Start with 32K for moderate hardness, then move upward if hardness is high, iron is present or the calculated regeneration interval becomes too short.
Example at 10 gpg
Two people using roughly 120–150 gallons per day create a daily hardness load of about 1,200–1,500 grains at 10 gpg.
Example at 20 gpg
At 20 gpg, that same water use creates 2,400–3,000 grains per day. Seven days can consume roughly 16,800–21,000 grains.
Why 32K can still work
Many 32K-class systems have enough usable capacity for a two-person household when set efficiently, especially at moderate hardness.
When to choose 48K
Move to 48K when hardness is very high, water use is high, iron adds load or you want a longer interval between regenerations.
Sizing and chemistry checks for What Size Water Softener for 2 People?
Water-softener decisions should be based on measured hardness, expected daily water use and peak service flow. “Grain capacity” is useful only when interpreted with the programmed salt dose and regeneration efficiency; the largest number on the product page is not automatically the most efficient operating point.
Iron changes the sizing problem. Some dissolved iron can be handled by ion exchange, but meaningful iron loading can foul resin and increase maintenance. When testing shows significant iron, manganese or sulfur, dedicated pretreatment is often more robust than asking the softener to solve every well-water problem.
Compare regeneration efficiency and long-term ownership
A well-sized unit should regenerate often enough to protect resin capacity without wasting salt and water. Compare usable capacity at the intended salt setting, estimated gallons between regenerations, service-flow rating and the amount of water used during regeneration. These operating details matter more over ten years than a small difference in purchase price.
Also consider valve support, replacement resin availability, bypass plumbing and local drain requirements. Salt-free conditioners belong in a separate category: they may reduce scale behavior in appropriate water but do not remove calcium and magnesium the way a true ion-exchange softener does.