Measure hardness in grains per gallon
Use a current water test. If your report uses mg/L as CaCO₃, divide by about 17.1 to convert to grains per gallon (GPG).
Estimate daily water use
Use your actual water bill if possible. If not, a rough planning figure is often 60–75 gallons per person per day, but household behavior varies widely.
Calculate daily grain load
Daily grain load = hardness (GPG) × daily gallons used.
If you want roughly seven days between regenerations, that household would need about 42,000 grains of working capacity before accounting for efficiency settings and reserve.
Check service flow separately
A resin tank can have enough grain capacity yet still be too small for simultaneous demand. Compare the system's service-flow rating with your likely peak: multiple showers, laundry, dishwasher and faucets running together.
For reference, SpringWell publishes 11 GPM service flow for SS1, 13 GPM for SS4 and 20 GPM for SS+. Those are useful examples of why two homes with similar hardness can still need different system sizes.
Account for iron and other water chemistry
Well water with iron, manganese or sulfur may need pretreatment or sizing adjustments. A softener can handle some iron under certain conditions, but heavy iron loading can foul resin and should not be treated as a simple hardness-only problem.