A model-based rate control mechanism involves the percentage of less probable symbols (LPS). The mechanism is first described in the context of bi-level video and subsequently extended for multi-level video by applying it to individual bit planes thereof. A model parameter represents a relationship between an LPS reduction ratio and a bit-count reduction ratio. The model parameter thusly facilitates transforming a targeted bit-count into an LPS reduction ratio. The LPS reduction ratio can be attained by adjusting a threshold band half-width wherein LPS pixels within the threshold band are converted to more probable symbols (MPSs). In a described implementation, a threshold band half-width is set so as to achieve a desired LPS reduction ratio. The desired LPS reduction ratio is determined based on a desired bit-count reduction ratio and the model parameter. The desired bit-count reduction ratio is determined using a bit plane complexity value and a targeted bit-count.