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One square foot of net free area for every 150 square feet of attic floor (IRC R806.2), split evenly between soffit intake and ridge exhaust. A 1,600 square foot attic needs 1,536 square inches — 768 each way. It halves to 1/300 only under the code exception: balanced vents, plus a vapour retarder in climate zones 6–8.
Attic ventilation is sized by net free area — the actual open area air can pass through, which is far less than the size of the vent itself. The code rule is one square foot of net free area for every 300 square feet of attic floor where there is a vapour barrier, and one for every 150 where there is not, split evenly between intake at the soffits and exhaust at the ridge. This works that out for your attic and then converts it into the units vents are actually sold in: feet of ridge vent, and the number of soffit vents.
| Attic floor | Total NFA | Intake half | Exhaust half | Ridge vent at 18 sq in/ft | 4x16 in soffit vents |
|---|---|---|---|---|---|
| 800 sq ft | 5.3 sq ft (768 sq in) | 384 sq in | 384 sq in | 21.3 ft | 15 |
| 1,200 sq ft | 8 sq ft (1,152 sq in) | 576 sq in | 576 sq in | 32 ft | 23 |
| 1,600 sq ft | 10.7 sq ft (1,536 sq in) | 768 sq in | 768 sq in | 42.7 ft | 30 |
| 2,000 sq ft | 13.3 sq ft (1,920 sq in) | 960 sq in | 960 sq in | 53.3 ft | 37 |
| 2,400 sq ft | 16 sq ft (2,304 sq in) | 1,152 sq in | 1,152 sq in | 64 ft | 45 |
| 3,000 sq ft | 20 sq ft (2,880 sq in) | 1,440 sq in | 1,440 sq in | 80 ft | 56 |
1/150 is the code default under IRC R806.2. The requirement halves to 1/300 only under the exception: 40–50% of the vent area is exhaust within 3 feet of the ridge with the rest as low intake, and, in climate zones 6, 7 and 8, a Class I or II vapour retarder is installed on the warm side of the ceiling. A vapour retarder on its own does not qualify.
| Vent | Rated | Net free area | Note |
|---|---|---|---|
| Ridge vent (shingle-over) | per linear foot | 18 sq in | Common rating; 12–20 depending on product |
| Ridge vent (aluminium) | per linear foot | 18 sq in | Similar range to shingle-over |
| Continuous soffit vent strip | per linear foot | 9 sq in | Perforated strip; varies widely with the perforation pattern |
| Vented vinyl soffit panel | per square foot | 10 sq in | Fully vented panel; centre-vent panels give far less |
| Rectangular soffit vent 4x16 in | each | 26 sq in | Common retrofit size |
| Round soffit vent 2 in | each | 3 sq in | Small; many are needed |
| Round soffit vent 3 in | each | 7 sq in | |
| Box / static roof vent | each | 50 sq in | Typical 50–60; do not mix with a ridge vent on one plane |
| Gable louvre vent 12x18 in | each | 100 sq in | Net of the louvre and screen |
| Turbine vent 12 in | each | 113 sq in | Rated when spinning |
Net free area is the open area air can actually pass through, and it is a fraction of the vent’s overall size — a soffit panel can be 80 square inches of surface with 9 square inches of net free area once the perforation pattern is accounted for. Every vent prints its own figure. Use that one, not these, for a final order.
Take the intake half of the net free area from the table above and divide it by the net free area of the vent you are buying. At the default 1/150 ratio a 1,600 square foot attic needs 768 square inches of intake, which is 30 rectangular 4x16 inch vents at 26 square inches each, or about 86 feet of continuous perforated soffit strip at 9 square inches per foot. Where the 1/300 exception applies, halve those figures.
The 1/150 ratio and its 1/300 exception come from the building code (IRC R806.2), not from any manufacturer. A branded ventilation calculator runs exactly the arithmetic above and then converts the result into the net free area of that brand’s own products. If you know the NFA of the vents you intend to buy — it is on the box — the answer is the same wherever you work it out.
This site is independent, sells no vents and is not affiliated with any manufacturer. Where a figure here differs from one on a product, the product is right: it is describing itself.
One square foot of net free area for every 300 square feet of attic floor where there is a vapour barrier, split half intake and half exhaust. A 1,600 square foot attic needs 5.33 square feet — 768 square inches — which is 384 square inches each way.
Divide the intake half of your net free area by the NFA of the vent you are buying. For a 1,600 square foot attic that is 384 square inches, which is about 15 rectangular 4x16 inch vents at 26 square inches each, or 43 feet of continuous soffit strip.
The open area air can actually pass through, after the louvres, screens and perforation pattern are taken out. It is much less than the vent’s overall size — a soffit panel can be 80 square inches of surface with 9 square inches of net free area.
Ridge vent is commonly rated at 18 square inches of net free area per linear foot, so a 1,600 square foot attic needing 384 square inches of exhaust takes about 21 feet. Check the rating on the product — it ranges from about 12 to 20.
The arithmetic is identical, because the 1/150 ratio and its 1/300 exception are building code (IRC R806.2) rather than anyone’s method. A branded calculator then converts the result into that brand’s product NFA. This site is independent and sells nothing.
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