What size water softener do I need?
Get your hardness from your water utility's report or a test kit. Well owners should also test for iron.
How the math works
- Compensated hardness = hardness (gpg) + 4 × iron (ppm). ppm ÷ 17.1 = gpg.
- Daily gallons = people × gallons per person.
- Capacity needed = compensated hardness × daily gallons × days between regenerations.
- Suggested size = smallest standard rated size whose usable capacity at your salt setting is at least that number.
- Salt per year (estimate) = grains removed per year ÷ grains per pound of salt (3,333 efficient, 2,000 full).
Method, iron factor and tank capacities: Nelsen Corporation, Softener Sizing Calculator (tank size vs. grain capacity table). Some installers use 3–5 gpg per ppm; 4 is Nelsen's factor. *75 gallons/person/day is the planning default used by Nelsen Corporation sizing method (75 gal/person/day) and Lowe's Water Softener Sizing Chart (75–80 gal/person/day); real use varies a lot, and your water bill gives a better number (monthly gallons ÷ 30). Salt figures derive from Excalibur Water, C800X10 softening resin: 30,000 grains/cu ft at 15 lb salt, 20,000 grains at 6 lb and are estimates: actual salt use depends on your valve, resin and settings.
Hardness scale
| USGS class | mg/L (ppm) as CaCO₃ | Grains per gallon (÷ 17.1) |
|---|---|---|
| Soft | 0–60 | 0.0–3.5 |
| Moderately hard | 61–120 | 3.6–7.0 |
| Hard | 121–180 | 7.1–10.5 |
| Very hard | over 180 | over 10.5 |
Classes: U.S. Geological Survey, Hardness of Water (checked 2026-10-27). Conversion: Maryland Geological Survey: 1 gpg = 17.118 mg/L.
Standard softener sizes
| Mineral tank | Rated (full-salt) grains | Usable at efficient salt setting |
|---|---|---|
| 8" x 44" | 24,000 | 18,000 |
| 9" x 48" | 32,000 | 24,000 |
| 10" x 44" | 40,000 | 30,000 |
| 10" x 54" | 48,000 | 36,000 |
| 12" x 48" | 64,000 | 48,000 |
| 13" x 54" | 80,000 | 60,000 |
| 14" x 65" | 110,000 | 84,000 |
Source: Nelsen Corporation, Softener Sizing Calculator (tank size vs. grain capacity table) (checked 2026-10-27). Brands differ; check your model's spec sheet.
See the full size chart by household and hardness → · Convert ppm, °dH and more · Estimate salt cost
FAQ
What size water softener do I need?
Multiply daily water use (people × about 75 gallons) by your hardness in grains per gallon, add 4 gpg per ppm of iron, then multiply by the days you want between regenerations (about 7). Pick the smallest softener whose usable capacity covers that number.
Is a bigger water softener better?
Not automatically. An oversized unit goes many days between regenerations; installers on r/WaterSofteners and r/WaterTreatment generally aim for about one regeneration a week. Bigger units cost more up front.
How do I convert ppm to grains per gallon?
Divide ppm (mg/L as calcium carbonate) by 17.1. For example, 171 ppm is 10 gpg.