Of course, you have to use the interior H x W x D dimensions, or else subtract the wall thickness from the exterior measurement.
If you are measuring in inches, divide the total cubic inches by 1728 to get the number in cubic ft.:
22” x 18” x 16” = 6336 cu. in.
6336 ÷ 1728 = 3.6 cu. ft.
As Dustin mentioned, you have to subtract the displacement of any braces from that figure (using the same method). Not sure how to figure the displacement of the driver – perhaps the manufacturer could tell you that.