It is convenient to measure foil weight and area and use known values of density to calculate an average thickness. It is also possible to measure the transmission of monochromatic x rays through the foil and use accepted values of the absorption coefficient to determine foil thickness. If this x‐ray transmission measurement is performed over a large area of an irregular foil, the resulting calculation of average foil thickness will not be in agreement with the weight‐area method. The purpose of this paper is to indicate the error introduced in the average thickness measurement by examining the disparity in transmission between a uniform foil and several nonuniform foils having the same average thickness.