understanding Minimum useable square vent dia.
 

Trying to understand the first formula from carstereo.com 1 driver @ 6mm xMax @ 33 hertz.= for square port min. Area 10.85" and length of side 3.29" does this mean roughly a 2.25x4.75 opening at 3.29 " long? If so how can it know that without air volume?

If you want to use this calculator, first choose what size driver(s) you are using, then enter how many are in your enclosure. Now, enter your Xmax figure (in millimeters), and the frequency that you need to tune your vent to. And, finally click the "Calculate Minimum" button to get your results. The "Minimum Diameter" box is for round ports, and the "Minimum Area" box is the minimum area required for a square vent.

3.29 * 3.29 = 10.85 (ish)

It is giving you the side dimension for a square vent.