How it works
Softener capacity is measured in grains of hardness removed between regenerations. Multiply daily water use by hardness to get grains per day, then by the days you want between regenerations.
Dissolved (ferrous, clear-water) iron also uses up resin capacity. Manufacturers add 3–5 gpg per ppm of iron; the default 5 follows A. O. Smith's manual. Rusty (ferric) or bacterial iron should be filtered before a softener.
Nominal sizes are rated at maximum salt dose; real capacity at efficient salt settings is lower. If the headroom shown is small, consider the next size up or check the manufacturer's capacity at your salt setting.
The formula
- Compensated hardness (gpg) = hardness + iron (ppm) × iron factor
- Grains per day = people × gallons per person per day × compensated hardness
- Capacity needed = grains per day × days between regenerations
- Suggested size = smallest common nominal size ≥ capacity needed
How to use this calculator
- Enter household size and daily water use.
- Enter hardness (gpg or mg/L) and dissolved iron.
- Choose how often you want the softener to regenerate.
- Read the capacity needed and suggested nominal size.
Frequently asked questions
What if I don't know my hardness?
Use a test kit or your utility's annual water quality report (Consumer Confidence Report). The lookup on this page shows hardness for your water system where we have it, labeled by confidence: measured by the utility, or a low-confidence county estimate of untreated water. Use it as a starting point and override it with your own test.
Is bigger always better?
Not necessarily. An oversized unit regenerates rarely, which some manufacturers advise against; follow the manufacturer's guidance.
Sources
- EPA WaterSense: Statistics and Facts: Average American uses 82 gallons of water a day at home
- EPA WaterSense: How We Use Water: About 70% of household water use is indoors
- USGS: Hardness of Water: Soft/moderately hard/hard/very hard classes in mg/L as CaCO₃
- A. O. Smith water softener owner's manual (AO-WH-SOFT series): Add 5 gpg of hardness per 1 ppm of ferrous iron (manufacturer guidance)
Sources last checked Oct 21, 2026Written and reviewed by the Tapped Out team against the sources above.How we calculate