HVAC airflow and duct calculator

Calculate actual duct airflow, mean velocity or required area with clear metric and imperial units.

Input units

Changing unit systems reinterprets the entered numbers.

Clear internal width

Clear internal height

Mean air velocity

m/s

Equal-area velocity samples

Use a set of velocity readingsReadings in m/sThese readings represent equal areas of the cross-sectionAn arbitrary set of point readings is not a valid mean traverse.

Round-up diameter increment

Optional. The rounded diameter's velocity is recalculated.

Actual airflow

500L/s

Airflow: 500 L/s · 1059.44 CFM
Internal area: 0.1 m²
Equivalent round diameter: 356.825 mm
Mean velocity: 5 m/s

A 375 mm round duct gives 4.527 m/s at this flow.

Use clear internal dimensions and a measured mean velocity. Results are actual volumetric flow at the measurement conditions.

Measuring duct airflow

Airflow is cross-sectional area multiplied by mean velocity. A 400 × 250 mm opening has 0.1 m² of area. At 5 m/s, it carries 0.5 m³/s, or 500 L/s.

A single center reading may miss slower air near the walls. The sample option averages readings only when you confirm that each represents an equal area. The round equivalent has the same area; it does not have the same pressure loss or noise performance.

More tools like this

Concrete volume and bag estimatorAdd slabs, footings and round piers to estimate concrete volume, whole bags and material cost from your product yield.Brick and block estimatorSubtract wall openings, include mortar joints and estimate whole units and packs with a visible waste allowance.Roof pitch and rafter calculatorConvert roof pitch between rise/run, degrees and percent, and calculate common-rafter centerline and overhang geometry.Stair rise and run plannerCompare equal-riser layouts, choose the top landing and check tread dimensions against your entered constraints.