What Size Water Softener Do You Need?
The arithmetic
Grains a day = hardness in gpg × 60 gallons of indoor water per person (EPA WaterSense) × people. Multiply by 7 for a week between regenerations, then round up to the next size you can buy: 24,000, 32,000, 40,000, 48,000, 64,000, 80,000 grains. Outdoor water is normally plumbed before the softener, so it does not count.
Worked sizes for a household of three
| Hardness | Grains a day | Unit | Salt a year |
|---|---|---|---|
| 5 gpg | 900 | 24,000 | 110 lb |
| 10 gpg | 1,800 | 24,000 | 219 lb |
| 15 gpg | 2,700 | 24,000 | 329 lb |
| 20 gpg | 3,600 | 32,000 | 438 lb |
Round up, not down
An undersized unit regenerates every two or three days, burns more salt and water per grain, and wears its valve faster. Oversizing a step costs little more up front. If a quote is for a smaller unit than the arithmetic gives, ask why.
Salt and water
We assume 3,000 grains removed per pound of salt, the middle between an efficiency-set unit and a full-dose one, at about $0.18 a pound. Each regeneration uses roughly 45 gallons. A demand-initiated valve, which regenerates on measured use rather than a timer, is the single biggest lever on both.
Where to go next
See the range across the country: Moore County, NC (0.8 gpg), Warren County, KY (9.7 gpg), Clay County, MO (31 gpg).
Well Water vs City Water: Why Hardness Differs Next Door · When You Do Not Need a Water Softener