The short answer
The grain rating is measured at a heavy salt dose; at an efficient setting expect about three quarters of it (Nelsen's full vs efficient table).
Rated vs usable
| Tank | Full salt | Efficient |
|---|---|---|
| 9x48 | 32,000 | 24,000 |
| 10x54 | 48,000 | 36,000 |
| 12x48 | 64,000 | 48,000 |
Resin volume
About 32,000 grains per cubic foot at full salt: a 48,000-grain unit holds about 1.5 cubic feet. Resin calculator.
Choosing a size
Divide your weekly load by 0.75 for the rating to look for. Size it; examples: Tier1 WS-165-132 32,000-Grain Water Softener, VEVOR BWsoftYT-5w 40,000-Grain Water Softener.
Products mentioned
Questions readers ask
What does 48,000 grains mean on a water softener?
It can remove about 48,000 grains of hardness between regenerations at a heavy salt setting; about 36,000 at an efficient one.
Is a higher grain rating better?
Only up to your weekly load; beyond that it costs more without benefit.