for a rectangular box, its simply Lenght x Height x Width. If the dimensions you provided are exterior dimensions, then subtract 2x the material thickness from each dimension prior to multiplying.
So for your stuff its:
(16.5 - (2 * 0.75)) x (12.5 - (2 * 0.75)) x (16.5 - (2 * 0.75)) = cubic inches
Then you can take cubic inches and divide by 1728 to get cubic feet
So for your stuff its:
(16.5 - (2 * 0.75)) x (12.5 - (2 * 0.75)) x (16.5 - (2 * 0.75)) = cubic inches
Then you can take cubic inches and divide by 1728 to get cubic feet