Net free area is not the size of the vent
Attic ventilation is sized off the attic floor area, and the rule is a ratio. The baseline is 1:150 — one square foot of ventilation for every 150 square feet of attic floor. That relaxes to 1:300 where a Class I or II vapour retarder is installed on the warm-in-winter side of the ceiling, or where the vents are balanced so that 40% to 50% of the required area sits in the upper portion of the attic with the rest at the eaves. A 1,200 ft² attic therefore needs 8 ft² of ventilation at 1:150, or 4 ft² at 1:300. Because vents are specified in square inches, multiply by 144: that 4 ft² is 576 square inches of ventilation to distribute around the roof.
The critical distinction is net free area (NFA) versus gross opening. NFA is the actual unobstructed opening left after the louvres, baffles and insect screen are accounted for — and that hardware eats a lot of it. Cutting a 16 × 8 inch hole gives you 128 square inches of gross opening, but the vent you screw over it might deliver only half to two-thirds of that as NFA. Manufacturers print the tested NFA per unit, or per linear foot for continuous soffit and ridge products; that printed figure is the only number you should divide your requirement by. Never size from the hole you cut, and be sceptical of old work: painted-over soffit louvres, screens clogged with dust, and insulation stuffed into the eaves are the three things that most often reduce a technically compliant attic to almost no airflow. Baffles at every rafter bay keep the insulation off the vent path.
Splitting the total matters as much as hitting it. Attic ventilation works by convection — cool air enters low at the soffits, picks up heat and moisture, and leaves high at the ridge — so the system needs both halves, and the low half must be at least as large as the high half. The convention is roughly 60% intake and 40% exhaust, and at minimum 50/50. If exhaust exceeds intake, the ridge vent cannot find enough make-up air at the eaves and starts pulling it from the easiest alternative: through ceiling penetrations, light fixtures and the attic hatch, drawing conditioned, humid household air straight into the attic. On a windy day it can also short-circuit, sucking air back in through one end of the ridge vent instead of drawing it up from the soffits. Mixing exhaust types makes this worse — a ridge vent combined with gable vents or a powered fan tends to feed one from the other rather than ventilating the whole attic. Pick one exhaust type, oversize the intake rather than the exhaust, and distribute both evenly along the roof.
Estimates for planning only. Ventilation requirements, the conditions that allow the 1:300 ratio, and the rules for unvented or conditioned attic assemblies vary by jurisdiction and by climate zone — confirm against the code enforced where you are building before ordering materials. Cathedral ceilings, spray-foam and other unvented roof assemblies are designed differently and are not covered by this calculation.