Tail morphology in primates is important for interpreting functional adaptation and phylogeny. Tail length is probably the most remarkable trait. Establishing usable methods to predict the tail length of extinct primates is part of the basis for the reconstruction of primate evolution, particularly of hominoids. Previous studies revealed that sacral morphology often predicts tail length. However, most previous studies have only attempted this by using categories (short, long, etc.). A problem with those studies is that the range of short tail length is wide. Accordingly, this study aimed to quantitatively estimate tail length in catarrhines with intermediate tail lengths. Sacra and proximal caudal vertebrae (first to third) of 89 hybrid individuals between Japanese macaque (Macaca fuscata) and Formosan rock macaque (Macaca cyclopis) were measured. These hybrid macaques were phylogenetically controlled but varied greatly in their relative tail length (tail length/head and body length = 18.8-88.8%), and thus were an excellent sample to obtain general regression formulae to estimate catarrhine tail length. A total of 15 predicting models were devised and five formulae performed well. The utility of these formulae were confirmed by application to 15 species/subspecies of catarrhine taxa.