An image forming apparatus includes a fixing device to fix a toner image onto a recording medium, a voltmeter to measure an input voltage from an external source, and a controller operatively connected to the fixing device and the voltmeter. The fixing device includes an endless, fixing rotator formed into a loop, a heater to heat the fixing rotator, a pressure pad disposed inside the loop, and a pressure rotator disposed opposite the pressure pad via the fixing rotator to press the fixing rotator against the pressure pad to form a fixing nip between the fixing rotator and the pressure rotator, through which the recording medium bearing the toner image is conveyed. The controller controls a heating operation of the heater and a fixing operation of the fixing device to fix the toner image onto the recording medium, based on the input voltage measured by the voltmeter.