The IEEE 802.15.4 standard specifies that a radio's PHY layer must provide an 8-bit integer value as an estimate of the received signal power. This value is commonly known as the Received Signal Strength Indication (RSSI) in the wireless sensor networks (WSN) community. While many protocols directly use the measurements that the radios provide, the standard only requires that the reported RSSI values should be linear and within ±6 dB of the actual RSSI values. However, ±6 dB is a wide error margin. For example, Packet Reception Ratio (PRR) can decrease from 100% to 0% with a 2 or 3 dB difference in the received signal strength. We examined two 802.15.4 compliant radios, the widely used TI CC2420 and Atmel AT86RF230, and discovered that they indeed introduce systematic errors when providing the RSSI readings. Therefore, we also developed a calibration scheme to effectively minimize the adverse effects associated with inaccurate RSSI values.