Measure the area in inches where you want to put a subwoofer box using the tape measure. In most trunks, there is lots of room for a subwoofer box, but where you put it is key to what kind of dimensions you have to work with. Write down the length, width and depth of the area you want to mount the box onto the paper with a pencil.
How to Figure Cubic Feet for a Subwoofer Box
Adding a subwoofer to the stereo system in your car is a good way to add lowend bass to your tunes, and start to truly feel your music. The key to making a subwoofer sound good in your vehicle is having a properlysized box, tuned to the requirements of your subwoofer. In this case, the box is built for a JL Audio 13.7 subwoofer and a sealed enclosure, but the process is similar for other subwoofers.
Things You'll Need
 Tape measure
 Pencil
 Paper
 Calculator
Instructions


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2
Look at the specifications for the subwoofer to find out the mounting depth of the speaker, the displacement of the subwoofer, and the recommended net volume of the enclosure. The mounting depth is the minimum depth that your box must be to fit the speaker, and the displacement is the amount of air the subwoofer itself displaces. The recommended net volume of the enclosure is the size of the box.


3
Add the displacement of the subwoofer to the recommended net volume using the pencil, paper and calculator. In the case of the 13w7, the displacement is .21 cubic feet, and the recommended net volume is 1.875 cubic feet, making a combined 2.085 cubic feet. This is what the internal measurements of your box must measure to properly fit the box.

4
Multiply the measurements you made in Step 1 together, to determine the available cubic feet in your trunk. Your measurements are in inches, so you are going to have to do math to determine the cubic feet. In this example, the trunk measures 36 inches long, 36 inches wide and is 24 inches deep. By multiplying each number together, the total is 31,104 cubic inches. To determine the cubic feet, you have to divide that number by 1,728, which is the amount of cubic inches in a cubic foot. In this example, that means there is 18 cubic feet available.

5
Take the volume that you determined in step 3 and multiply it by 1,728 to determine the cubic inches of your box, using the pencil, paper and calculator. In this example, that means the math is: 2.085 * 1728 = 3602.88 cubic inches.

6
Pick one measurement that you would like for the box. In this example, the box is going to go top to bottom in the trunk, so the measurement is 24 inches. The depth is going to be 11 inches, because the box is going to be shallow so that the trunk still has room. This is an arbitrary measurement, but it's a starting point for the box, and can be any number you like. If it ends up being too big or too small, you can adjust accordingly.

7
Multiply the numbers you have using the pencil, paper and calculator, and compare it to the answer from Step 5, to determine the missing variable in the equation. In this example, the math reads as follows:
24 inches tall 11 inches deep x = 3602.88 cubic inches.
264x = 3602.88
x = 13.65 inches
x = 13 13/20 inches 
8
Write down the numbers using the paper and pad you have determined to transfer to the size of the box. In this case, the internal dimensions of the box will measure 24 inches wide by 11 inches deep, and 13 13/20 inches long.

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References
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