Water Softener Grain Capacity Calculator
A water softener is sized by how many grains of hardness it must remove before it regenerates. The daily load is the number of people times the gallons each uses per day times the water hardness in grains per gallon. Multiply that by the days you want between regenerations and you have the grain capacity the unit needs to hold. This calculator runs that arithmetic from your household figures and the hardness from your own water test, so nothing is assumed.
Water softener sizing formula
Daily water use = people * gallons per person per day
Daily hardness load = daily water use * hardness (grains per gallon)
Capacity per cycle = daily hardness load * days between regenerations
Annual hardness = daily hardness load * 365
Hardness in grains per gallon comes from your water test. Multiplying daily use by hardness gives grains per day; multiplying by the regeneration interval gives the resin capacity required to last between cycles.
Water softener notes
- One grain of hardness equals 1/7,000 of a pound of dissolved calcium and magnesium.
- Indoor use is often planned at roughly 60 to 80 gallons per person per day.
- Weekly regeneration is a common balance between salt use and resin capacity.
- Add compensated grains per gallon for dissolved iron, often 3 to 5 grains per part per million.
- Have water tested for hardness and iron rather than assuming a value.
Water softener sizing: frequently asked questions
How do I size a water softener?
Find the daily hardness load: people times gallons used per person per day times the hardness in grains per gallon. Multiply by the days you want between regenerations to get the grain capacity the softener must hold. For 4 people at 75 gallons each at 15 grains per gallon over 7 days, that is 4 times 75 times 15 times 7, or 31,500 grains.
What is a grain of hardness?
Water hardness is measured in grains per gallon (gpg), where one grain equals 1/7,000 of a pound of dissolved hardness minerals, mainly calcium and magnesium. Softener capacity is rated in total grains it can remove between regenerations. Have your water tested to get the grains-per-gallon figure to enter here.
How much water does a person use per day?
Indoor household water use varies, but a common planning figure is around 60 to 80 gallons per person per day. Your actual use depends on fixtures and habits. Enter the per-person daily gallons that matches your household, or use your water bill to compute an average and divide by the number of people.
How often should a softener regenerate?
Many systems are set to regenerate roughly weekly, balancing salt efficiency against capacity. More frequent regeneration uses more salt; less frequent needs a larger resin tank. Enter the days between regenerations you prefer and the calculator sizes the capacity to last that interval.
Should I add iron to the hardness number?
Dissolved iron consumes softener capacity too. A common practice is to add about 3 to 5 grains per gallon of compensated hardness for each part per million of iron in the water. If your test reports iron, increase the grains-per-gallon input accordingly so the sizing accounts for it.
Official sources
- U.S. Geological Survey: Hardness of Water.
- U.S. Environmental Protection Agency: Safe Drinking Water Act.
Reviewed by the CalculatorHub team, edited by James Graham, 16 June 2026. See our methodology.