Calculating box volume requires simple math and knowing generally what you want from your system. You’ll need a tape measure, a pencil and paper, and a calculator. If you’re designing a box for a specific space, you can keep changing dimensions and calculating the box volume until you find the ideal box size and shape for your application.
HOW TO CALCULATE BOX VOLUME
First measure the external length, width, and height in inches, and write them down. If you’re an artistic sort, sketch out your box and write the dimensions next to where you measured them. You can also use free tools like SketchUp or Fusion360 to create and modify a 3D model of your enclosure.
Subtract material thickness from the external measurements:
¾” thickness x 2 sides = 1 ½ inches
Multiply length, width, and height in inches:
Internal volume in cubic inches = (height x width x depth)
Convert the volume from cubic inches to cubic feet:
Internal volume in ft3 = (Internal volume in in3) ÷ 1728
Below is an example of how to calculate volume for a box built from ¾” thick material with dimensions of 16″ by 34″ by 14″.
First, subtract the ¾” wood thickness: Internal dimensions = 14.5 in. x 32.5 in. x 12.5 in.
Calculate internal volume in cubic inches: 14.5 in. x 32.5 in. x 12.5 in. = 5890.625 in.3
Convert the volume from cubic inches to cubic feet: 5890.625 ÷ 1728 = 3.4 ft.3
CALCULATING NET VOLUME
The total available volume you just calculated is the box’s gross volume. The sub and port take up about 18% of that volume. That 18% no longer contains air to feed your woofer(s). Next you’ll need to subtract this 18% to get your net volume. Net volume is the figure you’ll need to use when determining box size and tuning.
Remove the 18% from the gross volume: 100% – 18% = 82% or .82
Net volume = .82 x internal volume
From the previous example: 3.4 ft.3 x .82 = 2.788 ft.3 of net volume
CALCULATING PORT AREA
You will need 16 square inches of port area per cubic foot of net volume. This figure is a good guideline for most applications. If you’re looking for extreme tuning, you may have to manipulate this to come up with a box design that’s physically possible.
Total Port Area: 2.788 ft.3 x 16 in.2 = 44.608 in.2 port area.
CALCULATING RECTANGULAR PORT DIMENSION
To calculate the port area, we need height and width dimensions. For a rectangular box with known dimensions, we already have a height. We just need to find the width of the port.
Example (cont.): 44.608 in.2 ÷ 16 in. height = 2.788 in.
Round up or down to the nearest 1/4”: 2.788″ rounds to 2.75″ in.
The longer the port length, the lower the enclosure tuning. An 18-inch port length will result in tuning at about 40 Hz. For bass-heavy music, you can use a port length at 22-24 inches.