Hardness Load Removed per Regeneration

Every softener sizing calculation starts here: how much hardness, by weight, has to come out of the water between one regeneration and the next. Concentration times volume, and the only trick is keeping the units honest. In North American trade practice the arithmetic is done entirely in grains — hardness in grains per gallon times gallons treated gives grains of hardness — so 20 gpg water and 1,000 gallons between regenerations is 20,000 grains, which is 20,000/7,000 = 2.86 lb of calcium carbonate, or 1.30 kg. That figure is what you compare against the resin bed's rated capacity.

The classic field mistake is sizing on average daily use and forgetting the peak. A family of four using 300 gallons a day looks like a 6,000 grain/day load on 20 gpg water, so a 30,000 grain softener seems to give five comfortable days — until laundry day, a filled hot tub, or an irrigation zone doubles the volume and the bed exhausts mid-shower with untreated hard water going straight to the heater. The other trap is forgetting to add clear-water iron, which the resin also removes: the rule of thumb is to add about 4 gpg of equivalent load per mg/L of dissolved iron before you size anything.