Calculating the HVAC duct size means that you're calculating the size of the ventilation ducts that are connected to your air conditioner system. Its important to install proper duct work so that the flow of cool air adequately reaches all the various areas in a building. One simple calculation can properly find the ideal duct size for any air conditioner system, whether in your home or place of business.
Determine the normal operational air flow rate of your air conditioning unit. This information, measured in cubic meters per second, will be available in the documentation that came with your air conditioner.
Determine the normal operational air speed of your air conditioning unit. This information will be measured in meters per second and can be found in the same places you found the airflow rate information in "Step 1."
Divide your air flow rate by your air speed. The number you are left with is the total area, in square meters, that your HVAC ducts need to be to properly ventilate the area you're trying to cool.