Water softener size = people × gallons per person × (hardness + 5 × iron), then × 7 days × 1.25 reserve. Four people using 80 gallons each at 10 grains per gallon (gpg) remove 3,200 grains a day, need 28,000 grains of capacity between regenerations, and land on a 32,000 grain softener that regenerates about every 8 days.
How the water softener size is calculated
A softener does one job: it swaps the hardness minerals in your water for sodium or potassium until its resin is full, then it regenerates with salt brine. Size it so that resin holds a week or so of your hardness load.
grains per day = people × gallons per person per day × (hardness in gpg + 5 × iron in ppm)
capacity needed = grains per day × days between regenerations × 1.25 reserve
Culligan's rule of thumb is 80 gallons per person per day, and other sizing guides use 75. Hardness comes from a test strip or your water utility's report, in grains per gallon (gpg) or parts per million (ppm, the same as mg/L). One gpg is 17.1 ppm, so a report of 171 ppm is 10 gpg. The days between regenerations default to 7, the once-a-week target in SoftPro's sizing guide, and the 25% reserve keeps you from running out of soft water on laundry day.
Worked example: 4 people, 10 gpg
| Step | Calculation | Result |
|---|---|---|
| Water per day | 4 people × 80 gal | 320 gal |
| Grains per day | 320 gal × 10 gpg | 3,200 grains |
| Load over a week | 3,200 × 7 days | 22,400 grains |
| With 25% reserve | 22,400 × 1.25 | 28,000 grains |
| Size to buy | next standard size up | 32,000 grains |
| Days between regenerations | 32,000 ÷ (3,200 × 1.25) | 8 days |
The 22,400 grains a week matches SoftPro's own four-person, 10 gpg example.
Water softener size chart
Grain capacity by household and hardness, at 80 gallons per person, a 7 day interval and 25% reserve:
| People | 5 gpg | 10 gpg | 15 gpg | 20 gpg |
|---|---|---|---|---|
| 1 | 24,000 | 24,000 | 24,000 | 24,000 |
| 2 | 24,000 | 24,000 | 24,000 | 32,000 |
| 3 | 24,000 | 24,000 | 32,000 | 48,000 |
| 4 | 24,000 | 32,000 | 48,000 | 64,000 |
| 5 | 24,000 | 40,000 | 64,000 | 80,000 |
| 6 | 24,000 | 48,000 | 64,000 | over 80,000 |
The smallest common residential size is 24,000 grains, so small households at moderate hardness all land there. A six-person house at 20 gpg needs more than one softener can offer, so look at a twin-tank unit.
Water softener capacity and resin
Softener capacity is quoted in grains, and it tracks the volume of resin. Culligan gives 1 cubic foot of resin as a typical 32,000 grain capacity, suitable for a family of five in its example: 5 people at 65 gallons and 11 gpg use 3,575 grains a day, so 32,000 grains lasts about 9 days. SoftPro adds that real-world capacity often runs 30 to 40% below the rating on the box, which is why this calculator carries a reserve instead of sizing to the label. If you know your resin volume, cubic feet of resin or a brine tank turns into grains at about 32,000 per cubic foot.
How to size for iron
Dissolved iron uses up the resin like extra hardness. Add 5 grains for every 1 ppm of iron: a test showing 10 gpg and 3 ppm iron is worked as 10 + 5 × 3 = 25 gpg. For the same four people that is 8,000 grains a day and 70,000 grains of capacity, which rounds to an 80,000 grain softener, the top of the common range. At iron levels like that, ask a water treatment dealer whether an iron filter ahead of the softener is the better plan.
What counts as hard water
The USGS scale by mg/L as calcium carbonate is soft up to 60, moderately hard 61 to 120, hard 121 to 180 and very hard above 180. In grains per gallon that puts 10.5 gpg, about 180 mg/L, at the edge of very hard. The calculator prints the class next to your hardness so you can see where a report falls.
The next questions
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