The precise estimation of efficiency of induction motors is crucial in industries for energy savings, auditing, and management. This paper presents a novel method for in situ induction motors efficiency estimation by applying the genetic algorithm and utilizing IEEE Form F2-Method F1 calculations with pretested motor data. The method requires a dc test, full-load operating point rms voltages, currents, input power, and speed measurements. The proposed algorithm uses a sensorless technique to determine motor speed. The algorithm is not only an in situ tool; it can also be used as an on-site efficiency estimation tool that might replace the expensive dynamometer procedure. The method was validated by testing 30 induction motors.