i have a macbook air that is listed as being size 13.3 (although actual measurement is 12" x 8.5"), so i initially bought the size 13.3 case. that one however left several inches of space around the edges of my laptop and the closure/edge of the bag. i didn't like the idea of my computer being able to slide around in the case so much, so i ordered the size 13 and it is perfect. this case is exactly what i was looking for to keep my laptop protected while inside another bag. it is still just fabric and foam, so don't expect miracles, but it is excellent for what it is.