A method and system control the adding or dropping of phases in a multiphase voltage regulator. The regulator has an efficiency and this efficiency of the regulator is calculated for a given number of phases being activated from an output voltage, input voltage, output current, and duty cycle of the regulator. The efficiency of the regulator is also calculated if a phase is added using the derivative of the duty cycle as a function of the output current. The efficiency of the regulator is further calculated if a phase is dropped using the derivative of the duty cycle as a function of the output current. From these operations of calculating, a phase is either added, dropped, or the phase is maintained at its current value to thereby optimize the efficiency of the regulator.