How to use the water softener capacity calculator
- Enter the number of people and the water each uses a day. The 75 gallons shown is an example, so use your meter or bill if you have it.
- Enter the water hardness from a test strip or your water report, and choose grains per gallon or mg/L.
- Open More options to add iron in the water, the capacity on your softener label and the reserve you want to keep back.
- Read the grains to remove per day and week, how many days a cycle lasts and the capacity that would give the interval you want.
Formula
hardness in grains per gallon = mg/L / 17.118
compensated hardness = hardness + iron ppm x iron allowance
grains per day = people x gallons per person x compensated hardness
usable capacity = capacity x (1 - reserve)
days between cycles = usable capacity / grains per day
One grain per gallon equals 17.118 mg/L, from 64.79891 mg per grain and 3.785411784 liters per gallon.
A family of four with 15 grain water
Four people using 75 gallons each use 300 gallons a day. At 15 grains per gallon that is 4,500 grains to remove per day and 31,500 per week. A 32,000 grain unit with a 15% reserve has 27,200 usable grains, so it lasts 27,200 / 4,500 = 6.04 days and regenerates about 60 times a year.
To go 7 days between cycles you would need 4,500 x 7 / 0.85 = 37,059 grains of capacity. With 256.77 mg/L of hardness, 2 ppm of iron at an allowance of 4 grains per ppm and 3 people using 60 gallons each, the hardness is 15 + 8 = 23 grains per gallon and the daily load is 4,140 grains.
Reading a hardness result
Hardness is the amount of dissolved calcium and magnesium in the water. Test strips are quick but rough. A lab report or the utility's water quality report gives a steadier number. Use the same unit the report uses, since mg/L and ppm are treated as equal here.
Iron uses up some of the softening capacity, so many makers add an allowance for each part per million. The 4 grains per ppm is an example. Use the number your equipment maker gives, or leave iron at 0 if your water has none.
What the reserve does
A softener regenerates on a schedule or when its capacity is nearly used. A reserve keeps part of the capacity back so hard water does not break through before the next cycle on a heavy day. A larger reserve means more cycles and more salt, and a smaller one risks hard water at the end of a cycle.
Related water tools
Size the heater with the Water Heater Size Calculator, check the cost of heating with the Water Heater Running Cost Calculator, and compare use with a Water Bill Calculator.
Assumptions and limits
- Every person uses the same amount of water each day.
- All the water you enter passes through the softener, including water used outdoors if you count it.
- Hardness, iron allowance, capacity and reserve are values you choose. The page does not know your equipment.
- The result is a sizing aid. Flow rate, resin type and salt settings also affect the right unit.
Frequently asked questions
How many grains does a family need to soften each day?
Multiply the number of people, the gallons each uses and the hardness in grains per gallon. Four people using 75 gallons each at 15 grains per gallon need 4,500 grains removed per day.
How do I convert ppm to grains per gallon?
Divide the ppm or mg/L by 17.118. A water report of 256.77 mg/L equals about 15 grains per gallon.
How often will my softener regenerate?
Divide the usable capacity by the grains to remove per day. A 32,000 grain unit with a 15% reserve and a 4,500 grain daily load regenerates about every 6 days.
Why does iron change the size?
Iron takes up capacity in the resin, so a softener sized only for hardness runs out sooner. The page adds a grains allowance for each ppm of iron that you can change to match your equipment.