How to use this calculator
- Enter attic floor area. Use the vented attic or ceiling area, not sloped roof surface area.
- Choose the ratio. Select 1:300, 1:150 or enter the project-required custom ratio.
- Set the upper split. Enter how much required NFA is assigned to upper exhaust vents; the balance becomes intake.
- Enter product NFA. Use manufacturer net free area per vent or per linear foot for intake and exhaust products.
- Check planned quantities. Compare required units, provided NFA, limiting utilization and upper/exhaust balance.
How it works
This roof ventilation calculator sizes attic net free area from the selected ratio:
required total NFA = vented floor area / ratio
The upper exhaust share assigns part of that total to ridge, high roof or gable exhaust vents. The remaining area is assigned to low intake or soffit vents.
required exhaust NFA = required total NFA x upper share
required intake NFA = required total NFA - required exhaust NFA
Required unit count or linear feet comes from dividing each required side by the manufacturer net free area for one vent or one linear foot.
Worked example
Verified against the live calculator
A 1,500 ft^2 vented attic using a 1:300 screen
needs 5.0 ft^2, or 720 in^2, of total
NFA. With a 50/50 split, intake and exhaust each need
360 in^2. At 9 in^2 per foot of soffit
intake and 18 in^2 per foot of ridge vent, that is
40 ft of intake and 20 ft of exhaust.
Frequently asked questions
How do you calculate attic ventilation net free area?
Use required total NFA = attic floor area / ventilation ratio. Then split that net free area between low intake vents and upper exhaust vents.
What is the difference between gross vent area and NFA?
NFA is net free area after screens, louvers and product restrictions. Use manufacturer NFA, not the gross hole or grille size.
Should I use 1:150 or 1:300?
This calculator lets you choose either common screen or a custom project ratio. The adopted local code and roof assembly conditions decide which ratio is allowed.
Can I use ridge vent NFA per linear foot?
Yes. Enter the manufacturer NFA per linear foot for exhaust, then enter planned exhaust linear feet. The required exhaust units result is also required linear feet.
Does this approve an attic ventilation design?
No. It is an NFA arithmetic screen. Blocked soffits, insulation baffles, vapor-retarder conditions, WUI/fire rules, product approvals and local code still need project review.
Method & assumptions
- Vented floor area is the attic or ceiling area served by the vents.
- Vent product data must be net free area after screens, louvers and restrictions.
- Counts can mean individual vents or linear feet when the product NFA is entered per foot.
- Balance checks compare provided intake and exhaust NFA; one side can bottleneck the other.
- Final work still needs adopted local code, climate and vapor-retarder conditions, blocked-soffit checks, insulation baffles, WUI/fire rules, roof shape and manufacturer installation instructions.
- For roof layout, use the rafter calculator. For roof drainage, use the gutter and downspout calculator.