Water softener sizing calculator
Grains per day sets the size. The salt setting sets how far each tank goes and how much salt and water it uses a year. Change any number and the table below follows.
Recommended size
48,000 grain
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| Size | Usable grains | Days between regenerations | Salt per year | Regeneration water per year |
|---|
How the numbers are worked out
Grains per day is people × gallons per person × compensated hardness. Each ppm of clear-water iron is counted as 4 gpg of hardness, the middle of the 3 to 5 that softener makers use, because iron takes up resin sites the same way calcium does.
Usable grains depend on the salt setting. Standard softener resin delivers about 20,000 grains per cubic foot at 6 lb of salt, 24,000 at 8 lb, 27,000 at 10 lb, 29,500 at 12 lb and 32,000 at 15 lb. The 15 lb figure is where nameplate ratings come from: a "48,000-grain" softener is 1.5 cubic feet at 15 lb. Lower settings remove more hardness per pound of salt and need more frequent regeneration.
Days between regenerations keeps one day's use in reserve, which is how a metered valve such as the Fleck 5600SXT decides when to regenerate. Seven to ten days is the usual target: often enough that the resin bed stays clean, far enough apart that salt and water use stay low. The recommendation is the smallest size that reaches seven days.
Regeneration water uses 35 gallons per cubic foot of resin, from Morton's published 31.6 gallons per regeneration for its 0.91 cubic foot M34 at the maximum salt dose. Backwash rates vary by valve and tank, so treat it as an estimate.
Sodium uses Penn State Extension's figure: a sodium softener adds 7.5 mg of sodium per quart for each grain per gallon it removes.
Very hard water or a large household that runs past the 2.5 cubic foot size points to a twin-tank system; the salt-based page compares them.
Compare salt-based softeners