How this calculator works
Building codes (such as the IRC in the US) set the minimum attic ventilation as a ratio of the attic floor area to the net free area of the vents: 1 square foot of vent per 150 square feet of attic. This can be relaxed to 1:300 if there is a vapor retarder on the ceiling, or if the vents are balanced with at least half of them high in the attic and the rest low at the eaves.
Worked example: a 40 × 30 ft attic (1,200 ft²) at the 1:300 ratio needs 1,200 ÷ 300 = 4 ft², or 576 in², split into 288 in² of soffit intake and 288 in² of ridge exhaust. At 18 in² per foot, the ridge vent is 16 ft long, and soffit vents with 40 in² each need 8 vents. At the stricter 1:150 ratio all those numbers double.
Tips before you buy
- Ventilation protects the roof from moisture in winter and reduces heat build-up in summer, which prolongs the life of the shingles and lowers cooling costs.
- Balance matters more than quantity. Exhaust without intake pulls conditioned air from the house, and intake without exhaust just stalls.
- Do not mix different kinds of exhaust vents (such as a ridge vent and a gable vent, or power fans), because they can short-circuit each other.
- Keep baffles (rafter vents) in place so insulation does not block the soffit airflow.
- Local codes decide the ratio you must meet, so check with your building department.
Tools & materials for this project
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Frequently asked questions
How much attic ventilation do I need?
The standard is 1 ft² of net free vent area for every 150 ft² of attic floor, or 1 per 300 where the vents are balanced between high and low (or there is a vapor retarder). Always split the area between intake and exhaust.
What is net free area?
The actual open space that air can flow through, after you subtract louvers, screens and baffles. It is printed on the vent packaging, and is always smaller than the vent's overall size.
Is a ridge vent better than a gable vent?
A ridge vent combined with soffit intake gives the most even airflow, and is generally the best system. Gable vents rely on the wind and ventilate only part of the attic.
Related calculators & guides
Estimates only. Results assume standard materials and simple shapes. Supplier products vary, and the final call on quantity, structure and safety is yours. Check your local code and your supplier's guidance before ordering.