Here’s the magic formula, and it’s easier than splitting a restaurant bill. Multiply your daily water use (from Step One) by the hardness number (from Step Two). Then multiply that result by 4—because you want the softener to run for a week without refilling.
Example: A family of four uses 320 gallons. Their hardness is 10 GPG. So: 320 x 10 = 3,200 grains per day. 3,200 x 7 days = 22,400 grains per week.
That number—22,400—is the minimum capacity you need. Look for a softener rated at about 24,000 to 30,000 grains. That gives you a little cushion, like ordering dessert after a big meal.
Water Softener Grain Chart & Calculator: Ultimate Cheatsheet
