Make a subwoofer enclosure plan. It does not include any volume displaced by woofers dividers and vents. Fill the container of known volume then pour the contents into your fiberglass enclosure.
You simply measure the height width and depth in inches multiply them together and then divide that number by 1728.
Subwoofer box calculator online. The volume equation shown here is used to calculate volume in cubic feet when height width and depth are measured in inches. Here is the formula we use. Make a subwoofer enclosure plan.