Quick take
Daily grain load = gallons used per day × compensated hardness in gpg. Required working capacity = daily grain load × target days between regenerations, plus reserve.
Step 1: measure hardness
Use a current hardness test in grains per gallon. If the report is in mg/L as CaCO3, divide by about 17.1 to convert to gpg.
Step 2: estimate daily gallons
Use actual meter data if available. If not, a planning estimate around 60–75 gallons per person per day can be used as a starting range rather than a guarantee.
Step 3: calculate daily grain load
For 300 gallons per day at 12 gpg, the daily hardness load is 3,600 grains.
Step 4: choose a regeneration interval
At 3,600 grains per day, seven days requires about 25,200 usable grains before reserve. Compare that requirement with usable—not merely nominal—capacity at the intended salt setting.