The short answer
A 1,500 sq ft attic needs 1,440 sq in (10 sq ft) of net free vent area under the IRC's 1/150 rule, or 720 sq in at 1/300 when 40 to 50% of it is high near the ridge (and, in climate zones 6 to 8, the ceiling has a vapor retarder). Split evenly, that's 40 ft of 18 sq in ridge vent and 80 ft of 9 sq in soffit vent at 1/150.
Vent area by attic size
Net free area and feet of vent, split evenly, with a 18 sq in per foot ridge vent and 9 sq in per foot soffit vent.
| Attic | 1/150 | Ridge, soffit | 1/300 | Ridge, soffit |
|---|---|---|---|---|
| 800 sq ft | 768 sq in | 22 ft, 43 ft | 384 sq in | 11 ft, 22 ft |
| 1,000 sq ft | 960 sq in | 27 ft, 54 ft | 480 sq in | 14 ft, 27 ft |
| 1,200 sq ft | 1,152 sq in | 32 ft, 64 ft | 576 sq in | 16 ft, 32 ft |
| 1,500 sq ft | 1,440 sq in | 40 ft, 80 ft | 720 sq in | 20 ft, 40 ft |
| 1,800 sq ft | 1,728 sq in | 48 ft, 96 ft | 864 sq in | 24 ft, 48 ft |
| 2,000 sq ft | 1,920 sq in | 54 ft, 107 ft | 960 sq in | 27 ft, 54 ft |
| 2,500 sq ft | 2,400 sq in | 67 ft, 134 ft | 1,200 sq in | 34 ft, 67 ft |
| 3,000 sq ft | 2,880 sq in | 80 ft, 160 ft | 1,440 sq in | 40 ft, 80 ft |
How it works
The formula
Code sets vent area as a share of the attic floor. Air comes in low at the eaves and leaves high at the ridge, carrying out heat and moisture, so the area is split between the two.
ridge vent (ft) = high share ÷ ridge rating
soffit vent (ft) = low share ÷ soffit rating
Assumptions
- Ratios from IRC R806.2 (2018 and 2021): 1/150, or 1/300 when 40 to 50% of the required area is within 3 ft of the ridge and, in climate zones 6 to 8, a Class I or II vapor retarder is on the warm side of the ceiling. Your ZIP sets the climate zone. Some older code editions allow 1/300 with either condition; check the edition your town uses.
- We split the area 50% high and 50% low, the most exhaust the 1/300 rule allows; intake should never be less than exhaust.
- Default ratings: 18 sq in per foot for a 12 in shingle-over ridge vent (Air Vent ShingleVent II, ICC-ES ESR-2071), 9 sq in per foot for continuous aluminum soffit vent (Air Vent SV201 and SV202). Type your own vents' ratings.
- Ridge vent covers both sides of the ridge at once: the feet shown are measured along the ridge, before the end cuts your vent maker calls for.
Worked example: 1,500 sq ft attic
- 1,500 ÷ 150 = 10 sq ft × 144 = 1,440 sq in
- Half high: 720 ÷ 18 = 40 ft of ridge vent
- Half low: 720 ÷ 9 = 80 ft of soffit vent
Too big or too small
- Too little intake: the ridge pulls from the house. Starved soffits make the exhaust draw conditioned air through ceiling gaps.
- Blocked soffits count for nothing. Keep insulation back with baffles so there's at least 1 in of air space above it at the eaves (IRC R806.3).
- Venting doesn't fix air leaks. Moisture problems in cold climates usually start at ceiling leaks; seal them first.
Questions people ask
How much attic ventilation do I need?
The International Residential Code (R806.2) calls for 1 sq ft of net free vent area for every 150 sq ft of attic floor. A 1,500 sq ft attic needs 10 sq ft, which is 1,440 sq in. With 40 to 50% of it high near the ridge, the code allows half that, 720 sq in, as long as in climate zones 6 to 8 the ceiling also has a Class I or II vapor retarder.
How many feet of ridge vent do I need?
Divide the high half of the vent area by the ridge vent's rating. At 1/150, a 1,500 sq ft attic needs 720 sq in high; a ridge vent rated 18 sq in per foot needs 40 ft. At 1/300 it's 20 ft.
Should I have more intake or exhaust?
At least as much intake as exhaust. The code allows 40 to 50% of the vent area up high for the 1/300 ratio, with the rest at the eaves; too much exhaust can pull air from the house below instead of from the soffits.
What is net free area?
The open area air can actually pass through, after louvers, screens and baffles. It is smaller than the vent's size: always use the net free area printed on the vent's label or spec sheet.
Can I mix ridge vents with box vents or gable vents?
Most vent makers say not to combine different exhaust types on one attic: a gable vent or a second exhaust can short-circuit the ridge vent, pulling air in through it instead of from the soffits.