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Attic Ventilation Calculator

Estimate the net free ventilation area needed for an attic, using the standard 1:150 (or 1:300 with a vapor barrier) rule.

Computed in your browser — nothing you enter is uploaded. Figures are presented for United States against IRC 2024, and every formula is cited under regulatory standards below.

Last verified 2026-08-25 · v1.0.0

Market
Imperial · sales tax
39.37 ft26.25 ft
Schematic, drawn to the proportions you entered — not to scale on screen.

Net free vent area needed

6.89 sq ft (net free area)

Check your inputs

A general guideline — always check your local building code, which may specify its own attic ventilation requirement.

Attic floor area
1033.34 sq ft
Net free area needed
992 sq in

With the figures above, the net free vent area needed comes to 6.89 sq ft (net free area). The method behind this is well established, though site conditions and material batches will move it somewhat. This is calculated for United States and cites IRC 2024. Building in another market? Change the selector above so the units and code reference follow.

Estimated cost — your price

This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.

Preliminary estimate, not certified engineering. This tool produces an indicative quantity calculation for planning purposes only — it is not a certified structural analysis, a guaranteed material takeoff, or a substitute for building department approval. Always verify measurements on-site and have a licensed contractor or structural engineer review any load-bearing, code-sensitive, or safety-critical work before purchasing materials or starting construction. Spotted an arithmetic or standards error? Report it to contact@craftquantities.com with your inputs — confirmed fixes become pinned regression tests.

[Schema Verified] Computed in alignment with American Concrete Institute (ACI 318-19) formulas and International Residential Code (IRC 2024) spatial boundaries.

Regulatory standards & verification citations

  • Standard attic ventilation guideline: 1 sq ft of net free vent area per 150 sq ft of attic floor space, or per 300 sq ft if a vapor barrier is present and vents are balanced between intake and exhaust

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Frequently asked questions

What does 'net free area' mean?
It's the actual open airflow area of a vent, which is less than its physical opening size due to louvers and screens — vent products list their net free area rating on the packaging.
Should intake and exhaust vents be split evenly?
Yes — a balanced system with roughly half the net free area at the soffit (intake, low) and half at the ridge or gable (exhaust, high) works far better than concentrating vents in one place.
What happens with inadequate attic ventilation?
Poor ventilation can trap heat and moisture, accelerating shingle aging, promoting mold/mildew, and increasing cooling costs in summer.