What net free area actually means
A vent has a hole in it, and then it has a screen, a baffle, a louvre and a filter. Net free area is what is left of the hole after all of that, and it is always smaller than the opening you cut. A 16 by 8 inch soffit vent has 128 square inches of outline and might deliver 26 square inches of net free area. This is the single most common reason a ventilation calculation done on the back of an envelope comes out wrong: people count openings instead of net free area.
The figure is published by the manufacturer for each specific product. It is not a property of the category. Two ridge vents that look identical on the shelf can differ by half. Look for the number on the data sheet, and if you cannot find it, that is information about the product.
The ratio, and why there are two of them
The commonly used figure is one square foot of net free ventilating area for every 150 square feet of attic floor. A reduced ratio of one to 300 is widely permitted where the ventilation is balanced between the upper and lower portions of the attic, or where a vapor retarder is installed on the warm side of the ceiling. Which of those applies to your house, and whether your jurisdiction has adopted them in that form, is a question for the building department that has authority over the work. Model codes are a starting point and the local authority decides. Treat both ratios here as arithmetic, not permission.
The practical difference is large. A 1,120 square foot attic needs 1,075 square inches at one to 150 and 538 at one to 300. On a 40 foot ridge, the first figure is not reachable with a ridge vent alone at any realistic net free area.
Balance matters more than the total
Attic ventilation works by buoyancy and wind: air enters low at the eaves and leaves high at the ridge. If the exhaust is larger than the intake, the ridge vent still moves air, but some of that air comes from the conditioned space below through every gap in the ceiling plane, dragging humidity with it. That is worse than a smaller balanced system. Intake should equal exhaust or exceed it, never fall below it.
| Situation | What happens |
|---|---|
| Intake equals exhaust | The intended flow, eaves to ridge, across the whole attic |
| Intake less than exhaust | Ridge pulls makeup air through the ceiling from the house |
| Intake greater than exhaust | Fine. Slightly pressurised attic, no reverse flow through the ceiling. |
| Ridge vent plus open gable vents | Short circuit. Ridge draws from the gable, and the far eaves go dead. |
The things that beat the arithmetic
Two failures make a correctly sized system do nothing. The first is insulation packed into the eave, which blocks the path at the top plate where the ceiling insulation meets the roof deck. Baffles hold that path open, and if there are none, the soffit vents ventilate the underside of the insulation and nothing else. The second is air leakage from the house into the attic: bath fans discharging into the attic instead of outside, open chases, recessed lights, and the attic hatch. Ventilation removes moisture that gets into the attic. Air sealing stops it getting there, and it is the higher-value job of the two.
What you actually have available at the eaves is settled by the soffit, so it is worth running the soffit and fascia calculator to see how much vented run exists before deciding the intake is fine. If the attic insulation is being redone at the same time, the depth and coverage side of that is in the insulation calculator. And if you are looking at ventilation because of stains on a ceiling, rule out an actual leak first with the water leak guide rather than assuming it is condensation.
Questions people ask
Is the 1 to 300 ratio always allowed?
No, and this page will not tell you whether it applies to your house. The reduced ratio is tied to conditions, most commonly a balanced split between upper and lower vents or the presence of a vapor retarder on the warm side of the ceiling, and jurisdictions adopt model code language with their own amendments. The authority having jurisdiction over your work decides. What is universally true is the arithmetic: the reduced ratio halves the required area, so which ratio you use changes the design completely and it is worth confirming rather than assuming.
How do I know the net free area of a vent I already have?
Find the model and look it up, because you cannot compute it from the outside dimensions. If the vent is unidentifiable, a conservative approach is to treat a louvred and screened vent as delivering somewhere between a fifth and a half of its outline area, and then to notice that this range is wide enough to make the answer meaningless. That is a reason to buy vents whose net free area is published rather than to guess at the ones on the house.
Should I close my gable vents when I install a ridge vent?
It is a common recommendation and the reason is airflow paths, not code. A ridge vent draws makeup air from the nearest opening. A gable vent high on the wall is a much easier path than a soffit vent forty feet away, so the ridge pulls from the gable and the eave end of the attic stops moving. Some ridge vent manufacturers explicitly call for gable vents to be closed, some do not address it. Follow the instructions for the vent you are installing.
Do powered attic fans change the calculation?
They change the problem rather than solving it. A powered fan moves far more air than passive vents, and if the intake cannot supply it, the fan pulls conditioned air out of the house through the ceiling, which costs money on both the fan and the heating or cooling it wastes. If you are considering one, the intake sizing becomes more important, not less. This page sizes passive net free area and does not model fans.
Does this apply to a cathedral ceiling or a conditioned attic?
Not directly. A vented cathedral assembly needs a continuous airway from eave to ridge in every single rafter bay, so the constraint is per-bay airway depth rather than a total area over the floor. An unvented conditioned attic is a different assembly altogether, with insulation at the roof deck and requirements around condensation control that depend on climate. Neither is what this calculator models. It assumes a conventional vented attic with insulation on the flat ceiling below.