Airflow CFM Calculator — Blower and Ventilation Requirements
Calculate required airflow (CFM — Cubic Feet per Minute) for blowers and ventilation systems using four industry-standard methods: Air Changes per Hour (ACH), Heat Load, Duct Velocity, or Room Volume with target ACH.
Method 1: Air Changes per Hour (ACH)
Formulas Used
Method 1 — Air Changes per Hour (ACH):
CFM = (Room Volume × ACH) ÷ 60
Room Volume (ft³) = Length × Width × Height
Method 2 — Sensible Heat Load:
CFM = Q ÷ (1.08 × ΔT)
Where Q = sensible heat (BTU/hr), ΔT = temperature difference (°F)
1.08 = 0.075 lb/ft³ (air density) × 0.24 BTU/lb·°F (specific heat) × 60 min/hr
Method 3 — Duct Velocity:
CFM = Duct Area (ft²) × Velocity (FPM)
Round: Area = π × (D/2)² ÷ 144 [D in inches → ft²]
Rectangular: Area = Width × Height ÷ 144
Method 4 — People-Based (ASHRAE 62.1):
CFM = (Occupants × CFM/person) + (Floor Area × CFM/ft²)
Assumptions & References
- Standard air density assumed at 0.075 lb/ft³ (sea level, ~70°F). Adjust for altitude or temperature extremes.
- The constant 1.08 in the heat load formula = 60 × 0.075 × 0.24 (specific heat of air at constant pressure).
- ACH recommendations per ASHRAE, SMACNA, and OSHA guidelines vary by space type.
- ASHRAE Standard 62.1 governs minimum ventilation rates for acceptable indoor air quality.
- Duct velocity recommendations per SMACNA HVAC Duct Construction Standards.
- CFM values should be increased by 10–20% for system losses, duct leakage, and filter resistance in real installations.
- This calculator does not account for humidity (latent load), altitude correction, or duct static pressure losses.
- 1 CFM = 0.000472 m³/s = 1.699 m³/hr for SI conversion.