Information concerning the size and shape of tunnel boring machine (TBM) debris is essential for designing efficient loading, hauling and dumping systems for TBM excavation. It is also very important in deciding how to recycle the TBM debris. However, only very limited information is available at present, and theoretical or calculation methods to estimate the size and shape are still open to discussion. In this paper, the size and shape of TBM debris are estimated by the well-known Nishimatsu's equation that is usually applied to roadheaders or shield-type machines with chisel bits. In this study, the equation was applied to the case of TBM excavation with disc cutters in which the shear failure or plane extends from a new groove to an adjacent pre-existing one. The estimated size and shape of TBM debris were found to be consistent with the measured results. Side forces applied to a disc cutter and the resultant stress on the cutter were considered. The maximum debris size encountered in tunnel excavation was also discussed assuming that it followed the Gumbel distribution. The results suggest that the proposed approach based on Nishimatsu's equation shows potential for future study.