The need for thin wings of high torsional stiffness leads to the use of box construction, in which both bending strength and torsional stiffness are provided by a single structural box. This report gives a method by which the wing weight for four types of box construction can be predicted from data available in the project stage of a new design. The equations are derived mainly on a theoretical basis, supported as far as possible by analysis of actual structures, and are evaluated for aluminium-alloy at "room" temperature.