A 32,000-grain-class unit can work for smaller households or moderate hardness. A 48,000-grain-class unit gives more buffer for higher hardness, more people and less frequent regeneration. Do not size from the sticker capacity alone—compare usable capacity at the actual salt dose.
The sizing formula
Estimate daily hardness load as gallons used per day × hardness in grains per gallon. Multiply by the number of days you want between regenerations to estimate required working capacity.
Example: four people at 10 gpg
If a four-person household uses roughly 240–300 gallons per day, the daily hardness load is about 2,400–3,000 grains at 10 gpg. Seven days of capacity would require roughly 16,800–21,000 usable grains before reserve.
Why 48,000 grain often gives more breathing room
Higher capacity can reduce regeneration frequency and provide more reserve for guests, peak usage or higher-than-expected hardness. Oversizing too far can also leave resin sitting for long intervals, so bigger is not automatically better.