Duct Air Velocity

This formula finds the average speed of air moving through a duct by dividing the airflow rate by the duct’s cross-sectional area. It works because flow rate is the volume of air passing a point each second, so spreading that volume over a larger area means the air moves more slowly, while a smaller area makes it move faster. The result is an average duct velocity, which is useful in real HVAC calculations even though the air speed can vary slightly across the duct.

Use this when you know the volumetric airflow and the duct size, and you want to estimate how fast the air is moving. It is common in ventilation design, duct sizing, airflow balancing, and checking whether duct velocity is in a safe or efficient range.

• Using duct diameter instead of cross-sectional area
• Mixing units, such as m³/min with m², without converting first
• Forgetting that the result is an average velocity, not the exact speed everywhere in the duct