Choose 64K when daily hardness load makes a 48K unit regenerate too often, or when household demand and peak flow justify the larger valve/resin configuration. Otherwise, 48K is often enough for typical family use.
Capacity should follow hardness load
A large household with 20+ gpg hardness can burn through usable capacity quickly even if total gallons are not extreme.
Regeneration interval matters
Sizing for roughly weekly regeneration is a common design target because it balances efficiency with regular resin cleaning. The exact target can vary by equipment and water chemistry.
Do not compare sticker grain numbers only
Nominal grain capacity is often measured at a high salt dose. Usable capacity at an efficient salt setting can be materially lower.
When 64K is clearly justified
Very hard water, five-to-six-person households, high-flow fixtures and combined iron/hardness loads can all push the project toward a larger system.