Airflow CFM Calculator — Fan and Vent Sizing
Calculate the required airflow in CFM (Cubic Feet per Minute) for fans and ventilation systems using room volume, desired air changes per hour (ACH), or sensible heat load.
Method 1: Room Volume & Air Changes per Hour (ACH)
Typical: Bedroom 4–6, Kitchen 15–60, Bathroom 6–10, Office 4–10
Method 2: Sensible Heat Load
Supply air temp vs. room temp difference
Method 3: Duct / Vent Sizing from CFM
Typical supply ducts: 600–900 FPM; return: 400–700 FPM
Formulas Used
Method 1 — ACH Method:
CFM = (Room Volume [ft³] × ACH) / 60
Where Room Volume = Length × Width × Height (all in feet), and ACH = Air Changes per Hour.
Method 2 — Sensible Heat Load:
CFM = Q / (1.08 × ΔT)
Where Q = sensible heat load (BTU/hr), ΔT = temperature difference between supply air and room (°F), and 1.08 is the air constant derived from: 0.075 lb/ft³ (standard air density) × 0.24 BTU/lb·°F (specific heat of air) × 60 min/hr.
Method 3 — Duct Sizing:
Area (ft²) = CFM / Velocity (FPM)
Round Duct Diameter (in) = √(4 × Area / π) × 12
Assumptions & References
- Standard air density assumed at 0.075 lb/ft³ (sea level, ~70°F).
- Specific heat of air: 0.24 BTU/lb·°F.
- ACH constant of 1.08 is widely used in HVAC load calculations (ASHRAE).
- Recommended ACH by space type (ASHRAE 62.1 / ACGIH): Residential bedrooms 4–6, kitchens 15–60, bathrooms 6–10, offices 4–10, server rooms 20–30, commercial kitchens up to 60.
- Recommended duct velocities: supply ducts 600–900 FPM, return ducts 400–700 FPM, exhaust ducts 400–600 FPM (SMACNA guidelines).
- Standard round duct sizes follow SMACNA sheet metal standards.
- This calculator does not account for duct friction losses, static pressure, or altitude corrections.
- References: ASHRAE Handbook — Fundamentals; SMACNA HVAC Duct Construction Standards; ACGIH Industrial Ventilation Manual.