When installing a subwoofer into your car audio system you need to build a box or enclosure for the speaker. Unless you are using a freeair subwoofer, the speaker is designed to perform inside a box with a certain amount of air space. If you look at the owners' manual of the subwoofer, you will find the recommended air space in cubic inches that the subwoofer will need to perform. To find the airspace of a subwoofer box you will need to do a little math.
Things You'll Need
 Tape measure
 Calculator

Measure the height, width and length of the inside of the box in inches. Write down the measurements.

Multiply the height, width and length. This will give you the total volume of the inside of the box in total cubic inches.

Divide the number by 1,728. This will give you the the box's volume calculation in total square feet, the number you will find in your subwoofer's owner's manual. For example, if the box is 14 inches tall, 14 inches wide, and 14 inches deep, you multiply the dimensions together and get 2,744 cubic inches. If you divide this product by 1,728, you'll get 1.58 cubic feet.