Measure the length and width of room for which you need the duct work.
Calculate the square footage of the room by multiplying the length of the room by the width.
Calculate the cubic feet per minute (CFM) of the room. In heating and cooling, 1 CFM is necessary per square foot. So a 400-square-foot room needs duct work capable of producing a 400 CFM.
Convert your CFM calculation to the right duct size. When heating with gas or oil, 100 CFM is equal to about 8,000 British thermal units (BTU) per hour. A standard 8-inch deep duct equals 100 CFM.
Locate a duct calculator to make the final conversion (see Resources). The round duct needed for 400 CFM requires sections 10 inches in diameter.