How to Size a Gable Vent
Two different questions get mixed together. One is how big the vent should look on the wall. The other is how much air the attic needs. This page separates them. Neither answer replaces the drawing on the product you order.
Looks
General guidance for appearance only. It is not a ventilation calculation.
Airflow
Attic ventilation is measured as net free area, the actual open path through the slats, not the overall width and height of the frame. A common guideline in US codes is about 1 square foot of net free area per 150 square feet of attic floor, or 1 per 300 when intake and exhaust are balanced. That wording is a familiar rule of thumb, not a promise that it applies to your house.
This page does not calculate net free area. Read the net free area on the product page for the vent you are considering, then confirm it with local code or your builder. A closed louver has no air path, so net free area does not apply to it. Compare open gable louvers if air is the point.
This is general guidance, not engineering or code advice. Roof pitch, intake at the soffit, and what is behind the wall all change the right size. A triangle vent also has to match the roof pitch, which this sketch does not check.
More from the manufacturer: the Spectis sizing guide. Related pages: shapes and the install overview.