Calculate your Softening requirements

Multiply the number of people in your household by the average amount of water each person uses per day. Typically most people use around 60 gallons per day including their share of laundry, dishwashing, showering, flushing toilets, etc.

3 people X 60 gallons per day = 180 gallons

Larger families typically use more water than smaller families. Consider how many people in your household shower, do laundry, and cook.

Hardness Levels

Water hardness is measured in grains per gallon (gpg) when sizing Softeners. Water reports may use mg/L. To find GPG just divide the mg/L by 17.1
Water with 0–3 gpg is considered soft, 4-10 is moderatly hard, while 11–15 gpg is considered hard. Anything over is very hard water.
Multiply that number by the hardness of your water in grains per gallon.

180 gallons per day X 20 GPG = 3600 Grains

How to size

Water softeners need to regenerate themselves once its capacity is used up and can no longer retain hard water ions.
The recommend time between regenerations is around 3-7 days. Multiply your daily required Grains removal by the number of days between regenerations.

3600 Grains per day X 5 days = 18000 Grains (or 18k)