Also remember that port within the thickness of the material used to contruct the enclosure is not counted as it does not take up space within the enclosure ...
For example,
You have a box made of 3/4" MDF with a slot port down the middle that is 12.5" high, 3" wide, and 12" long. When calculating port displacement, you would add 1.5" to width (two 0.75" MDF pieces to construct the walls of the port) and subtract 0.75" from the length.
So, port displacement = 12.5*4.5*11.25/1728 = 0.36621 ft^3
You should always draw a diagram out on paper (Top view) as it makes it very hard to make mistakes when you're actually looking at it ...