Handheld Trane Ductulator This handy device is the ultimate tool for sizing supply and return duct systems. Using the equal friction design method, this HVAC duct calculator can be used to calculate the equivalent round duct dimensions from rectangular duct dimensions. Additionally, this rotating duct size calculator also makes it easy to calculate the size of the duct that your system needs by inputting the desired friction loss and matching it up with the CFM. With I-P units and SI unit conversions, this ductwork-sizing calculator is the ultimate tool to make calculations simple, easy and error-free. Don't have room in your toolbox? This HVAC-duct-sizing calculator comes with a protective sleeve that holds ASHRAE-recommended design air velocities! Questions? Ask our experts today!