Equations for the efficiency of a single-acting impact forming machine are derived by representing it as an equivalent single-degree-of-freedom spring-mass system. The deformation load is approximated by various shapes of the force pulse. The effects of the mass ratio, period ratio and the machine-structure parameters on the process efficiency are studied. The interaction between the input momentum and energy is explored. The influence of damping on the efficiency is also investigated. The analysis shows that for achieving a high level of efficiency the time duration in which deformation is completed should be equal to or greater than the natural period of the machine. It is also found that damping improves efficiency when deformation is completed in a time reasonably shorter than the natural period of the machine. Maximum efficiency is attained when all the momentum associated with the falling platen is transferred to the billet