Why We Built This Calculator
Attic ventilation is one of the most misunderstood parts of residential construction. Most homeowners don't know they need a specific number. They assume a few vents are "good enough." Contractors often quote "add two roof vents" without running any numbers. The result is thousands of homes with inadequate ventilation, leading to ice dams, moisture damage, and shingles that fail a decade early.
IRC Section R806.2 sets the standard: your attic needs a minimum ratio of net free area to floor area, either 1/150 (without a vapor retarder) or 1/300 (with one). The formula is simple. But most people have never seen it, and most contractors aren't going to show their work.
We built this tool so homeowners, inspectors, and contractors can get the right number in under a minute, and it's completely free.
Code-Accurate Results
Every calculation follows IRC Section R806.2 exactly. It's the same formula used by building inspectors across 49 states. No proprietary adjustments, no padding. Just the code.
Transparent Methodology
We show our work. The formula, variables, and assumptions are all explained. You should understand why you got a number, not just what it is.
Educational Content
Beyond the calculator, we publish detailed guides on ventilation code, vent types, seasonal considerations, and common mistakes.
Our Editorial Process
Every calculation and piece of content on this site is verified against the 2021 International Residential Code (IRC), the ASHRAE Handbook of Fundamentals, and guidance from the National Roofing Contractors Association (NRCA). We don't publish anything we can't source.
Blog articles are reviewed for factual accuracy before publication. Cost estimates are updated annually based on regional contractor data. Formula constants are cross-checked against the current edition of the IRC whenever the code updates.
We flag any content that reflects conditions specific to a region or climate, and we note where local code amendments may differ from the base IRC. If you find an error, contact us and we review every report.
Get in Touch
Found an error, have a question about IRC ventilation requirements, or want to suggest a topic for our blog?
contact@atticventilationcalculator.com