Duct Airflow Velocity Calculator
Calculate the air velocity inside a duct given the volumetric flow rate and duct cross-sectional dimensions (rectangular or circular).
Formula
V = Q / A
- V = Air velocity (FPM — feet per minute)
- Q = Volumetric flow rate (CFM — cubic feet per minute)
- A = Duct cross-sectional area (ft²)
Circular duct area: A = (π / 4) × d²
Rectangular duct area: A = W × H
Dimensions in inches are divided by 144 to convert in² → ft².
Unit conversion: 1 FPM = 0.00508 m/s
Assumptions & References
- Recommended velocity range of 500–2,500 FPM is based on ASHRAE Fundamentals Handbook (Chapter 21 — Duct Design) for low-velocity HVAC supply ductwork.