1 - how hard is your water2 - how much water do you use in a day?
These two factors influence how often it will cycle. You don't want it cycling everyday as that will waste water and salt.
The ultra simple calculation is to multiply the hardness (in grains) by the average water usage and double that. This is the minimum size softener you want.
In simpler terms, for hard water (20 grains or more) and a family of 4 get a larger unit.
Other factors are also involved. Get a unit that cycles "on demand". It detects when it needs to cycle and will cycle that night. This is better than cycling on a set schedule.
Larger units also have more bells and whistles.
You can often get the larger units on sale.
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