Attic Ventilation Calculator
Enter your attic floor area and the code ratio and this tool returns the net-free vent area you need, split evenly between soffit and ridge, and how many vents that takes.
Calculator
1,200 sq ft of attic at 1:150 needs 8.0 sq ft (1,152 sq in) of net-free vent area — split 50/50 low and high, that is about 9 soffit vents (2021 IRC R806; confirm with your building official).
A healthy attic breathes: outside air enters low at the soffits, rises as it warms, and leaves high at the ridge, carrying away summer heat and winter moisture. Building codes size that airflow by net-free area (NFA) — the actual open area of a vent after its louvers and screen, not its outside dimensions. The 2021 IRC (R806) requires vent NFA of at least 1/150 of the attic floor area, or 1/300 if you provide balanced high-and-low venting plus a ceiling vapor retarder.
This calculator turns your attic area into a required NFA in both square feet and square inches, splits it 50/50 between low (soffit) and high (ridge) as good practice dictates, and estimates how many vents that takes from the NFA rating of a single vent.
Formula
NFA (sq ft) = area ÷ ratio
NFA (sq in) = NFA (sq ft) × 144, split 50% soffit / 50% ridge
soffit vents = ⌈ (NFA sq in ÷ 2) ÷ per-vent NFA ⌉
The ratio is 150 or 300 per the code option you choose. Per-vent NFA is the rating stamped on the vent you plan to use — a screened vent has less NFA than its opening, so use the manufacturer's figure.
Worked example
A 1,200 sq ft attic under the standard 1:150 rule:
1,200 ÷ 150 = 8 sq ft = 1,152 sq in
Split evenly, that is 576 sq in low and 576 sq in high. With soffit vents rated 65 sq in each:
576 ÷ 65 = 8.9 → 9 soffit vents
and a matching run of ridge vent (or equivalent high vents) for the other 576 sq in. Choosing the 1:300 option instead would halve the required area to 4 sq ft — but only if you also have a vapor retarder and truly balanced venting.
Balance and baffles
Balance matters more than raw area. The 50/50 split between intake (soffit) and exhaust (ridge) is what makes the air actually move. If intake is short, a ridge or power vent can pull air from the house instead of the soffits. When in doubt, err toward slightly more intake than exhaust.
Do not bury the soffits: install baffles so blown-in insulation cannot slide down and block the intake path. Mixing exhaust types (for example, a ridge vent plus gable fans) can short-circuit the flow — pick one exhaust strategy. And keep the loose-fill depth from the insulation-depth calculator clear of the vents.
Frequently asked questions
How much attic ventilation do I need?
Under the 2021 IRC you need net-free vent area of at least 1/150 of the attic floor area, or 1/300 with a ceiling vapor retarder and balanced high/low vents. For a 1,200 sq ft attic that is 8 sq ft (1,152 sq in) at 1:150, split half low and half high.
What is net-free area?
Net-free area (NFA) is the actual open area of a vent for airflow after subtracting the louvers and insect screen. It is always smaller than the vent's outside size, so use the NFA rating printed on the vent, not its overall dimensions.
What is the difference between the 1:150 and 1:300 rule?
1:150 requires vent NFA equal to 1/150 of the attic area. The code allows 1/300 — half as much — only if you install a Class-I or II vapor retarder on the warm-in-winter side of the ceiling and split the venting so 40–50% is high and the rest low. Choose 1:300 only when you meet both conditions.
Should soffit and ridge vents be balanced?
Yes. Aim for a roughly even split — half the net-free area at the soffits (intake) and half at the ridge (exhaust). Too little intake makes a ridge or power vent draw conditioned air out of the house. If anything, provide a little extra intake.