A method and system for determining a whole-body averaged specific absorption ratio (SARwb) in a person comprises positioning an exposimeter on the person's body and providing a chamber forming an electromagnetic (EM) cavity and comprising a radiofrequency emitter and receiver; determining a first reverberation time of decay of EM power and a free-space incident power density in the chamber when the person is absent, and determining, a second reverberation time of decay of EM power in the chamber and a reference received power, received by the exposimeter, when the person is present in the chamber; and determining an absorption cross section of the person taking the first and second reverberation time and a volume of the chamber into account, determining a calibration factor relating a received power on the exposimeter to the SARwb, measuring a received power using the exposimeter, and determining the SARwb by applying the calibration factor.