You need to know 4 things when you size your water softener. These are: 1. What is the hardness of your water in grains per gallon 2. How many people will be living in the houshold. 3. Clear water iron (iron in solution) in parts per million PPM.
Ex. Without iron in water: A family has 3 people with 28 grains per gallon of hardness with no iron.
3 (people} +1 (1 for appliances) x 28 grains per gallon hardness x 70gallons (aveage water usage per person) x 6 days between regeneration=