Attic Ventilation Calculator
Estimate a target net free vent area and a balanced intake/exhaust count from attic floor area, a selected ratio, and the net-free area listed for each vent.
How this calculator works
Vent products list net free area (NFA), which accounts for screens and louvers that reduce the open area. Enter the listed NFA per vent, not the outside dimensions of the vent. The calculator converts the selected attic-area ratio to square inches and estimates the number of vents needed for the intake and exhaust shares.
target net free area (sq ft) = attic floor area ÷ selected ratio intake and exhaust areas = total area × selected share vent count = ceil(required area (sq in) ÷ product NFA per vent)
Worked example
For a 1,200 sq ft attic using a 1:300 planning ratio, target NFA is 1,200 ÷ 300 = 4 sq ft (576 sq in). At an even split and 144 sq in NFA per vent, the estimate is 2 intake vents and 2 exhaust vents.
Treat ratios and balance as planning inputs
The 1:150 and 1:300 options are common ventilation planning ratios, but the applicable method depends on the roof assembly, vapor control, climate, attic layout, and local requirements. The default 50/50 split is an estimate. Verify the required ratio, balance, and vent placement for your building.
Do not mix incompatible vent layouts
The calculation does not check for short-circuiting between nearby intake and exhaust vents, blocked air paths, insulation obstructions, or roof geometry. A qualified building professional or local code official can help confirm a suitable layout. This count is not a code determination or a moisture-control guarantee.
Frequently asked questions
What does net free area mean for attic vents?
Net free area is the unobstructed ventilation area after screens, louvers, and other restrictions are accounted for. Use the product’s stated NFA, not the vent’s outside dimensions.
Should I use a 1:150 or 1:300 ratio?
The required ratio depends on building conditions and applicable rules. These are planning choices in the calculator, not a universal recommendation; confirm the correct method for your attic.
How should intake and exhaust ventilation be divided?
Many planning approaches aim for balanced intake and exhaust, but the required distribution and placement depend on the roof and vent system. Verify product instructions and local requirements.
Does the calculator confirm building-code compliance?
No. It estimates net free area and vent counts from your inputs. It does not evaluate code requirements, obstructions, vent placement, or the performance of the complete roof assembly.