A control and method are disclosed to determine the relative duct sizes of a plurality of ducts leading to a plurality of zones in a multi-zone HVAC system. In a disclosed method, the dampers leading to each of the zones are operated such that one damper is held more open than the remaining dampers, and a system component is monitored as air is blown through the duct. In particular, a blower speed may be monitored. Once the blower speed is monitored, for one damper being open, with the remaining dampers being relatively closed, another damper is opened and the first is closed. This process continues until relative information is gathered for each of the zones. This relative information is then utilized to determine the relative sizes of the ducts leading to each of the zones as a percentage of the total duct size. The relative duct size information is then utilized to perform various control methods.