Attic Ventilation Area Calculator
Net free ventilation area is often expressed relative to attic floor area. The chosen ratio is an assumption, and the split can help plan balanced intake and exhaust openings.
How to use this calculator
- Enter the floor area, chosen ratio denominator and intended intake share.
- Check that units, denominator definitions and time periods are consistent before comparing scenarios.
- Run a second scenario when an assumption is uncertain.
Formula used by attic ventilation area calculator calculator
Total net free area = attic floor area / selected ratio; intake and exhaust areas follow the entered percentage split.
- Total net free area = attic floor area / selected ratio; intake and exhaust areas follow the entered percentage split.
- Use the required ratio and definitions in your local code or design guidance. Product gross opening area is not the same as rated net free area.
- The tool performs arithmetic on the values entered and does not validate assumptions against outside records.
Worked examples
Worked example
120 m² attic using a 1:300 ratio, split equally between intake and exhaust.
Total net free area = 120 ÷ 300 = 0.4 m² (4,000 cm²); each half is 2,000 cm².
The calculator estimates the total and split net free ventilation area.
Understanding the result
Net free ventilation area is often expressed relative to attic floor area. The chosen ratio is an assumption, and the split can help plan balanced intake and exhaust openings.
Use the required ratio and definitions in your local code or design guidance. Product gross opening area is not the same as rated net free area.
Important notes
Use the required ratio and definitions in your local code or design guidance. Product gross opening area is not the same as rated net free area.
Treat this output as an estimate tied to the stated inputs. For regulated, safety-critical, tax or professional decisions, verify applicable official guidance and specifications.
Frequently asked questions
Does this tell me the required vent count?
No. Divide the area by each product’s rated net free area, following the manufacturer and code.
Should intake and exhaust always be exactly equal?
Design requirements vary; use the split prescribed for the project.
How should I interpret the result?
Use the required ratio and definitions in your local code or design guidance. Product gross opening area is not the same as rated net free area.
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This page provides a calculation tool plus explanatory information so you can understand what the result means and when the underlying method may not fit your situation.