The Trane Ductulator is an accurate , easy-to-use calculating device that aids in laying out air-handling systems , sizing ducts and checking duct systems. Complete with both standard and metric calibrations. This hand-held rotating calculator is used for sizing supply and return duct systems using the equal friction design method. It includes scales for friction loss per unit length , air volume , air velocity , round duct diameter , and rectangular duct diameters. .