To calculate duct size, a Ductulator calculator can be used. In each room, the heat loss or gain needs to be determined to see how much either warm or cold air, should be delivered to it. The standard duct size is eight inches and is designed for one hundred CFM or cubic feet per minute. For an air conditioner, the one hundred CFM is 6000 BTU while the CFM for a furnace is 8000 BTU. More
. information is at http://www.warmair.net/html/duct_dynamics.htm. The Ductulator can be found at http://docs.bentley.com/en/HVAC/htfhelp89.html
