In this paper, we present efficient bit allocation methods for stereo image coding purpose. Since the common idea behind most of the existing stereo compression schemes consists of encoding a reference and residual images as well as a disparity map, we mainly focus on the bit allocation issue between the reference and residual images. Generally, this problem is solved in an empirical manner by looking for the optimal rates leading to the minimum distortion value. Thanks to recent approximations of the entropy and distortion functions, we propose accurate and fast bit allocation schemes appropriate for the open-loop- and closed-loop-based stereo coding structures. Experimental results show the benefits which can be drawn from the proposed bit allocation methods.