard. Scale problems in water heaters and boilers begin in earnest in the hard range, and most residential softeners are installed somewhere above 7 gpg.
Sizing works directly in these units. A softener's capacity is quoted in grains, so a 32 000 grain unit serving a household using 300 gallons a day at 15 gpg removes \(300 \ imes 15 = 4500\) grains per day and regenerates roughly every seven days at full capacity, or every five if it is run at a realistic salting efficiency rather than its maximum rating. Add 4 to 5 gpg of apparent hardness for every 1 ppm of dissolved iron, since iron loads the resin the same way calcium does. That single calculation is why the unit persists: it turns a water analysis into a salt bill.
Two traps. This is the US gallon, and an imperial gallon would put the factor at 14.25 mg/L rather than 17.118, which is why British hardness figures from older sources do not convert with the North American number. And the value is expressed as calcium carbonate equivalent, so a water at 10 gpg contains 171 ppm as CaCO3, which is only about 68 ppm of actual calcium ion.