Additional volume will include any space taken up by the speaker structure braces or anything else that takes up space in the enclosure. Internal dimensions can be found by subtracting the thickness of wall and dividers from an external dimension. You simply measure the height width and depth in inches multiply them together and then divide that number by 1728.
If you are using a slotted port select square under port type then answer yes under are you using a slotted port.
Use the appropriate formula to find volume in cubic inches. Calculating Internal Enclosure Volume To find the internal volume of an enclosure internal dimensions must be used. By measuring the length of the pistons handle sticking out of the box it is easy to calculate the volume at which the speaker sounds the best. E nter the number of ports you are using the internal airspace of your box then tuning frequency you wish to use and then hit calculate button.