Use the following formula to calculate the proper size:

1. Multiply the number of people in your family times 80 (gallons of water used per day, national average); or use this calculator; or look at your water bill like this one (don't forget some of it is for irrigation)
2. Multiply the answer by your water hardness in grains per gallon (from the Water Quality Report). If iron is present, add 3 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 most water softeners, you will have a choice of salt settings:

Example:

5 people in home, 15 grains per gallon hardness.

Step 1.    5 people x 80 gpd = 400 total gpd
Step 2.    400 gpd x 15 grains = 6000 grains per day

For the most efficient, use low salt setting of 6lbs/CF of resin:

• 1.0 cubic ft. @ 20,000 grains / 6000 = 3.3 days between regenerations (30 days x 6/3.3 = 64lbs of salt used in a month)
• 1.5 cubic ft. @ 30,000 grains / 6000 = 5 days between regenerations (30 days x 6/5 = 36lbs of salt used in a month)
• 2.0 cubic ft. @ 40,000 grains / 3500 = 11.4 days between regenerations (30 days x 6/11.4 = 16lbs of salt used in a month) 

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, 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.)