Calculate Attic Volume
Attic volume is calculated  - LxWxH. When the roof pitch is the same on both sides of the roof you can multiply the length of the roof section times half the total (building) width times the height of the roof. This works because the two halves make a rectangular whole.

For a roof section 42' long by 28' wide by 6.5' high, the calculation would be:

42 X 14 X 6.5 = 3,822 CFT

Then divide the total cubic feet by the coverage per vent unit to get the total number of vents required.