Stereoisomers often have different physiological characteristics, and stereocontrol , which includes diastereoselective reduction of carbonyls, is important in the synthesis of pharmaceuticals and natural products. Here, we have developed a model that offers features that have been difficult to achieve simultaneously using conventional methods: reflective of the experimental values of selectivity, applicable to both chained and cyclic ketones, and can quantitatively predict diastereoreductive selectivity. The method, named Moving Sphere Model (MSM), predicts diastereoreductive selectivity by quantifying steric hindrance by the overlapping volume of ketone substrate atoms and a virtual sphere moving in both reaction directions. The reaction mechanism was predicted by examining how the consistency between the calculated selectivity by MSM and the experimental values of selectivity changes when the values of the radius r of the virtual sphere and reaction direction θ are changed. Thus, a reaction mechanism in which only hydride contributes to steric hindrance was inferred for the reduction of ketones by NaBH4 in MeOH solvent. Meanwhile, for the reduction of ketones with NaBH4 in iPrOH solvent, a reaction mechanism in which the solvent introduced to the reducing agent also contributes to steric hindrance was inferred. The MSM enabled us to calculate the angle of nucleophilic attack to carbonyls based on experimental facts, to infer the reaction mechanism of the reduction of ketones with NaBH4 in alcohol solvents, and to predict the reduction selectivity of ketones with NaBH4 in MeOH solvents.