Daily grain load
A practical estimate is hardness in gpg × gallons of softened water per day. Penn State Extension demonstrates sizing with 75 gallons per person per day in its example.
Nominal vs usable capacity
A softener sold as “48,000 grain” may not deliver that amount at an efficient salt dose. Compare the actual capacity and salt-dose data for the specific unit.
Iron and manganese
Do not automatically add arbitrary “hardness equivalents” for iron or manganese. Treatment suitability depends on concentration, chemistry and manufacturer limits. Our main planner flags EPA secondary-guideline exceedances for further evaluation.
Next step
Run your numbers through the Water System Planner, then verify any product against its actual service-flow, pressure-drop, capacity and certification data.