Result
Preliminary calculation: This tool is for quick sizing and information. In real projects, load calculation, selection and commissioning require engineering calculations and a site survey. Results are approximate values.
Formulas
A [m²] = V̇ [m³/h] / (3600 × v [m/s])Round duct diameter d = √(4A/π)Rectangular equivalent diameter: De = 1.3 (a·b)^0.625 / (a+b)^0.25Friction increases as the aspect ratio of a rectangular duct moves away from 1:1; the ideal ratio is between 1:1 and 1:3.
Air change rate (ACH) examples
The required ACH varies with the space and regulations. The value in this tool is for calculation convenience; for official requirements, refer to the relevant standard and regulation. You can also use the thermal load of air calculator to find the capacity equivalent of the flow.
Frequently asked questions
How is air flow calculated?
Multiply the volume by the number of air changes per hour: V̇ = volume × ACH.
What happens if the velocity in the duct is too high?
Noise, pressure loss and fan energy increase. For comfort, a range of 5–8 m/s is generally targeted in the main duct.
How do I convert a rectangular duct to a round one?
The equivalent diameter formula is used; the equivalent diameter is the basis for friction loss calculations.