The Owner's Manual says 21 cu ft. With door storage, it's too hard to re-calculate. So I'd believe it.
The useable space on each shelf is from side to side is about 26 1/2 inches. From the back to the inside of the door is about 18 inches. The bottom shelf is about 26 1/2 inches wide by about 18 inches deep. The bins in the door begin just above the bottom shelf. So the top 3 shelves are 26 1/2 inches wide by 13 inches deep. The bins in the door protrude into the freezer about 4 1/2 inches.
The compressor and refrigeration unit is in the bottom back, so there is an internal hump across the bottom back which is about 4 inches deep (front to back) and about 12 inches high. This space is subtracted from the back side of the wire basket.
Th inside usable height is about 68 inches.