What size water softener do I need?
Grain capacity from your household size and hardness, with the salt trade-off shown.
A softener is sized by how many grains of hardness it has to remove between regenerations. That is household size times daily water use times hardness, and the arithmetic is simple enough to check by hand.
Worked example: 4 people, 15 grains per gallon
The rating on the box is the maximum salt dose
This is the part most sizing guides skip. A softener's grain rating is measured at the highest salt dose the unit can run, about 15 lb of salt per cubic foot of resin, which yields roughly 30,000 grains per cubic foot. Run the same unit efficiently at 6 lb per cubic foot and it delivers closer to 20,000 grains per cubic foot instead.
So a "32,000 grain" softener is really about one cubic foot of resin. It will deliver 32,000 grains only if you are willing to spend 15 lb of salt to get there.
| Setting | Salt per cu ft | Grains per cu ft | Grains per lb of salt |
|---|---|---|---|
| High efficiency, 6 lb/cu ft | 6 lb | 20,000 | 3,333 |
| Standard, 9 lb/cu ft | 9 lb | 24,000 | 2,667 |
| Maximum, 15 lb/cu ft | 15 lb | 30,000 | 2,000 |
Grains per pound of salt is the efficiency number, and it falls by nearly half between the gentlest and the heaviest setting. A slightly larger unit run at a low salt dose is cheaper to operate than a smaller one worked hard, which is the single most useful thing to know before buying.
Iron changes the answer
Dissolved iron is removed by the same resin, and the industry convention is to add 5 grains per gallon of effective hardness for every 1 ppm of iron, and 4 for manganese. At 2 ppm of iron, 10 grain water behaves like 20 grain water and the softener needs twice the capacity.
Sizing table
| Hardness | 1 person | 2 people | 3 people | 4 people | 5 people | 6 people |
|---|---|---|---|---|---|---|
| 5 gpg (86 mg/L) | 16,000 | 16,000 | 16,000 | 16,000 | 16,000 | 16,000 |
| 10 gpg (171 mg/L) | 16,000 | 16,000 | 16,000 | 24,000 | 32,000 | 32,000 |
| 15 gpg (257 mg/L) | 16,000 | 16,000 | 24,000 | 32,000 | 40,000 | 48,000 |
| 20 gpg (342 mg/L) | 16,000 | 24,000 | 32,000 | 48,000 | 64,000 | 64,000 |
| 25 gpg (428 mg/L) | 16,000 | 32,000 | 40,000 | 64,000 | 80,000 | 80,000 |
| 30 gpg (514 mg/L) | 16,000 | 32,000 | 48,000 | 64,000 | 80,000 | 96,000 |
Figures are the nearest size actually sold, rounded up from the calculated requirement. Add iron compensation separately.