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This paper proposes a method to estimate transformer hotspot heating due to half-cycle saturation caused by geomagnetically induced currents (GICs). The method is based on fitting a closed-form analytical function to the calculated or measured thermal response of a particular transformer design, typically provided by the manufacturer, at its hotspot locations to a step dc current excitation. This fitted function can then be used to simulate the hotspot temperature profile for transformers of that particular design when subjected to GIC having an arbitrary profile over time. Using this approach, examples are presented of the winding and metallic hotspot thermal responses of the two transformers to recorded GIC time sequences, to illustrate how hotspots may be impacted on during a geomagnetic disturbance event.