An apparatus for measuring air flow in a duct includes a sensor fittable into connection with the duct. The sensor includes an ultrasound transmitter, at least two ultrasound receivers, and a control unit to which the ultrasound transmitter and ultrasound receivers are connectable. The apparatus further includes means for measuring temperature. In a method, with the apparatus, the temperature of the air and/or of a sensor in a duct is measured with means for measuring temperature. A sensor calibration measurement at the temperature in question is performed with the apparatus if a calibration of the sensor at the measured temperature and/or within a certain predefined temperature range has not been performed earlier from the environs of the measured temperature, and temperature compensation data for the measurement result of air flow, the data being formed on the basis of calibration measurement, is determined and/or recorded in memory with the apparatus.