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Attic Ventilation Calculator

Size attic ventilation to code — net free area required, the 50/50 intake and exhaust split, and how many feet of ridge vent or soffit vents to buy — with U.S. pricing, a vent comparison, and a downloadable PDF.

Follows the IRC 1/150 and 1/300 rules. Vents are rated in net free area (NFA) — the actual open square inches, not the size of the hole. Intake must always equal or exceed exhaust. Estimates use 2024-2025 U.S. prices — for planning purposes only.

Attic & Vents
1 Attic Size
ft
×
ft
The attic floor footprint — the space you are venting, not the roof surface.
sq ft
ft
Used to check whether a continuous ridge vent can actually fit the exhaust you need.
2 Code Rule & Balance
50 / 50
50/50 is the standard split. Going to 60/40 in favor of intake is fine — the reverse is not.
Switches to CFM sizing (0.7 CFM per sq ft). A fan needs a lot of intake, or it pulls conditioned air out of the house.
3 Vent Products & Cost
$/ linear ft
$/ linear ft
$total
Pro ridge vent + soffit cut-in ~$600 - $1,500. Set 0 for DIY.
Ventilation Estimate
Net Free Area Required
—
Enter the attic size to begin
Intake NFA Needed —
Exhaust NFA Needed —
Attic Area —
Full Cost Breakdown
Total NFA required —
Intake vents —
Exhaust vents —
TOTAL ESTIMATE—
Intake to buy —
Exhaust to buy —
Cost / sq ft of attic —
Cost Breakdown Chart
Estimated cost by category ($)
Exhaust Options — Your Attic
OptionRatingQuantity NeededFits The Roof?Est. Cost

To cover 288 sq in of exhaust NFA. Never mix ridge vent with gable louvers or a powered fan — the ridge becomes an intake and pulls rain and snow in.