EfficientComfort.net and Informer Technologies offer free duct sizing software. The programs available on these companies' websites include DuctSizer and ResDuct. EfficientComfort.net also offers AllDuctCalc, a free application that calculates duct size, airflow, friction and other parameters.
DuctSizer is a simple program for performing size and pressure calculations on straight oval, round and rectangular ducts. It also includes a reference library.
ResDuct is a Web application that calculates minimal duct sizes in accordance with the standards of the Air Conditioning Contractors of America Association. The program works with metal, fiberboard, PVC and flexible duct types. It also includes an airflow velocity calculator.