Estimating the block size is a major task for the quarry economy. Two approaches such as volumetric joint count and weighted joint density exist in the literature to assess the block size. However, due to the complex nature of discontinuities in the rock masses, this parameter could not be predicted easily everytime. Especially, when working in the rock masses having a wide discontinuity spacing, it is too difficult to perform a scanline survey. In this study, to overcome this difficulty, the photoanalysis method was considered to obtain the data required to construct a predictive model for weighted joint density. Considering the obtained data, a Mamdani fuzzy inference system was construct and its performance was assessed. As a result, a model proposed for predicting the weighted joint density in the present study.