opific

Water softener size calculator

Affiliate disclosure: links to softener brands on this site may earn us a commission at no cost to you. The calculator's math does not change based on who pays us.

Water softener size calculator

Enter your water test numbers and household size. The result is the smallest standard unit that regenerates about once a week at a salt-efficient setting.

Soft < 1 · Moderate 3–7 · Hard 7–10 · Very hard > 10 gpg
Each person is assumed to use 75 gallons per day
Leave 0 for city water. Each 1 ppm adds about 4 gpg of load.
Used to check flow rate, not capacity
Advanced (manganese, usage, regeneration interval)
Each 1 ppm adds about 2 gpg of load
US average is 75–100; use 60 for a frugal household
7 is the usual target; longer intervals need a bigger tank

Recommended size

64 000 grain (2.0 ft³)
15 gpgcompensated hardness
4,500grains removed per day
31,500grains per 7-day cycle
12 lbsalt per regeneration
51 lbsalt per month (≈ 1.3 bags)
No red flags. A 64 000 grain (2.0 ft³) unit set to about 12 lb of salt per regeneration will cover your household with a weekly cycle.
How this was calculated
  1. Compensated hardness = hardness + 4 × iron ppm + 2 × manganese ppm = 15 gpg.
  2. Daily load = 15 gpg × 4 people × 75 gal = 4,500 grains.
  3. Cycle load = daily × 7 days = 31,500 grains.
  4. Usable capacity at 6 lb salt per ft³ is about 20 000 grains per ft³, so 31,500 grains needs 2 ft³ of resin. The "64,000 grain" label on the box assumes a wasteful 15 lb/ft³ salt dose.

This calculator uses standard industry sizing rules (1 gpg = 17.1 ppm; iron ≈ 4 gpg per ppm; 20 000 usable grains per ft³ at 6 lb salt). It is guidance, not a substitute for a lab water test. We do not own or install these systems; we read specifications, certifications and thousands of customer reports.

Frequently asked questions

What size water softener do I need for a family of 4?

At 15 gpg hardness and 75 gallons per person per day, a family of 4 removes about 4 500 grains per day, or 31 500 per week. At a salt-efficient 6 lb/ft³ setting that needs about 2 ft³ of resin, sold as a '64 000 grain' unit.

Why is the calculator's answer bigger than the '32 000 grain' unit the store recommends?

Nominal grain ratings assume about 15 lb of salt per cubic foot of resin, which is wasteful. At the efficient 6 lb setting a 1 ft³ (32 000 grain) tank delivers about 20 000 usable grains. Sizing for efficiency means a larger tank, less salt and fewer regenerations.

How does iron change the size?

Each 1 ppm of dissolved iron adds roughly 4 grains per gallon of load. Above about 3 ppm, a dedicated iron filter before the softener is the right fix; softener resin fouls otherwise.

What the inputs mean