How the math works
Softenry mathAbout 32,000 grains per cubic foot of standard resin at full salt, and about 24,000 at an efficient setting (Nelsen Corporation's full vs efficient table). One cubic foot is about 28.3 liters.
Questions
How much resin is in a 48,000-grain softener?
About 1.5 cubic feet.
How much resin should I buy to re-bed my softener?
The same volume your tank holds; leave the free space above the bed the maker calls for.