First, an apology for the horrible pictures of our octagon vents. It is almost impossible to take a straight picture when there are 8 sides on the product to line up. Some even call this a stop sign vent.
What makes these so inexpensive is the careful thought in the construction of the louvers. Using true .019 aluminum and limiting the width of the vent louvers to their strength ability. Super lightweight construction.
Well there are eight of them. This means the corners of these gable louvers are set on a 22.5 degree angle for a finished casting. And each side is identical in length. Measure the opening height and width for an octagon vent. Nothing else.