Abstract This paper proposes an improved indicator to estimate the voltage stability margin in electric power systems, defined as the Simplified Voltage Stability Index (SVSI), which uses voltage measurements and some assumptions from the Thevenin model. The proposed technique uses information about the current operating condition incorporating only the voltage phasor measurements and the power system’s topology. The proposed index has been evaluated on the IEEE 30-bus and 118-bus test systems, considering several operating conditions and contingencies as line outages. The validation of the proposed index was performed by comparing it to other referenced voltage stability indexes. Simulation results show that the proposed index gives a good indication of the voltage stability of a power system; it is also simple and computationally less expensive than the indices it was compared to.