The measurement of the Sound Strength (G) room acoustic parameter requires a calibration of the measuring system, so either the sound power of the source or the sound pressure level at a distance of 10 m in free field is known. Two procedures are described in ISO 3382-1: Calibration in a reverberation chamber and calibration in free field in a large anechoic chamber. Both methods require that all conditions in the measuring chain (amplification gains, sound card, cables etc.) remain fixed during calibration and in-situ measurements. This can be impractical mainly because measurements may be performed much later than calibration and at a distant location, and it cannot be guaranteed that the conditions in the measuring chain remain unchanged. In this paper a Two-step calibration procedure is proposed that removes the requirement of fixed conditions of the measuring chain. The procedure adds a second step to the normal reverberation room or free field calibration methods, in order to allow compensation for any shift in octave bands from the calibration to the in-situ measurement condition. The procedure is implemented in the ODEON Room Acoustics Software v.13.03 as a Two-step calibration to be applied in connection with the sine-sweep method for room acoustic measurements.