
Minimum Vent Diameter 
The first two formulas shown here represent the minimum usable port diameter for subwoofer speaker enclosures  they are set up to avoid power compression and vent noise. The first one is the one I use for bandpass designs (from "The Loudspeaker Design Cookbook" by Vance Dickason  I divide the tuning frequency by two before I use it in this formula though), and the second is a more conservative formula from the works of Richard Small, which is useful for vented speaker designs. 
dv  is the required diameter of the port in inches. 
Fb  is the tuning frequency of your enclosure in Hertz. 


Vd  is the volume displaced by the driver (in cubic meters) traveling through it's full excursion (peaktopeak). To figure out Vd for a speaker, find the Sd value for your driver in the table below, and multiply that number times the Xmax (in meters) of your speaker. 
Driver Diameter 
Sd (M^{2}) 
18" 
0.1300 
15" 
0.0890 
12" 
0.0530 
10" 
0.0330 
8" 
0.0220 
6.5" 
0.0165 
6" 
0.0125 
5.25" 
0.0089 


Vent Length 
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. 