Water Softener Capacity Calculation
Ideally, a water softener should be sized so that it does not regenerate any more often than 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). 6 days between regenerations is probably best - especially if iron is present. For the majority of homes, 1 cubic foot unit is more than enough capacity.
Use the following formula to calculate the proper size:
Note: With most water softeners, you will have a choice of salt settings:
5 people in home, 15 grains per gallon hardness.
For the most efficient, use low salt setting of 6 lbs/CF of resin:
Note: One 40-lb bag of salt costs $5
The best choice would be the 1.5 cubic foot water softener (closest to 6 days).
You can use this Excel Sizing Spreadsheet for more options.
Expected water softener capacity with different salt settings
When setting up your water softener, you usually have a choice of salt usage settings. The higher the salt setting the more "powerful" the force is on the water softening resin and therefore it will more completely remove the hard minerals from the water. As you lower the salt setting the force decreases and starts letting more hard minerals pass though the softener.
Softeners are usually assumed to be set at 15 lbs per cubic foot (ft3) of resin. At that setting, your softener will deliver the softest water (<1 p.p.m.). As you lower the salt setting the amount of hardness that can pass though the softener will be higher (2-3 grains) but the amount of gallons water you can run through per pound of salt used to regenerate the softener will increase.
For instance if we take water with 10 grains of hardness and run it thought a 15 lb regenerated 30K softener (1 CuFt of Resin), we could expect to get about 3000 gallon or about 200 gallons per pound of salt. If we take the same softener and regenerate it with 6 pounds of salt, then we would make 2000 gallons of slightly hard water or about 333 gallons per pound of salt. For residential application having up to 4 grains of hardness would still be considered soft water. The only time you would use a 15 pound salt setting is for applications requiring less than 1 p.p.m. (1 grain - 17.1 p.p.m.)