Airflow CFM Calculator
Calculate airflow in cubic feet per minute (CFM) using either the Velocity Method (duct velocity × area) or the Air Changes Per Hour (ACH) Method (room volume ÷ 60 × ACH).
Formulas Used
Velocity Method:
CFM = Velocity (FPM) × Area (ft²)
Rectangular duct area: A = (Width × Height) / 144 (converts in² → ft²)
Circular duct area: A = π × (Diameter / 2)² / 144 (converts in² → ft²)
ACH Method:
CFM = (Room Volume (ft³) × ACH) / 60
Room Volume: V = Length × Width × Height
Assumptions & References
- Velocity Method assumes uniform air velocity across the entire duct cross-section.
- Duct dimensions are entered in inches and converted to square feet (÷ 144) for the area calculation.
- The ACH Method divides by 60 to convert air changes per hour into per minute (CFM).
- Typical residential HVAC duct velocities range from 400–900 FPM; commercial systems 1,000–2,500 FPM.
- ACH recommendations per ASHRAE Standard 62.1 and ASHRAE Handbook — HVAC Applications.
- Results assume standard air conditions (sea level, ~70°F). Altitude or temperature corrections may be needed.
- No duct leakage, friction losses, or system static pressure losses are accounted for in this calculator.