Due to space the Fi unit takes up, it is not as adjustable as other collars. The larger the collar size, the more adjustable it becomes. The smaller sizes are less adjustable.

A 12” collar should be adjustable about 11-13”.

Here are some sample sizes:

13” adjusts ~ 12-14” | 14" adjusts ~ 13-15" | 15" adjusts ~ 14-16" | 16" adjusts ~ 15-18" | 17" adjusts ~ 16-19" | 18" adjusts ~ 17-20" | 19" adjusts ~ 18-21" | 20" adjusts ~ 18-23" |