A backpack can be either a personal item or a carry on bag depending on its size. Most carry-on bags are in the region of 56 x 35 x 22 cm / 22 x 14 x 9” (height, width, depth) and personal items are usually restricted to 45 x 35 x 20 cm / 18 x 14 x 8”. A smaller, fashion backpack may fall under a personal item category while a larger one would be considered a carry-on bag. Other airlines don’t clarify personal item sizes other than saying it should be around the size of a “purse, briefcase or laptop computer”.