A single air velocity reading is rarely enough to calculate duct airflow because velocity is not uniform across the duct. Air usually moves at different speeds near walls, bends, dampers and the center of the duct. A credible volume-flow result uses a suitable traverse or other validated measuring method to estimate mean velocity across the actual duct area.
Area weighting matters when the velocity profile is uneven
If equal-area traverse points show velocities of 1.2, 1.8 and 3.0 m/s, their simple mean is 2.0 m/s; however unequal-area points require weighting rather than an ordinary average. Multiply the representative mean by the actual unobstructed duct area. Document bends, dampers and insufficient straight run as sources of uncertainty. For a practical multipoint procedure, see testo 405i multipoint traverse for duct air-velocity measurement.
Why the center is often misleading
In many practical ducts the velocity profile is distorted by friction and upstream components. A reading at the center may exceed the area average, especially in developing or disturbed flow. It is also possible for elbows, fans and dampers to create asymmetric, swirling or even locally reversed flow. A single centerline value cannot quantify those patterns.
From mean velocity to volume flow
The usual relationship is Q = v̄ × A, where Q is volume flow in m³/s, v̄ is representative average velocity in m/s and A is internal area in m². Converting m³/s to m³/h requires multiplying by 3,600; converting to CFM requires multiplying by approximately 2,119. A duct dimension taken from an outside insulation jacket is not the clear internal area. For additional guidance on local velocity and flow-rate measurements, see the Vane Anemometer Guide.
A professional traverse workflow
- Select a test plane with as much suitable straight duct as the measurement standard requires and document nearby disturbances.
- Measure the internal geometry and determine a suitable traverse-point pattern for its cross-section.
- Use an instrument whose velocity sensitivity fits expected conditions and check orientation.
- Measure at each specified point with the fan operating at a stable, documented setting.
- Compute the required average according to the chosen traverse method, then calculate Q using the measured area.
- Repeat checks when readings are erratic or the flow is highly disturbed. Change the test position or method if the plane is unsuitable.
Worked example: centerline versus average
Hypothetical: duct area is 0.20 m². One center reading is 5.0 m/s, but a properly weighted traverse gives a mean of 3.8 m/s. Using the center alone gives 3,600 m³/h; the traverse gives 2,736 m³/h. The single-point estimate is 864 m³/h higher, or about 31.6% above the traverse estimate. These illustrative values are not field measurements.
Testo probe for duct traverses
The testo 405i hot-wire Smart Probe measures air velocity and temperature and supports multi-point/timed traverse workflows through the Smart App. Its calculation of volume flow depends on entered duct dimensions and representative sampling. Probe selection, contamination, flow direction and measurement range still matter.
When a duct traverse is not sufficient
Complex swirl, very short straight runs and low air speeds may require another method, correction or more suitable test plane. Report the approach and its limitations rather than presenting one calculated CFM value without the underlying measurements. When a traverse is unsuitable, HVAC flow hood measurements explains another approach for measuring volumetric airflow, including setup limitations and errors.