Duct air velocity calculator

Enter airflow in cubic metres per second and area in square metres to receive velocity in metres per second, feet per minute, and kilometres per hour. It uses the continuity relationship v = Q / A, making it useful for quick HVAC design checks, commissioning calculations, equipment comparisons, and repeatable engineering workflows without relying on charts or manual unit conversion.

Enter airflow and the true internal area

Start with the volumetric airflow rate that actually passes through the duct section being evaluated. The input is expressed in cubic metres per second, so convert values reported in litres per second, cubic metres per hour, or cubic feet per minute before submitting them. Then enter the duct's internal cross-sectional area in square metres. Use clear inside dimensions rather than nominal outside dimensions, because insulation, liners, seams, and wall thickness can reduce the space available to the air. For a rectangular duct, area is internal width multiplied by internal height. For a round duct, area is pi multiplied by the square of the internal radius. The calculator intentionally accepts area directly so it also works with oval, irregular, or manufacturer-specified sections. Both inputs must be positive finite values. A zero or negative area has no physical meaning here, while a missing airflow cannot establish a velocity. Keeping the input basis explicit makes results easier to audit and prevents hidden assumptions about duct geometry.

Understand the continuity calculation

The calculation applies the steady-flow continuity relationship: average velocity equals volumetric airflow divided by cross-sectional area. When airflow is supplied in cubic metres per second and area in square metres, the quotient is metres per second because the square-metre terms cancel. The result also includes feet per minute and kilometres per hour as convenient equivalent speeds; these are conversions of the same calculated value, not separate estimates. If the airflow stays constant while the area is halved, velocity doubles. If the area stays constant while airflow increases by twenty percent, velocity also increases by twenty percent. This direct proportionality makes the tool useful for comparing duct options. The reported value is a cross-sectional average, however, not a map of the local velocity profile. Real flow is affected by wall friction, fittings, dampers, bends, transitions, and turbulence. Measurements taken at a single point can therefore differ from this average, especially near disturbances where the flow has not become reasonably uniform.

Use the result in design and commissioning

Treat the calculated speed as a transparent engineering checkpoint. During early design, compare alternative duct areas at the required airflow to identify sections that may carry unusually high or low velocities. During commissioning, pair a measured or balanced volumetric flow with the verified internal area to calculate the corresponding average speed and compare it with project criteria. Higher velocity can permit smaller ducts, but it may also increase pressure loss, regenerated noise, drafts, and sensitivity to fittings. Lower velocity can reduce some of those effects while requiring more space and material. This calculator does not decide which velocity is acceptable because appropriate limits depend on system type, acoustic goals, pressure budget, occupancy, duct location, and governing standards. It also does not calculate friction loss, fan power, Reynolds number, or fitting losses. Use specialized methods for those checks. For repeatable automation, the API returns the normalized inputs, three velocity units, and the exact formula label for straightforward logging, testing, and downstream calculations at $0.002 per request.