Free area is the total minimum area of the openings in the supply outlet or return inlet (usually in square inches) through which air can pass. Also known as the "see-through area". (See also "effective area"). The free area should NOT be used to determine the amount of air that can pass thru the grille when it's being forced thru by a fan. Free area only applies to natural (gravity) air transfer thru the grille (i.e. combustion air into an appliance room).