Spirit requirements are 18 x 14 x 8. I would give this "Samsonite Wheeled Underseater Large, Black" 5 stars except the fact the actual, non-comperssible dimenshions are hindered by the base of the bag. The actual dimensions as measure by by myself with an accurate tape measure ARE:
height, non-compressible, 17" (ok) width, non-compressible, 13" (ok) depth, non-compressible because of plastic rame and wheels sticking out, 10" (NOT OK) Do NOT buy this bag if you're trying to fit it into an 18 x 14 x 8 space. Returning bag to sender. Very sad that neither the mfr / amazon / nor customer reviews could accurately measure this.