Understanding your result
The headline is the net free area required, with the intake and exhaust split. The stats repeat the intake, the exhaust and the area in square feet. The breakdown shows the rule, the division, the split and a first estimate of ridge vent footage and soffit vents.
The numbers to carry on are the intake and the exhaust: the soffit vent calculator turns the intake into vents or feet of strip, and the ridge vent calculator or the box vent calculator turns the exhaust into product. The attic ventilation table lists the requirement for common attic sizes.
How we calculate this
The rule is a ratio of vent opening to attic floor, chosen by the code to keep attics dry in winter and cooler in summer. The 1:300 relaxation recognises that balanced vents move air through the whole attic and that a vapor retarder keeps moisture out in cold climates; the calculator applies the rule you pick and warns when the split is outside the 1:300 balance. The guide to the 1:150 rule covers the history and the exceptions.
The assumptions behind the numbers
| Assumption | Default | Where it comes from |
|---|---|---|
| Rule | 1:300 | IRC R806.2, with balanced vents |
| Balance for 1:300 | 40 to 50% exhaust in the upper attic | IRC R806.2 exception |
| Intake share | 50% | Ventilation makers’ guidance (at least as much intake as exhaust) |
| Ridge vent rating | 18 sq in per ft | A common ridge vent rating (12 to 20) |
| Soffit vent rating | 56 sq in each | A common 16 × 8 in aluminum undereave vent |
Assumptions last reviewed October 7, 2026.
The calculator does not design unvented attics, does not size powered fans (the attic fan calculator does), and does not check the insulation, which is sized with the insulation calculator.
Two worked examples
A 1,500 square foot attic, balanced
1,500 square feet, 1:300, 50 percent intake.
- NFA: 1,500 ÷ 300 = 5 sq ft = 720 sq in
- Intake 360 sq in; exhaust 360 sq in
- Ridge vent at 18 sq in per foot: about 20 ft; soffit vents at 56 each: about 7
A 2,200 square foot attic under 1:150, intake-heavy
2,200 square feet, 1:150, 60 percent intake.
- NFA: 2,200 ÷ 150 = 14.67 sq ft = 2,112 sq in
- Intake 1,267 sq in; exhaust 845 sq in
- Ridge vent: about 47 ft
More intake than exhaust is the safe direction: the attic stays at slightly positive pressure and does not pull air from the house.
Where to find your inputs
Attic floor area. The ceiling area below the attic: the house footprint inside the walls, without overhangs.
Rule. 1:300 when the vents are balanced between soffit and ridge, or with a vapor retarder in zones 6 to 8; otherwise 1:150.
Common mistakes
- Counting vent size, not net free area. Use the rating on the product.
- Exhaust without intake. Soffits blocked by insulation do nothing; add baffles.
- Mixing exhaust types. One type per roof.
- Using the roof area. Use the attic floor.
- Ignoring local amendments. Some jurisdictions modify R806.
- Forgetting the insulation. Ventilation and insulation are sized together.
Questions people ask
- How much ventilation does an attic need?
- The International Residential Code (R806.2) requires a net free area of 1 square foot per 150 square feet of attic floor, or 1 per 300 when 40 to 50 percent of it is exhaust in the upper part of the attic (balanced), or when a class I or II vapor retarder is installed on the warm side of the ceiling in climate zones 6 to 8. A 1,500 square foot attic needs 1,440 square inches under 1:150 or 720 under 1:300.
- What is net free area?
- The total unobstructed area of a vent's openings, after the screen, louvres and baffles: what air can actually pass through. A 16 by 8 inch soffit vent has a net free area of perhaps 56 square inches, not 128. Vent makers print it on the product.
- How should ventilation be split between intake and exhaust?
- Roughly half and half, with at least as much intake as exhaust. Too much exhaust and too little intake makes the exhaust vents draw air from the house through ceiling leaks; the code's 1:300 allowance requires the exhaust share to be 40 to 50 percent.
- Can I mix ridge vents and gable vents?
- It is not recommended. Two exhaust types on one roof short-circuit: the gable vent becomes an intake for the ridge vent and the soffits stop drawing. Use one exhaust type with soffit intake.
- Does a cathedral ceiling need ventilation?
- Vented cathedral ceilings need a clear 1-inch air space above the insulation in every rafter bay from soffit to ridge; unvented assemblies with insulation in contact with the deck are allowed under R806.5 with specific insulation. The calculator covers attics with a floor.
- Do I use the attic floor area or the roof area?
- The attic floor area, which is usually the area of the ceiling below it, not the roof surface and not the house footprint with overhangs.