Box Size: (Vb)
The volume of the chamber you plan on porting. Remember, if you are biulding a 3 cubic foot box, for two 12" subs and it is divided, then each chamber would be about 1.5" cubic feet. If you have the Number System option set to metric, then VB would be in litres. Standard, of course, would be cubic feet.
Tuning Frequency: (Fb)
The frequency at which to plan to tune the port of the chamber. If you are calculating bandpass enclosure vent(s), vent MUST be calculated using chamber's parameters! (ex. Vf and Ff)
Port Diameter and Vents: (Dia and Vents)
The inside diameter of the port you plan to use. A good rule of thumb is: select a port size (diameter) that is equal to or larger than 25% of the cone diameter of the woofer you are using.
If you have the Number System option set to metric, then Dia would be in centimeters. Standard, of course, would be inches.
Numeric System: (Number system)
In Metic setting, Vb should be entered in Liters, Dia in Centimeters and the results will return in Centimeters.
In Standard setting, Vb should be entered in Cubic Feet, Dia in Inches and the results will return in Inches.