The quantification of tear volume trapped under a flexible contact lens in situ is important for an understanding of both optical and physiological lens behavior. Mathematical treatment is used to calculate the flexed back radii of flexible contact lenses in situ from the various hypotheses available. For each hypothetical radius the tear volume under the lens is calculated. The method is limited in accuracy due to several underlying assumptions. Twelve lens situations are analyzed, and the results indicate a tear volume of the order of 10 microliters.