Duct Size Calculator

This calculator sizes HVAC ductwork three ways. Give it an airflow in CFM and a target velocity and it returns the exact round diameter, the next standard size up, the velocity you actually get in that size and the friction loss per 100 feet; give it a rectangular size instead and it returns the velocity, the equivalent round diameter and the same friction figure. The third mode turns a room's dimensions and an air changes per hour figure into the airflow that room needs. Friction comes from the standard equation for galvanized steel carrying standard air, so flexible duct and long fitting runs will lose more than the number shown. Sizing a whole system still needs a proper load calculation and duct design.