Ridge vent is sized by airflow rating and usable ridge length together. Determine required exhaust net free area, divide by the selected vent's NFA per linear foot, then confirm that enough eligible ridge and balanced intake ventilation are available.
Divide required exhaust NFA by the selected ridge vent's published NFA per linear foot, then confirm the roof has enough usable ridge and matching intake NFA.
Published: September 20, 2026
Last reviewed: September 20, 2026
Published by BuildYard. The guide reflects BuildYard's calculation methodology and editorial standards and does not replace project-specific professional advice.
Start from the attic ventilation requirement that applies to the assembly.
Allocate the intended exhaust share before converting airflow to ridge length.
Ridge vents publish net free area ratings per linear foot.
Divide required exhaust square inches by that rating rather than by the vent's physical width.
Only ridge length that can be opened and vented according to the installation instructions should be counted.
Short hips, intersections, fire separations, or structural details can limit usable length.
Exhaust cannot perform as intended without adequate low intake airflow.
Size soffit or other intake vents from their published NFA and keep insulation from blocking the air path.
Convert required linear feet to the stocked vent sections or rolls and round up.
Follow slot-width, end-clearance, fastening, weather, and shingle-cap instructions for the selected system.
BuildYard conversion example using 18 sq in NFA per linear foot as a planning product rating; replace with the exact selected vent rating.
| Step | Result |
|---|---|
| Required exhaust NFA | 384 sq in |
| Planning vent rating | 18 sq in/linear ft |
| Required ridge vent | 21.34 linear ft |
| Purchase/layout | Round up and verify usable ridge |
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Estimate attic intake and exhaust net free area from attic floor area, applicable ventilation ratio, balance, and product NFA ratings.
Read guide →RoofingEstimate roof sheathing fasteners from panel layout, supported edges, intermediate framing, specified spacing, and a purchase allowance.
Read guide →RoofingEstimate starter shingles from eave and rake lengths, product coverage, required overlaps, corners, and whole-bundle rounding.
Read guide →Current manufacturer data, supplier specifications, drawings, field measurements, applicable codes, permit requirements, and qualified professional guidance can take priority over a general-purpose planning estimate.