Attic Vent Calculator
Required net free vent area split between soffit and ridge vents.
What it does
Attics breathe through screened openings, and codes size those openings by floor area. The standard rule gives 1 square foot of net free vent for every 150 square feet of attic, so a 1200 ft2 attic owes 8 ft2, or 1,152 in2. The tool above divides any attic and splits the result between low and high vents.
The imperial default is the classic handout case: 1200 divided by 150 is 8 ft2, times 144 is 1,152 in2, halved to 576 in2 of soffit intake with 576 in2 of ridge exhaust. Metric mode runs the same ratio on 110 m2 for 0.73 m2 split into 3,667 cm2 each side.
Two authorities carry the page. The International Residential Code sets the 1:150 minimum with a 1:300 exception for balanced vents plus a ceiling vapor retarder, and the federal Building America Solution Center works the same 30 by 42 foot house to 4.2 ft2 at 1:300 with a 50/50 high low split.
Balance matters more than raw area. Air should enter low at the soffits and leave high at the ridge, riding convection from eave to peak. Piling every vent at the ridge short circuits the flow, and insulation shoved into the eaves chokes intake no matter what the math allows.
How to use
- Pick imperial (ft) or metric (m).
- Type the attic floor area.
- Pick 1:150, or 1:300 with balanced vents and a retarder.
- Read total vent area first.
- Split it half soffit intake, half ridge exhaust.
Formula
- NFA
- Net free open area after louvers and screens.
- 1:150
- Standard minimum vent ratio.
- 1:300
- Allowed with balanced vents plus vapor retarder.
Vent ratings print net free area, which runs near 60% of gross opening for screened units. Always size from the rating, never the frame dimensions.
Worked example
Example: 1200 ft2 at 1:150
Imperial defaults: 1200 ft2, ratio 150.
- Divide: 1200 / 150 = 8 ft2 of vent.
- Convert: 8 x 144 = 1,152 in2.
- Split: 1,152 / 2 = 576 in2 low and high.
- Meaning: 576 in2 soffit intake with 576 ridge exhaust.
- Tool shows 8 ft2, 1,152 in2, 576 in2 soffit + ridge.
Reference table
| Attic | Total | Each side |
|---|---|---|
| 800 ft2 | 5.33 ft2 (768 in2) | 384 in2 |
| 1200 ft2 | 8 ft2 (1,152 in2) | 576 in2 |
| 1500 ft2 | 10 ft2 (1,440 in2) | 720 in2 |
| 2000 ft2 | 13.33 ft2 (1,920 in2) | 960 in2 |
Limits
- Minimums only; hot climates often want more.
- Assumes clear air paths past insulation baffles.
- Local amendments can override these ratios.
- Cathedral ceilings measure sloped area instead.
Common mistakes
- Sizing from gross frame size instead of NFA rating.
- Piling vents high with no soffit intake.
- Blocking eaves with insulation.
- Claiming 1:300 without the retarder and balance.
- Sealing vents shut for winter.
Try it yourself
Quick checks. Answers sit tucked under each question.
1260 ft2 at 1:300?
1260 / 300 = 4.2 ft2, 605 in2, 302 in2 each side.
1500 ft2 at 1:150?
1500 / 150 = 10 ft2, 1,440 in2, 720 each.
110 m2 at 1:150?
0.73 m2, 7,333 cm2, 3,667 cm2 each side.
Questions
How much vent for 1200 ft2?
8 ft2 (1,152 in2) at 1:150, split 576 and 576.
What is net free area?
True open airflow area after louvers and screens.
When is 1:300 allowed?
Balanced high low vents plus a ceiling vapor retarder.
Why split high and low?
Convection needs intake low and exhaust high to sweep the attic.
Do screens change the count?
Yes. Size from the NFA rating, near 60% of gross.
Who confirms my ratio?
The local building official; amendments vary.