Determine the amount of intake and exhaust volume using the online calculator at www.airvent.com. You can do this manually by finding out the attic square footage and dividing by 150. Then divide this number in half to determine appropriate exhaust volume. Next, deduct the peak ridge net free area (NFA) from overall exhaust area required to determine overall area required on hip ridges. Divide this by 48 to determine total amount of four foot units needed and then divide the number of four foot units required by the number of hips to evenly distribute the exhaust area for optimum performance.