The formula used to determine the amount of CFM, or cubic feet per minute, a room needs for heating begins by multiplying the room’s volume by the number of times the heated air gets changed hourly. Then divide the resulting number by 60 to produce the amount of CFM required. To determine the room’s volume for the first step in the formula, the room’s length, width and height must be multiplied together. When this volume is multiplied by the hourly air changes, the resulting number is the amount of cubic feet of air that passes through that room hourly. The cubic feet is then divided by 60 in the final step, since there are 60 minutes in an hour.The resulting CFM refers to how many cubic feet of air must be fed into that room every minute.