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