Attic Ventilation Calculator
Quick answer: Per IRC §R806.2, attics need 1 sq ft of net free ventilation area per 150 sq ft of attic floor (the 1/300 rule applies only to balanced systems with continuous soffit intake, ridge exhaust, and a vapor retarder). A 40 × 30 ft attic (1,200 sq ft) needs 8.0 sq ft = 1,152 sq in of net free area — 576 sq in intake plus 576 sq in exhaust, about 32 linear feet of ridge vent.
Enter your attic's length and width, then check the box only if you have a balanced system (continuous soffit intake plus ridge exhaust with a vapor retarder). Results update instantly with required net free area, the 50/50 intake-exhaust split, and ridge vent footage.
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Estimate per IRC §R806.2. Ridge vent footage assumes 18 sq in net free area per linear foot — a common manufacturer specification; check your vent product's rating.
How to use this calculator
Measure the attic floor length and width in feet — the vented attic area, not the whole roof footprint. Leave the checkbox unchecked unless you truly have continuous soffit intake, ridge exhaust, and a ceiling vapor retarder.
Use the results to size soffit vents (intake) and ridge/gable vents (exhaust). Match real vent products by their rated net free area per unit, since nominal opening sizes overstate actual airflow.
How this is calculated
Definitions: area = attic floor length × width (sq ft); NFA = net free ventilation area; 1 sq ft = 144 sq in.
- Attic area = length × width
- Required NFA = area ÷ 150 (standard) or area ÷ 300 (balanced system with vapor retarder)
- NFA in sq in = NFA in sq ft × 144
- Intake = exhaust = 50% of total NFA (balanced split)
- Ridge vent linear feet = exhaust NFA (sq in) ÷ 18 sq in per linear foot
Assumptions: the attic floor is rectangular; the 18 sq in per linear foot ridge-vent rating is a common manufacturer specification, not a code figure — use your product's rated NFA. Estimates for planning; local code amendments take precedence.
Worked example
40 × 30 ft attic = 1,200 sq ft. Standard 1/150 rule: required NFA = 1,200 ÷ 150 = 8.0 sq ft = 1,152 sq in. Balanced 50/50 split: 576 sq in intake + 576 sq in exhaust. Ridge vent footage = 576 ÷ 18 ≈ 32 linear ft.
Reference data
| Rule | Ratio | When it applies |
|---|---|---|
| 1/150 | 1 sq ft NFA per 150 sq ft attic floor | Default requirement |
| 1/300 | 1 sq ft NFA per 300 sq ft attic floor | Balanced system: continuous soffit intake + ridge exhaust, with vapor retarder |
Source: International Residential Code (IRC) §R806.2 (2021/2024 editions).
Method & sources
The 1/150 and 1/300 ratios come directly from IRC §R806.2's minimum vent-area requirements; this page performs exact division of your attic area by the applicable ratio and converts to square inches and ridge-vent footage. The 18 sq in per linear foot ridge-vent figure is labeled as a common manufacturer specification — confirm against the specific vent product you plan to buy.
Frequently asked questions
What is the difference between the 1/150 and 1/300 attic ventilation rules?
The 1/150 rule requires 1 square foot of net free ventilation area for every 150 square feet of attic floor space. The 1/300 rule allows half that — 1 square foot per 300 square feet — but only when the system is balanced, meaning continuous soffit intake plus continuous ridge exhaust, and a vapor retarder is installed on the warm side of the ceiling. Per IRC §R806.2, both ratios must be net free area, split between upper and lower portions of the attic.
Why must attic intake and exhaust be balanced?
Because attic ventilation works on airflow, not just openings: cool air enters low at the soffits and warm moist air exits high at the ridge. If exhaust exceeds intake, the system can pull conditioned air — and moisture — from the living space into the attic. If intake exceeds exhaust, air stagnates and moisture accumulates. A roughly 50/50 split keeps air moving in the intended direction.
Can power vents replace passive vents to meet code?
Building code is written around passive net free area — the square-inch openings the code tables count. Mechanical vents such as power attic fans move air differently and are generally not counted as net free area toward the 1/150 or 1/300 requirement. This page computes only passive net free area per code; check with your local building official before relying on powered ventilation for code compliance.
Does this replace a code official's review?
No. This calculator applies the IRC §R806.2 net free area ratios as a planning estimate. Local amendments, roof design details, and existing vent products all affect what an inspector will accept. Confirm final vent sizing with your local building department.