Bidirectional transmission (BT)-based protocols have been proposed to achieve uplink/downlink fairness in IEEE 802.11 systems. To successfully guarantee the fairness in unsaturated carrier sensing multiple access (CSMA) systems, it is important to accurately estimate the number of backlogged nodes considering both of the uplink (UL) and downlink (DL) traffic. In this letter, we propose a backlog estimation mechanism for the bidirectional media access control to guarantee UL/DL fairness in an unsaturated CSMA environment. We derive the optimal transmit probability and the optimal DL transmit opportunity for BT as a function of the estimated backlog size. The proposed algorithm ensures the near-perfect UL/DL fairness while improving the overall system performance.