RFID has been developed for many applications such as object identification, object traceability, geometric localization, etc. In order to locate an object labeled by a tag, several characteristics of the tag responses have been studied. This yielded theoretical models whose accuracy is not sufficient in practice for targeted applications. In this paper, we investigate how to localize a tag based on the set of received signal strength indicator (RSSI) values measured for a large range of transmission powers. Using an extensive measurement campaign, we establish that the RSSI is a linear function of the transmission power of the RFID reader. Moreover, a minimum transmission power is necessary for activating the tag which also depends on its localization. As a result, the statistics of the RSSI for different transmission powers may be synthetized through a triple. We then show how this triple may be exploited for localizing a tag.