What size counts as underseat or personal-item luggage?

Most U.S. airlines cap personal items around 18 x 14 x 8 inches, though budget and international carriers can be stricter. The bags here range from compact 14- to 16-inch totes and spinners to an 18 x 14 x 8 backpack, so always confirm your airline's published limits before buying, since reviewers frequently note that a bag fitting one carrier may be rejected on another.