Here’s a little spreadsheet that helps to figure out the dimensions of a speaker box. What you do is enter a bunch of dimensions, and it tells you the actual internal volume. I’m sure someone must have done this before but I couldn’t find one. Here’s the file:

• Box dimension calculator

Here’s a screenshot:

Here’s how to use it.

1. In the Enclosure section, enter the best-guess dimensions, and the thickness of the wall material.
2. In the Round Port section, enter the diameter and length of the port as calculated by your box design program. If it’s a sealed box, enter 0 for the length.
3. If you want to make the port part of the box, use the Brace Port section. To make it work, set the Internal Dimension in the Side to Side column to the width of the port. If it’s the full width of the box, set the Material thickness to 0.
4. In the Bracing section, put in the size and number of braces that you intend to use in each dimension.
5. In the Driver section, put in some dimensions to get an approximation of the volume used by the driver.
6. Adjust the dimensions until you get the right volume at the bottom right.

Simple! Let me know if it’s useful.