The Air Duct Calculator App shows a slide chart that affords the configuration of rectangular, circular and oval air ducts in a minimum of time without any calculating effort.
With a given speed you simply adjust the flow rate. The appropriate duct sizes show up simultaneous on the slide chart itself and in the table on the top to make the calculation even more convenient. You can also send the results via e-mail. With a given pressure drop you procced in the same way.
With parameters for temperature, height over sea level and roughness the calculated results can be converted to operating conditions.