Garage Ventilation and Exhaust Fan Sizing Calculator
Calculate the required exhaust fan size (CFM) for your garage based on dimensions, ceiling height, and usage type to ensure safe and adequate ventilation.
Formulas Used
1. Volume: V = Length × Width × Height (ft³)
2. ACH Method CFM: CFMACH = (V × ACH) ÷ 60
3. Vehicle Method CFM: CFMvehicles = Number of Cars × 100 CFM/car
4. Required CFM: CFMrequired = max(CFMACH, CFMvehicles)
5. Design CFM (with 20% safety factor): CFMdesign = CFMrequired × 1.20
6. Duct Diameter: d = √(4 × (CFMdesign ÷ 500 FPM) ÷ π) × 12 inches
7. Fan Power Estimate: W ≈ CFMdesign × 0.75 W/CFM
Assumptions & References
- Standard residential garages require a minimum of 6 ACH per ASHRAE 62.2 and IRC guidelines.
- Active workshops or garages with frequent vehicle idling should use 10–15 ACH.
- The 100 CFM per vehicle allowance is based on ACGIH Industrial Ventilation guidelines for CO dilution from idling vehicles.
- A 20% design factor accounts for duct friction losses, motor inefficiency, and filter resistance.
- Recommended duct air velocity of 500 FPM minimizes noise while maintaining adequate airflow.
- Fan power estimate of 0.75 W/CFM is typical for residential/light-commercial exhaust fans.
- Make-up air (supply) should be provided equal to exhaust CFM to avoid negative pressure; a louvered vent or passive inlet is recommended.
- For attached garages, local building codes (e.g., IRC R302.5) may require additional fire-rated separation and dedicated ventilation.
- References: ASHRAE 62.2, ACGIH Industrial Ventilation Manual, IRC Chapter 3, SMACNA HVAC Duct Construction Standards.