In air ducts, which formula correctly expresses volumetric airflow rate?

• A. Velocity divided by cross sectional area
• B. Cross sectional area divided by velocity
• C. Cross sectional area times velocity
• D. Cross sectional area times density

Answer: (C)

Volumetric airflow rate is the amount of air volume passing through a cross-section per unit time. If air moves through a duct with cross-sectional area A at velocity v, the volume crossing each second is A × v (assuming velocity is roughly uniform across the area). That makes Q = A × v the correct expression.

Units line up: velocity (m/s) times area (m^2) gives m^3/s, which is a volume per time. The other options don’t have the right units or meaning: velocity divided by area isn’t a flow rate, area divided by velocity isn’t a flow rate, and area times density yields a different quantity (mass-related, not volume per time). For example, with A = 0.5 m^2 and v = 4 m/s, Q = 2 m^3/s. If velocity varies across the cross-section, you’d use Q = ∫ v dA, but for a uniform velocity, it simplifies to A × v.