When you're choosing a water softener, you need to factor in the number of people in your household and how hard your water is. To calculate the unit size:
Multiply the number of people in your house by 80, which is the number of gallons the average person uses per day.
Use a water test kit to determine the grains of hardness in your water.
Multiply the grain amount with the total number of gallons to be used.
This number gives you an idea of the unit you need to handle your hard water requirements.