The amount of upset during friction welding is a quite important criterion of optimum friction welding conditions because it closely concerns the mechanical properties of a welded specimen. However, it is very difficult to estimate the amount of upset before friction welding, because the friction welding process is extremely complicated. If the amount of upset during friction welding can be estimated beforehand, it is expected to extend the usefulness and application limits of friction welding. In this study, first of all, a nonuniform upsetting process firing friction welding under various friction welding conditions were correctly measure. Next, the formulation of experimental equations for the amount of upset in connection with friction welding conditions and kinds of steel (SC steel) was made. Two experimental equations were calculated. One is based on the amount of upset considering a nonuniform state during friction welding, and the other is based on the total amount of upset after friction welding. Both equations agreed well with experimental results.