How the math works
Softenry mathCompensated hardness = hardness in gpg + 4 × ferrous iron in ppm. Grains a day = people × gallons per person × compensated hardness. Grains between regenerations = grains a day × days. A softener fits when its usable capacity at an efficient salt setting covers that load and its stated service flow covers your peak flow.
Softenry mathUsable capacity: grain ratings are full-salt figures, about 32,000 grains per cubic foot of resin. At an efficient salt setting a tank delivers about three quarters of its rating, so we use the rating × 0.75, or resin volume × 24,000 when the volume is stated. Units that only keep up at a high salt setting are listed separately.
IronUp to 2 ppm of ferrous iron, any softener is listed. From 2 to 5 ppm, only softeners whose maker states an iron rating at or above yours. Above 5 ppm, or with red water or a sulfur smell, treat the iron first with an iron filter. Oxidized (red) iron fouls softener resin.
LimitsThe bathroom peak-flow figures are planning estimates. If you know your home's peak flow, compare it with each unit's stated service flow directly.
Questions
What size water softener do I need for a family of four?
At 15 grains per gallon and 75 gallons a person, four people need about 4,500 grains a day, or 31,500 grains a week. At an efficient salt setting that takes a softener rated around 42,000 grains or more.
Why not just buy the biggest softener?
Sizing to about a week of your load is the usual target. A much larger unit costs more up front, and on a timer it regenerates on schedule whether the resin needs it or not, which wastes salt and water. A metered unit regenerates on gallons used instead.
Does iron change the size I need?
Yes. Each ppm of ferrous iron takes up resin capacity like about 4 grains per gallon of hardness, so 2 ppm of iron in 15 gpg water sizes like 23 gpg.