To figure out the square inches of port area needed per cubic ft of volume, do I just multiply the vent width by vent length?

And then I just take that number and divide it by how many cubic ft the box is?

This is what I'm just guessing as I'm not sure of this.

Please help.

Welcome ....

Take your vent height x vent width = square inch of port area!

Port Height x Port Width = In^2 of port area

Port Height x Port Width x Port Length = Ft^3 or port volume

The Length of the port determines the tuning frequency.

To figure out the square inches of port area needed per cubic ft of volume, do I just multiply the vent width by vent length?

to figure out the amount of port area needed is going to vary by woofer. you want enough room for the sub to breathe. too small and the port velocity increases which can cause noise.

back in the day i used 10 sq inches per cubic foot of box as a guideline. but that was dealing with smaller woofers. with todays high xmax woofers more port area is recommended, usually around 15 sq in per cubic foot of volume.

so if you build a box that is 5 cubic feet in volume, you will want somewhere between 50 and 75 sq inches of port. the port opening can be 5x10" or 4"x12.5" or any other combination you want. just dont do something stupid like 1"x50"

Remember that is a rule of thumb, not exact.

PORT Size Calculations and Formulas for WOOFER and Subwoofer BOXES

There is no "rule of thumb" that always works. The only real way to know is to model the enclosure and determine the air speed in the port.