copied from another website... (dont remember where lol)
This formula is also from "The Loudspeaker Design Cookbook". It tells you how long you need to make the vent.
Fb - is the tuning frequency of your enclosure in Hertz.
Lv - is the length of your port in inches.
R - is the inside radius of your vent tube.
Vb - is the internal volume of your enclosure in cubic inches. To convert cubic feet to cubic inches, multiply by 1728.
If you want to use multiple ports, divide your enclosure volume by the number of ports you want to use, then use the result of this calculation as your Vb in the formula below to find out how long each port should be (a tip from JL Audio™).
If you want to calculate square vents, the formula below will give you the value of R to use in the formula above.
In the formula above, a is the area of your square vent (height x width), and (Pi) is approximately 3.141592.