Sizing Water Softener

Sizing a Water Softener – Calculator, Formula & Chart

If you are looking for a water softener, then you should make sure to understand the sizing for water softener. Generally 32,000 grain units ( 1 cubic foot of resin) is really good capacity for your home, but it is possible if you have a large family ( 6 or more), large house ( 41000 sq ft+) or very hard water ( over 16 grains).

Ideally water softener should be sized so that it does not generate any more often that every 3 days you waste water and salt, also not should go longer than 15 days before regenerating. 7 days b/w is probably best – especially when you water is coming from a well where iron is present. 

Calculation for Sizing a Water Softener

  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).

High Salt-Efficiency Water Softener Sizing Guide

0.75 Cubic Feet of Softener Resin = 15,000 Grains Capacity
1.0 Cubic Feet of Softener Resin = 20,000 Grains Capacity
1.5 Cubic Feet of Softener Resin = 30,000 Grains Capacity
2.0 Cubic Feet of Softener Resin = 40,000 Grains Capacity
3.0 Cubic Feet of Softener Resin = 60,000 Grains Capacity
4.0 Cubic Feet of Softener Resin = 80,000 Grains Capacity


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