Attic ventilation calculator
An attic needs 1 sq ft of net free vent area for every 150 sq ft of attic floor under the International Residential Code (section R806.2), split about evenly between intake at the soffits and exhaust at the ridge. A 1,200 sq ft attic needs 1,152 sq in: 576 in, 576 out. Type the attic floor and this calculator gives the vent area at 1/150 or 1/300, then the feet of ridge vent, box vents and soffit vents from each product's rating. Free, no account.
The attic
Type the attic floor area: length times width, inside the walls. The code ratio and the vents' ratings do the rest.
50 = balanced
Exhaust vent ratings
Net free area, in square inches. Starting values are Air Vent's and Alside's listed ratings; type the number on the product you install.
Intake vent ratings
16 × 8 in vent
Soffit vent strip
Vented soffit panel
- 1,200 sq ft ÷ 150 × 144
- 1,152 sq in
- Exhaust: ridge vent at 18 sq in a foot
- 32 ft
- or roof (box) vents at 50 sq in each
- 12 vents
- Intake: undereave vents at 56 sq in each
- 11 vents
- or continuous soffit vent at 9 sq in a foot
- 64 ft
- or vented soffit at 7 sq in a sq ft
- 83 sq ft
Check which edition of the code your area adopted, and any local changes to it.
Worked out in your browser from the numbers you type. Nothing is looked up or saved. No prices are shown here. Confirm every number on site.
How to calculate attic ventilation
Vent area is rated as net free area (NFA): the open area air can pass through, in square inches. Work it out in five steps.
- Measure the attic floor: length times width, inside the walls.
- Divide by 150 (or by 300 when the code's exception applies) for square feet of NFA, then multiply by 144 for square inches.
- Split it between intake low on the roof and exhaust high on the roof, half and half, or with a little more intake.
- Divide the exhaust by the ridge vent's NFA per foot, or by each roof vent's NFA.
- Divide the intake by each soffit vent's NFA, or by the vented soffit's NFA per square foot.
ExampleA 1,200 sq ft attic at 1/150
- 1,200 sq ft ÷ 150 × 144
- 1,152 sq in
- Intake, half
- 576 sq in
- Exhaust, half
- 576 sq in
- Ridge vent at 18 sq in a foot
- 32 ft
- or roof vents at 50 sq in each
- 12 vents
- Undereave vents at 56 sq in each
- 11 vents
- or continuous soffit vent at 9 sq in a foot
- 64 ft
ARMA's shortcut, attic floor ÷ 2 for each side at 1/150, gives 600 sq in. It runs a little high, which ARMA says is fine for estimating.
Vent area by attic size
At 1/150, every 1,000 sq ft of attic floor needs 960 sq in of net free area: 480 in and 480 out. At 1/300 it's half that.
Air Vent recommends 1/150 with half the vent area as intake and half as exhaust, and notes that some bare-minimum codes use 1/300 (Air Vent).
Net free area needed, square inches
| Attic floor | 1/150 total | Each side | 1/300 total | Each side |
|---|---|---|---|---|
| 1,000 sq ft | 960 | 480 | 480 | 240 |
| 1,200 sq ft | 1,152 | 576 | 576 | 288 |
| 1,500 sq ft | 1,440 | 720 | 720 | 360 |
| 2,000 sq ft | 1,920 | 960 | 960 | 480 |
| 2,500 sq ft | 2,400 | 1,200 | 1,200 | 600 |
| 3,000 sq ft | 2,880 | 1,440 | 1,440 | 720 |
Rounded up to the next square inch. With an even split, intake and exhaust are each half.
Vents and their ratings
Each maker rates its vents for net free area. These are the calculator's starting values, checked October 2026, with what each takes for a 1,000 sq ft attic at 1/150. Use the rating on the product you install.
| Vent | Side | Rating | 1,000 sq ft attic |
|---|---|---|---|
| Air Vent ShingleVent II ridge vent | Exhaust | 18 sq in per ft | 27 ft |
| GAF Cobra Rigid Vent 3 ridge vent | Exhaust | 18 sq in per ft | 27 ft |
| Air Vent Airhawk RVG50 slant back roof vent | Exhaust | 50 sq in each | 10 vents |
| Air Vent 16 × 8 in undereave vent | Intake | 56 sq in each | 9 vents |
| Air Vent continuous soffit vent | Intake | 9 sq in per ft | 54 ft |
| Alside Alliance T4 vented soffit | Intake | 7 sq in per sq ft | 69 sq ft |
1/150 or 1/300
IRC section R806.2 sets 1/150 and allows 1/300 only when both conditions of its exception are met. Check which edition your area adopted and any local changes.
1/150: the code minimum
- 1 sq ft of net free area for every 150 sq ft of the vented space.
- 960 sq in for each 1,000 sq ft of attic floor.
- Air Vent recommends it for today's tighter homes, split half intake and half exhaust.
1/300: only when both are true
- In climate zones 6, 7 and 8, a Class I or II vapor retarder is on the warm-in-winter side of the ceiling.
- 40% to 50% of the vent area is in the upper part of the attic, no more than 3 ft below the ridge, with the rest in the bottom third.
- 480 sq in for each 1,000 sq ft of attic floor.
Skip the attic: the footprint from an address
When you can't get into the attic, ARMA says the footprint of the house from an aerial view can stand in for the attic floor (ARMA). Enter an address and Measured Bid's free roof measurement reads every roof plane and its pitch from aerial data.
Flattened, our sample house's 4 planes cover 2,164 sq ft, so it needs about 2,078 sq in of vent area at 1/150. The footprint includes the overhang, so it runs a little larger than the attic floor.
Planes
- North, 5/12962 sq ft
- South, 5/12962 sq ft
- East, 5/12210 sq ft
- West, 5/12210 sq ft
Put it on the roof estimate
Add the ridge vent and soffit vents to the reroof as their own lines. In Measured Bid you price the roof per square from an address, with tear-off, waste and steep-roof adjustments.
FAQ
How much attic ventilation do I need?
The International Residential Code (section R806.2) asks for 1 sq ft of net free vent area for every 150 sq ft of attic floor. That's 960 sq in for each 1,000 sq ft of attic. Split it about half and half between intake low on the roof and exhaust up high. Check the code your area adopted.
What is the 1/150 rule, and when can I use 1/300?
1/150 is the code minimum: divide the attic floor area by 150 for the square feet of net free area. IRC R806.2 drops it to 1/300 only when both of its conditions are met. In climate zones 6, 7 and 8, a Class I or II vapor retarder is on the warm-in-winter side of the ceiling. And 40% to 50% of the vent area is in the upper part of the attic, no more than 3 ft below the ridge, with the rest in the bottom third.
How many feet of ridge vent do I need?
Divide the exhaust area by the ridge vent's net free area per foot. Air Vent's ShingleVent II and GAF's Cobra Rigid Vent 3 are both rated 18 sq in per foot. A 1,200 sq ft attic at 1/150 needs 576 sq in of exhaust, so 32 ft of ridge vent.
How many soffit vents do I need?
Divide the intake area by each vent's net free area. Air Vent rates its 16 × 8 in undereave vent at 56 sq in, so 576 sq in of intake takes 11 of them. Its continuous soffit vent gives 9 sq in per foot, so the same attic takes 64 ft.
How many box vents do I need?
Divide the exhaust area by each vent's rating. Air Vent's Airhawk slant back roof vent (RVG50) is rated 50 sq in, so 576 sq in of exhaust takes 12 of them. ARMA says to put exhaust vents as close to the peak as you can.
Can intake and exhaust be unequal?
Yes, as long as intake is the bigger one. ARMA says that when the two can't be balanced, it's better to have more intake than exhaust. GAF says exhaust at the ridge should never exceed the soffit ventilation.
Does roof pitch change how much ventilation I need?
The code doesn't use pitch. ARMA passes on a rule of thumb from vent makers: add 20% for 7/12 to 10/12 roofs and 30% for 11/12 and steeper, because a steeper roof holds a bigger attic. The calculator has it as an option.
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