A formula giving the effective shearing rigidity of either flat or curved panels in the buckled state is developed from approximate theoretical considerations making use of the concept of effective width. Cases of panels buckled by compression, shear, or combined compression and shear are considered. The formula contains an experimentally determined exponent and gives values both for an effective shear modulus to be used in determinations of stress distributions and for a reduced effective shear modulus to be used in instability calculations. An experimental check is established.