Water Softener Sizing Calculator
The grain capacity a whole-house water softener needs for your household, hardness and iron.
Result
Enter water hardness to see the result.
https://calcgallery.com/home-systems/water-softener-sizing-calculator/
Results are estimates for planning only. Actual needs vary with site conditions, products and workmanship. Always confirm quantities with your supplier and follow local building codes and manufacturer instructions. Read the full disclaimer
How to use the Water Softener Sizing Calculator
- Enter the number of people in the home.
- Enter the water's hardness from a test or your utility, in grains per gallon or mg/L, and any dissolved iron.
- Choose how many days you want between regenerations.
- Read the grain capacity, and compare it with softeners' rated capacity at the salt setting you plan to use.
How it is calculated
water a day = people × water per person per daycompensated hardness = hardness + iron (mg/L) × hardness added per mg/L of irongrains a day = water a day × compensated hardnesscapacity = grains a day × days between regenerationsA grain is 1/7000 of a pound, so a grain per gallon is 1/7000 lb of hardness, as calcium carbonate, in each US gallon. Softeners are rated by the grains they remove before they must regenerate.
Assumptions and limits
The page assumes all the water used indoors is softened. Leaving outside taps and toilets on hard water, or softening only hot water, lowers the load. Water per person and the iron factor are published design figures you can change under More options. This is a sizing estimate for comparing softeners, not a water treatment design; high iron, manganese or other contaminants may need other treatment, and plumbing and discharge rules vary locally.
With your numbers
- Water a day = 4 people × per person = 300 gal
- Grains a day = 300 gal × 10 gpg = 3,000 gr
- Capacity = 3,000 gr × 7 days = 21,000 gr
Quick reference
Arithmetic from your figures; a softener’s rated capacity depends on its salt setting, so compare at the setting you will use.
| Job | Grain capacity between regenerations |
|---|---|
| 4 people, 10 gpg, weekly | 21,000 gr |
| 4 people, 10 gpg, 1 mg/L iron | 31,500 gr |
| 2 people, 20 gpg, 3 mg/L iron, every 4 days | 21,000 gr |
| 5 people, 15 gpg, weekly | 39,375 gr |
| 3 people, 25 gpg, every 10 days | 56,250 gr |
| 6 people, 12 gpg, 2 mg/L iron | 69,300 gr |
| 3 people, 250 mg/L, weekly | 23,002 gr |
| 4 people, 400 mg/L, every 5 days | 35,051 gr |
Where the numbers come from
The fixed figures this calculator relies on, and the published source each one was checked against. How the calculators are built and checked
2 sources
Water Quality Association, "Residential Water Treatment Design", excerpted from the WQA QuickCourse How to Design a Residential Water Treatment System (WQA article, PDF dated March 2025), p. 9: "The current estimate for daily volume water usage per person is 75 gallons", with endnote iii, printed on p. 16: "Water conservation measures could reduce the daily water usage volume per person to 65 gallons, or even 60 gallons." https://wqa.org/wp-content/uploads/2025/03/Article-2-Residential-Water-Treatment-Design-w-tables-2025.pdf. The same 75 gallons is in Penn State Extension, "Water Softening" (Robillard, Sharpe and Swistock, updated September 8, 2025): "75 gallons (10 gpg) x 4 = 3,000 grains per day used", https://extension.psu.edu/water-softening
Used for
- Water one person uses in a day, for sizing a whole-house softener: 75 US gallons, the Water Quality Association’s design estimate.
A.O. Smith whole house water softener owner’s manual (models AO-WH-SOFT-PRO-300, AO-WH-SOFT-450T and AO-WH-SOFT-PRO-500, sold through Lowe’s), pp. 4–5, "Ferrous Iron and Compensated Hardness": "To determine compensated hardness in standard measurements you should increase your water hardness setting by 5 grains per gallon (86 mg/L) for every 1 ppm (1mg/L) of ferrous iron", worked as "Actual Hardness 20 (gpg); Ferrous Iron times five (x5) 15; Compensated Hardness (20+15) 35", https://www.aosmithatlowes.com/media/1169/ao-wh-soft-500_ownersmanual.pdf. Other makers differ: Hellenbrand owner’s manual p/n 109122 Rev. E (4/17/18), "Add 3 grains per gallon of hardness for each ppm iron", https://www.hellenbrand.com/wp-content/uploads/2015/11/Manual-Owners-H125-Super-HP-109122.pdf; Water-Right Impression series manual, "(1 ppm = 4 gpg)", https://www.water-right.com/wp-content/uploads/2023/08/Water-Right-IM-IMP-Softener-Manual.pdf. No Water Quality Association figure could be opened.
Used for
- Hardness added to a softener’s setting for each 1 mg/L (1 ppm) of dissolved ferrous iron: 5 grains per gallon (86 mg/L), one maker’s figure. Other makers use 3 or 4.
Common questions about the Water Softener Sizing Calculator
- What size water softener do I need?
- Multiply the water your home uses in a day by its hardness to get the grains of hardness removed each day, then by the days between regenerations. Penn State Extension’s example is four people using 75 gallons each at 10 grains per gallon, 3,000 grains a day, so a 20,000 grain softener regenerates about every six to seven days.
- How much water does each person use?
- The Water Quality Association uses 75 gallons per person per day as its design estimate and notes that water-saving homes may use 60 to 65. EPA WaterSense puts the national average at 82 gallons at home. If your water meter or bill shows your own use, enter that under More options.
- How does iron change the softener size?
- Dissolved iron uses up the resin’s capacity faster than hardness does, so makers add hardness for each mg/L of iron to get a compensated hardness. The figure varies: A.O. Smith’s manual adds 5 grains per gallon for each 1 mg/L, Water-Right’s uses 4 and Hellenbrand’s 3. Use your softener maker’s figure. Iron that has already turned to rust particles is filtered, not softened.
- How do I convert mg/L to grains per gallon?
- One grain per gallon is 17.1 mg/L, since a grain is 64.8 milligrams and a US gallon 3.785 litres. Water tests report hardness either way, as calcium carbonate, and the page takes both.
- Should I buy a bigger softener to be safe?
- Not by much. WQA’s best practices for installers say to avoid oversizing equipment and overtreating water, which waste salt and water. A softener’s rated capacity also depends on how much salt each regeneration uses, so compare models at the same salt setting.