Seat height is 18", HOWEVER, that is when measured to the top of the soft, uncompressed seat cushion.
When you sit on the soft cushion, it is compressed significantly, and approaches the height of the chrome frame's tubing. The height/top of that tubing from the floor is 16.25 to 16.5 inches, and so I'd say that when sitting on the cushion, the height of the chair's seat is somewhere around 17 inches.