Test the hardness of your home’s water using a test kit to determine the grain capacity you need in a water softener. After finding out the measure of water hardness in parts per millions, or ppm, divide this number by 17.1 to determine the hardness in grains (of minerals) per gallon in the water.
Contact a local water-softener business to confirm your water hardness level, as many of these companies will do this for free. Also, check sites of companies such as Quality Water Treatment or The Good Water Company, which feature U.S. water hardness maps (see Resources section). These sites offer an idea of the hardness of water in your particular geographic region.
Multiply the water hardness level, which should be in grains per gallon, by the amount of water you use every day, to learn how many grains must be removed from your water. Note that one individual typically uses about 80 gallons of water daily, which includes water used for showering, doing laundry or consumption. If you have three people living in your home, you can estimate that you use 240 gallons of water each day. If your water hardness level is 10, plan to have 2,400 grains removed every day.
Look for a water-softener system that removes more grains per gallon than the amount you calculated -- so that the system will go longer between regenerations. For example, most water softener systems’ coverage is between 20,000 and 100,000 grains per gallon, which would be enough to last you about two weeks or more if you need 2,400 grains removed from your water daily.
Check to see if your chosen water softener is capable of removing the particular contaminants present in your home's water. For example, consider purchasing a water softener that specifically handles small amounts of hazardous substances, such as manganese or iron if this is your area of need.