In the design of the mechanical ventilation of air duct system the motor fun is usually designed to have enough capacity to exchange the whole air in the assigned compartments with fresh out-door air at required frequency with all terminal air ports fully opend. The dimensions and laying of the air ducts must be determined so as to satisfy the requirement. This is an arduous task. We have derived a new basic formula applicable to the calculation of the ventilation ducts, assuming that the all air resistances, including duct friction, bend and orifice resistances, are proportional to the square of air velocity in the duct, and also adopting the equivalent length of duct which makes the calculation easy. This formula has analogy to that of electric circuits, and the calcultaion can be made in the same way as in electric circuits.