Attic ventilation calculator: how much attic ventilation do I need?
1,152 sq in of net free area
8 sq ft · 1/150 of 1,200 sq ft of attic · half low and half high is 576 sq in each · for a 40 × 30 ft attic, IRC R806.2
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1,152 sq in of net free area
8 sq ft · 1/150 of 1,200 sq ft of attic · half low and half high is 576 sq in each
Source: 2021 International Residential Code, R806.2 Minimum vent area (2021 Seattle Residential Code, chapter 8), read October 21, 2026. Data sheet (PDF)
A 40 by 30 ft attic, 1,200 square feet, needs 1,152 sq in of net free area: IRC 2021 R806.2 sets 1/150 of the vented space, 8 square feet, and allows 1/300 only when both conditions of its exception are met.
A worked example
A 52 by 27 1/2 ft attic in climate zone 6 that meets both conditions of the exception, vented with soffit vents marked 65 sq in and a ridge vent marked 18 sq in per foot needs 687 sq in of net free area. 1,430 square feet ÷ 300 is 4.77 square feet, 687 square inches; half low and half high is 343.5 each: 6 soffit vents of 65 square inches and 20 feet of ridge vent at 18 per foot.
What is the 1/150 rule for attic ventilation?
One square foot of net free vent area for every 150 square feet of attic floor, by IRC 2021 R806.2: 8 square feet, or 1,152 square inches, for a 1,200 square foot attic. The code allows 1/300, half that, only when upper vents carry 40 to 50% of the area within 3 feet of the ridge and, in climate zones 6 to 8, a Class I or II vapor retarder is installed.
How many soffit vents do I need?
Half the net free area, divided by the area printed on one vent. For a 1,200 square foot attic at 1/150 that is 576 square inches low: 9 vents marked 65 square inches, or 72 feet of continuous soffit vent marked 8 per foot.
What else it is called
Attic ventilation is sized as net free area, NFA, and searched as an attic ventilation calculator, soffit vent calculator, how many soffit vents do I need or the 1/150 rule. Intake vents sit low, at the soffit; exhaust vents sit high, at or near the ridge.
Net free area by attic size
| Attic floor | 1/150, sq in | 1/150, sq ft | 1/300, sq in | 1/300, sq ft |
|---|---|---|---|---|
| 600 sq ft | 576 | 4.00 | 288 | 2.00 |
| 800 sq ft | 768 | 5.33 | 384 | 2.67 |
| 1,000 sq ft | 960 | 6.67 | 480 | 3.33 |
| 1,200 sq ft | 1,152 | 8.00 | 576 | 4.00 |
| 1,500 sq ft | 1,440 | 10.00 | 720 | 5.00 |
| 2,000 sq ft | 1,920 | 13.33 | 960 | 6.67 |
| 2,500 sq ft | 2,400 | 16.67 | 1,200 | 8.33 |
Minimum net free ventilating area by IRC 2021 R806.2, rounded up to the square inch. The 1/300 columns apply only when both conditions of its exception are met: a Class I or II vapor retarder in climate zones 6 to 8, and 40 to 50% of the area in upper vents within 3 ft of the ridge.
Where the numbers come from
- Vent area: 2021 International Residential Code, Section R806.2: «The minimum net free ventilating area shall be 1/150 of the area of the vented space», or 1/300 when both conditions of its exception are met: in climate zones 6, 7 and 8, a Class I or II vapor retarder on the warm-in-winter side of the ceiling; and not less than 40 nor more than 50 percent of the required area in upper vents no more than 3 feet below the ridge, with the rest in the bottom third of the attic. R806.1 sets vent openings of 1/16 to 1/4 inch. Read in the 2021 Seattle Residential Code, where R806 carries no Washington amendment, and checked against the 2021 IRC as adopted in New Jersey. Read October 21, 2026. Read the code chapter (PDF)
- The rest is arithmetic: square feet of attic floor ÷ 150 (or 300), × 144 for square inches, rounded up; vents = half the area ÷ the net free area printed on your vent, low and high. Half and half is one way to meet the exception; under the 1/150 rule the code sets no split.
Net free area is the open area of a vent once its louvers and screen are counted, not its outside size: read it on the product or its box. This page sizes the vent area, not the airflow of a powered fan.
By Javier Miralles · Last updated . How this site is made.
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