We proposed an equation formulating the relationship between the resonant frequency of a quartz crystal and the mass loading. The equation developed by Sauerbrey concerning quartz crystal microbalance (QCM) was improved by a new approach based on the mechanical strength of materials. This equation is seemingly trivial but greatly improves the accuracy with respect to the frequency-mass relationship to frequency shifts of 10% or more in the case of selected materials. Methods to treat the effects of crystal mounting and non-uniformity of deposited films are elucidated.