Ideally, a water softener should be sized so that it does not regenerate any more often that every three days (wastes water and salt), nor go longer than 14 days before regenerating (can cause compacting of resin, and fouling with sediment or iron). 7 days between regenerations is probably best - especially if iron is present. For the majority of homes, our 1 cubic foot unit is more than enough capacity. There are conditions that would create a need for a larger unit: larger family (6 or more) and/or very hard water (over 15 grains).

Use the following formula to calculate the proper size:
1. Multiply the number of people in your family times 70 (gallons of water used per day, national average).
2. Multiply the answer by your water hardness in grains per gallon (to convert mg/l or ppm to grains, divide by 17.1). If iron is present, add 5 grains for every ppm (mg/l) of iron (iron MUST be dissolved iron - it appears clear from the tap but leaves reddish-brown stains).
3. This is your "grains per day" number. Divide this number into each of the softener capacities until you find the best size.

Note: With the Autotrol Logix computer-control valve, you can choose between three settings:

"S" Standard Salt Usage Setting (most widely recommended)

"L" - Low Salt Usage Setting

"H" - High Salt Usage Setting

Calculation example:
5 people in home, 10 grains per gallon hardness.

1. 5 people X 70 gpd = 350 total gpd
2. 350 gpd X 10 grains = 3500 grains per day

0.75 cubic ft. @ 19,000 grains / 3500 = 5.4 days between regenerations
1.0 cubic ft. @ 25,000 grains / 3500 = 7.1 days between regenerations
1.5 cubic ft. @ 38,000 grains / 3500 = 10.8 days between regenerations
2.0 cubic ft. @ 50,000 grains / 3500 = 14.3 days between regenerations

The best choice would be the 1 cubic foot unit (closest to 7 days).

You can download a Sizing Calculator spreadsheet (MS Excel) by clicking HERE.

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