An encoded video bitstream is composed of two main components: the coefficient bits representing the discrete cosine transform coefficients, and the header bits representing the header information (e.g., motion vectors, prediction modes, etc.). Compared with previous video standards, the H.264 Advanced Video Coding (AVC) standard has some unique features: (1) the header bits take up a considerable portion of the encoded bitstream; (2) the header bits vary significantly across macroblocks (MBs); and (3) a large number of MBs are quantized to zero and produce zero coefficient bits (zero-coefficient MB). These unique features make most existing rate estimators inaccurate for decision-making processes related to rate-distortion calculation for rate control. This paper analyzes the characteristics of the H.264/AVC bitstream, and reveals that both the header bits and the occurrence of the zero-coefficient MBs are strongly related with motion-compensated residues obtained by INTER16×16. Therefore, two statistical models are proposed for estimating the header bits and separating the zero-coefficient MBs. Based on the proposed models, a rate control scheme is developed for buffer-constrained constant-bit-rate video coding. Experimental results show that the resultant scheme achieves an average of 0.53 dB peak signal-to-noise ratio (PSNR) gain over the original JM6.1e, and less than 2% bit-rate inaccuracy.