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Calculator · 2026

Roof ventilation calculator

How much attic ventilation do I need?

The IRC minimum is 1 sq ft of net free vent area per 150 sq ft of attic, or 1 per 300 when the code's exception applies. A 1,500 sq ft attic needs 1,440 sq in at 1/150 (720 at 1/300), split evenly between intake and exhaust.

How it is calculated

  1. Net free area (NFA) = attic floor area ÷ 150, or ÷ 300 under the exception.
  2. Convert to square inches (x 144).
  3. Split 50/50 between intake (soffit or eave) and exhaust (ridge or high on the roof), ARMA's balance; if you cannot balance it, favor intake.
  4. Divide each by the NFA printed on the vent product to get the number of vents (or linear feet of ridge vent).

Always use the product's rated net free area, not its overall size: screens and louvers block part of the opening. Poor ventilation shortens a roof's life: see how long a roof lasts.

Sources for the figures on this page

IRC R806.2

  • UpCodes, R806.2 Minimum Vent Area (Colorado DFPC Residential Code 2021 = IRC 2021), retrieved 2026-09-28: The minimum net free ventilating area shall be 1 / 150 of the area of the vented space.
  • UpCodes, R806.2 Minimum Vent Area (Colorado DFPC Residential Code 2021 = IRC 2021), retrieved 2026-09-28: Exception: The minimum net free ventilation area shall be 1 / 300 of the vented space provided both of the following conditions are met: In Climate Zones 6, 7 and 8, a Class I or II vapor retarder is installed on the warm-in-winter side of the ceiling. Not less than 40 percent and not more than 50 percent of the required ventilating area is provided by ventilators located in the upper portion of the attic or rafter space. Upper ventilators shall be located not more than 3 feet (914 mm) below the ridge or highest point of the space, measured vertically. The balance of the required ventilation provided shall be located in the bottom one-third of the attic space.

Intake and exhaust balance (ARMA)

  • ARMA: The Attic Needs Ventilation, but How Much Exactly? (page dated Revised September 2022), retrieved 2026-09-28: “What if it’s not possible to balance the attic ventilation system 50 percent intake/50 percent exhaust?” If it cannot be balanced it’s better to have more intake than exhaust because it has been industry experience most attics lack proper intake ventilation
  • ARMA: The Attic Needs Ventilation, but How Much Exactly? (page dated Revised September 2022), retrieved 2026-09-28: Attic floor square footage ÷ 4 = square inches of EXHAUST and square inches of INTAKE Net Free Area needed.

Code text is quoted from a published adoption of the 2021 IRC; your jurisdiction's adopted code governs. Written by the Thomas Town Digital editorial team.

Attic ventilation questions

How much attic ventilation do I need?

The IRC (R806.2) sets the minimum net free ventilating area at 1/150 of the vented attic area. For a 1,500 sq ft attic that is 10 sq ft, or 1,440 sq in.

When is 1/300 allowed?

Under the R806.2 exception, 1/300 is allowed when both of its conditions are met: in Climate Zones 6, 7 and 8 a Class I or II vapor retarder is installed on the warm-in-winter side of the ceiling, and 40 to 50 percent of the required area is provided by upper vents no more than 3 feet below the ridge, with the rest in the bottom third of the attic. Check the exact wording in your adopted code.

Should intake and exhaust be balanced?

ARMA (the Asphalt Roofing Manufacturers Association) works from a 50 percent intake / 50 percent exhaust balance, and says that if it cannot be balanced it is better to have more intake than exhaust.

What is the quick rule of thumb?

ARMA: attic floor square footage ÷ 4 = square inches of exhaust and square inches of intake net free area needed. That about matches the 1/300 ratio split in half (floor area ÷ 4.17).