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What size Softener do I need? -- Water Softener Capacity Calculation

For the majority of homes, our 32,000 grain unit (1 cubic foot of resin) is more than enough capacity. There are conditions that would create a need for a larger unit: larger family (6 or more), larger house (4000sq ft+) and/or very hard water (over 15 grains).

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 goes 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 32,000 grain unit (1 cubic foot of resin) 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 Logix computer-control valve, it is possible to program it for more efficient use of salt by setting the capacity at a lower amount. For example, a one cubic foot softener has a peak capacity of 32,000 grains at a salt setting of 18.5 lbs. per regeneration. This provides 1729 grains of softening per pound of salt (32,000/18.5). You can program this for a capacity of 24,000 grains at a salt setting of 8.5 lbs. of salt. This would provide a more efficient 2823 grains per pound of salt, nearly doubling the efficiency. Granted, it will regenerate slightly more often, but since it only uses about $0.10 of water per regeneration, this is not really a major consideration. This being said, the following capacity options list both the peak and the efficient levels to make a better decision.

1.0 cubic foot of resin - 32,000 grain peak, 24,000 efficient
1.5 cubic feet of resin - 48,000 grain peak, 32,000 efficient
2.0 cubic feet of resin - 64,000 grain peak, 40,000 efficient

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

1 cubic ft. @ 24,000 grains / 3500 = 6.8 days between regenerations
1.5 cubic ft. @ 32,000 grains / 3500 = 9 days between regenerations
2.0 cubic ft. @ 40,000 grains / 3500 = 11 days between regenerations

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

More information on about Water Softeners.

Learn about Whole House Water Filters with our Whole House Filter Buying Guide.

Read our Water Testing Guide here and our Well Water Testing Information Supplement.

View a graphic comparing contaminant and filtration size and learn about Reverse Osmosis Systems.

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