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
- Airflow is assumed to be uniform across the entire duct cross-section (plug flow).
- Duct walls are assumed smooth and straight with no obstructions at the measurement point.
- Recommended velocity range of 500–2,500 FPM is based on ASHRAE Fundamentals Handbook (Chapter 21 — Duct Design) for low-velocity HVAC supply ductwork.
- Higher velocities (up to 4,000 FPM) may be acceptable in industrial or high-velocity systems.
- Return air ducts typically operate at lower velocities (400–1,500 FPM) to minimise noise.
- This calculator does not account for pressure drop, friction losses, or fittings.
- Reference: ASHRAE Handbook — Fundamentals (2021), Chapter 21; SMACNA HVAC Duct Construction Standards.