Just a piece of anecdotal evidence: a few days ago I measured a 4'X2'X2' All-Glass aquarium stated to have 120 gal capacity. External dimensions gave 131 gal. Internal dimensions, allowing for a 1/2" free space between water surface and frame top, gave 120 gal. So at least in this case the manufacturer's figure is rigth. -Ivo Busko Baltimore, MD