Before the duct cross-sectional area can be calculated you must determine two elements.
Qcfm = Amount of Ventilation air and Combustion air (combine system) in cfm.
Va = Desired inlet air velocity [Not to exceed 2,000 feet per minute (609.6 m/min)]
Once these two elements have been determined then the following formula can be used to determine the minimum cross-sectional for both intake and exhaust ducts or openings.
Av = Duct cross sectional area in square inches (mm)
Qa = Quantity of air flow in cubic feet per minute (m3/min)
Va = Velocity of air in the duct in feet per minute (m/min)