TL;DR
The attic ventilation ratio is the code formula setting how much net free vent area an attic needs relative to its floor area: 1 to 150 by default under IRC R806, relaxed to 1 to 300 when 40 to 50 percent of the venting is high (ridge) with the rest low (soffit) or where a ceiling vapor retarder is present. Net free area is the unobstructed opening after screens and louvers, printed on the vent's label, not the vent's overall size.
What it means
The attic ventilation ratio is the code formula setting how much net free vent area an attic needs relative to its floor area: 1 to 150 by default under IRC R806, relaxed to 1 to 300 when 40 to 50 percent of the venting is high (ridge) with the rest low (soffit) or where a ceiling vapor retarder is present. Net free area is the unobstructed opening after screens and louvers, printed on the vent's label, not the vent's overall size. Balanced intake and exhaust matter as much as the total, with intake never less than exhaust. Roofers list the calculation when quoting ridge vent or adding soffit vents during a reroof.
Where it sits in the glossary
Attic ventilation ratio is part of the Trade jargon group inside the ProFix Directory glossary. Browse every term in this category from the glossary index.
Why Ohio homeowners should know it
This is a term Ohio homeowners encounter when reading contractor quotes, hiring paperwork, or inspection reports. Understanding it well enough to ask one good follow-up question is usually all the protection a homeowner needs.
ProFix Directory keeps definitions short on the index page and saves the longer context — Ohio-specific rules, where the term comes from, and which ProFix tools touch it — for these per-term pages so the term is easy to cite and easy to share.
ProFix tools that touch this term
See also
License: CC-BY-4.0 — quote freely with attribution to ProFix Editorial Team / ProFix Directory.