Abstract: A method of calibrating a heat flux sensor for measuring body temperature of an individual, and a heat flux sensor. The method includes measuring heat flux with the heat flux sensor applied to a part of the body of the individual, acquiring a reference temperature value for the heat flux sensor, the reference temperature being measured at a side of the heat flux sensor facing away from the body, and acquiring a body temperature value for the individual. Further, the method comprises determining an overall heat transfer coefficient for the sensor and the individual by using the measured heat flux, the acquired reference temperature value, and the acquired body temperature value.