Attic Ventilation Net Free Area Calculator
Enter the attic floor area to find the net free ventilation area required, how to split it between high and low vents, and how big the vents must be.
Method reviewed October 26, 2026. Version 1.0.0. How it works
Result
- Total net free area
- 605in²
- High (ridge, roof or gable) vents: net free area
- 303in²
- Low (soffit or eave) vents: net free area
- 302in²
605 in² of net free area: about 303 in² high and 302 in² low.
- When the 1/300 ratio is allowed depends on your code edition (vent placement, and in cold climates a ceiling vapour retarder). Keep at least 1 in of space between insulation and roof sheathing, with baffles at the eaves.
- Total net free area
- 4.2ft²
High vents
- High (ridge, roof or gable) vents: net free area
- 303in²
- High vents: gross vent area at this free-area %
- 505in²
Low vents
- Low (soffit or eave) vents: net free area
- 302in²
- Low vents: gross vent area at this free-area %
- 504in²
Show the working
- Net free area = attic area ÷ 3001,260 ÷ 300 = 4.2 ft²
- Convert to square inches (× 144), rounded up605 in²
- High vents = share × total (rounded up); low vents = the rest303 in² high, 302 in² low
- Gross vent area = net free area ÷ free-area fraction (rounded up)303 ÷ 0.6 = 505 in²; 302 ÷ 0.6 = 504 in²
The rule
DOE’s Building America Solution Center describes the code requirement for vented attics: the net free ventilating area must be at least 1/300 of the attic floor area when vents are balanced high and low, or 1/150 where low (soffit or eave) vents cannot be provided. About half of the area should be high in the roof, at least 3 feet above the eave vents, and the rest low.
When the reduced 1/300 ratio is allowed depends on the code edition — for example, conditions on vent placement or a ceiling vapour retarder in cold climates. Check the code your jurisdiction has adopted.
Net free area versus vent size
Net free area is the open area after screens and louvres. Most screened vents have about 60 percent free area, so the gross vent size is the net free area divided by 0.6. Many vents print their net free area on the label; use it if you have it.
Worked example from the Solution Center: a 30 × 42 ft attic is 1,260 ft². At 1/300 that is 4.2 ft² or 605 in², of which 303 in² goes high — 505 in² of 60-percent-free vents.
Worked example
These are the example values the calculator opens with, and the result the engine calculates for them.
Inputs
- Attic floor (ceiling) area
- 1,260 ft²
- Vent ratio
- 1/300 — balanced high and low vents
- Share of vent area placed high
- 50 %
- Vent free-area percentage
- 60 %
Result
- Total net free area
- 605 in²
- Total net free area
- 4.2 ft²
605 in² of net free area: about 303 in² high and 302 in² low.
Assumptions
- Net free area = attic area ÷ 300 (balanced high and low vents) or ÷ 150 (no low vents), as described by DOE’s Building America Solution Center; gross vent area = net free area ÷ the vent’s free-area fraction.
Limitations
- Local code governs the ratio and when the reduced 1/300 ratio is allowed.
- Not for unvented (sealed) attics, which are designed differently.
Frequently asked questions
Should I add more vents than required?
Balanced high and low venting matters more than extra area. Keep at least 1 inch between insulation and roof sheathing and use baffles at the eaves so soffit vents stay open.
Do I use the roof area or the floor area?
The attic floor (ceiling) area below the attic. For a cathedral ceiling, use the sloped ceiling area.
Does this apply to sealed attics?
No. Unvented attics with insulation at the roof deck are designed differently and do not use these ratios.
Sources
- Calculating Attic Passive Ventilation — U.S. DOE Building America Solution Center (PNNL). Net free vent area not less than 1/300 of the attic area (2015 IRC R806.1) or 1/150 where low vents cannot be provided; about 50 % of it high, at least 3 ft above the eave vents; most screened vents have 60 % free area; example 30 × 42 ft → 4.2 ft² = 605 in², 303 in² high, 505 in² of vents; 1/150 → 8.4 ft² = 1,210 in².
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