40 gpg converts to roughly 684 mg/L as CaCO₃. The table uses 75 softened gallons/person/day, seven days and a 20% planning reserve.
| People | Daily grains | Planning grain band |
|---|---|---|
| 1 | 3,000 | 32,000 |
| 2 | 6,000 | 64,000 |
| 3 | 9,000 | 80,000 |
| 4 | 12,000 | 120,000 |
| 5 | 15,000 | >120,000 — custom/twin sizing |
| 6 | 18,000 | >120,000 — custom/twin sizing |
Important limitation
Do not treat the planning band as a promise that every product carrying that nominal grain label will deliver the same usable capacity. Check salt dose, certified/claimed capacity, service flow and pressure drop.
What 40 gpg means in practice
At about 684 mg/L as CaCO₃, this falls in the USGS very hard range. Hardness is mainly a scale and soap-use issue, not a drinking-water safety rating. Whether a softener is worthwhile depends on your appliances, hot-water equipment, cleaning preferences and how much of the home's water you actually plan to soften.
Capacity and flow are separate checks
For a four-person example at 75 softened gallons per person per day, 40 gpg creates about 12,000 grains/day of softening load. A seven-day interval plus the site's 20% planning reserve is about 100,800 grains. That number helps frame resin capacity, but it does not tell you whether the valve and resin bed can pass enough water during simultaneous showers, laundry or tub filling. Always compare the product's service-flow and pressure-drop data with your peak-flow target.
Before choosing equipment
If this is private-well water, confirm hardness with a current lab test and also review iron, manganese, pH and any locally relevant health contaminants. High iron, manganese, sediment or sulfur-like odor can change treatment order and may call for pretreatment rather than simply choosing a larger softener. For municipal water, your utility's Consumer Confidence Report is a useful starting point.
When comparing softeners, use the manufacturer's usable capacity at the intended salt dose rather than relying only on the large nominal grain number in the product name. See the planner methodology for the assumptions behind these tables.