The detailed probability of grain breakage influences the micromechanical behaviours of granular soil under repeated impact loads. This study investigated grain breakage in single-sized and multi-sized calcareous sands under repeated low-energy impact loads. By introducing the dye tracing and particle image segmentation methods in impact tests, various-sized fragments and residual grains were distinguished and further quantitatively analysed. The experimental results showed that the probability of grain survival decreased with the increase in the repeated cycles of impact (total input work); large grains played a more important role in the latter breakage stage as the repeated cycles of impact increased. Compared with the commonly used index Bg, the breakage index Bs based on the detailed probability of grain survival was much closer to the actual amount of grain breakage for single-sized samples and was slightly larger than Bg for multi-sized samples. The detailed probability of grain survival and the indexes could be used for investigating mechanisms of grain breakage under repeated impact loading.