Connect flow, area and speed
Average velocity equals volume flow divided by cross-sectional area. With flow in m³/s and area in m², the result is m/s. For a circular duct, area is π multiplied by the square of the internal diameter, divided by four.
Use compatible units and the relevant internal area. A nominal product size should not replace a confirmed dimension in a calculation.
Work through an illustrative comparison
Consider a flow of 0.20 m³/s through a duct with an internal diameter of 0.20 m. Its area is approximately 0.0314 m², giving an average velocity of about 6.37 m/s. At the same flow through a 0.25 m internal diameter, the average is about 4.07 m/s.
These examples show the relationship only. They are not recommended design velocities or a duct selection. The duct sizing guide explains the wider project information needed.
Read a measurement record carefully
| Item | Why it matters |
|---|---|
| Location | The reading must relate to a defined duct section. |
| Method | Air velocity varies across the section; the assessment method matters. |
| Dimensions | Area is needed to relate average velocity to volume flow. |
| Operating state | Fan controls and connected equipment affect the condition measured. |
Keep different measurements separate
Duct velocity and cupboard face velocity describe different locations. Neither should be substituted for the other. Keep the unit, position and operating state with each figure.
For an equipment review, pair the required volume flow with the system pressure requirement. A competent tester or designer can interpret the measurements against the installation’s intended duty.