How this calculator works
A water softener removes hardness measured in grains. To size it, work out how many grains of hardness your household uses each day, multiply by the days you want between regenerations, and choose the next size up.
Worked example: four people using 75 gallons each per day in 15 gpg water use 4 × 75 × 15 = 4,500 grains per day. Over 7 days that is 31,500 grains, so a 32,000-grain softener is the size to buy.
Softener size by household and hardness (7 days between cycles)
Water use of 75 gallons per person per day, no iron.
| Hardness | 2 people | 3 people | 4 people | 5 people | 6 people |
|---|---|---|---|---|---|
| 5 gpg | 16,000 | 16,000 | 16,000 | 16,000 | 16,000 |
| 10 gpg | 16,000 | 16,000 | 24,000 | 32,000 | 32,000 |
| 15 gpg | 16,000 | 24,000 | 32,000 | 40,000 | 48,000 |
| 20 gpg | 24,000 | 32,000 | 48,000 | 64,000 | 64,000 |
| 25 gpg | 32,000 | 40,000 | 64,000 | 80,000 | 80,000 |
| 30 gpg | 32,000 | 48,000 | 64,000 | 80,000 | 80,000+ |
Tips before you buy
- Test your water first. A strip or kit costs a few dollars, and gives you the hardness in grains per gallon or ppm. Your water utility may publish it.
- Undersized softeners regenerate often, wasting salt and water. Oversized ones can leave salt sitting too long. The right size regenerates about once a week.
- Iron above about 0.3 ppm stains fixtures, and heavy iron can foul the resin. A softener with iron-removal resin, or a separate filter, may be needed.
- Check the flow rate of the unit against your peak demand. Houses with many bathrooms need a higher flow valve.
- Soft water leaves more soap lather and no scale, but may not suit people on low-sodium diets for drinking. Keep the drinking tap unsoftened if needed.
Quick answers by size
Tools & materials for this project
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Frequently asked questions
What size water softener do I need for a family of 4?
With average use (75 gallons each), a family of 4 needs a 24,000 grain softener at 10 grains per gallon of hardness, and 32,000 grains at 15. Very hard water (20 grains per gallon or more) needs 48,000 grains or more.
How do I measure water hardness?
Use test strips or a drop-count kit, or read your utility's water quality report. It is given in grains per gallon (gpg) or mg/L: divide mg/L by 17.1 to get gpg.
What counts as hard water?
Under 3.5 gpg is soft, 3.5 to 7 moderately hard, 7 to 10.5 hard and over 10.5 very hard.
Related calculators & guides
Estimates only. Results assume standard materials and simple shapes. Supplier products vary, and the final call on quantity, structure and safety is yours. Check your local code and your supplier's guidance before ordering.