In the absence of glycolytic support, mammalian sperm derive their energy for motility from a densely packed array of mitochondria at the base of the flagellum known as the midpiece. Using data on the morphometric dimensions of over 200 mammalian species, we found that an allometric relationship exists between midpiece length (Lm) and flagellum length (Lf). Specifically, the length of the mid-piece varies approximately as the 3/2 power of the flagellar length although the proportionality constant is different for eutherian and marsupial sperm. In contrast, when we corrected for the fraction of the midpiece that was taken up by mitochondria, a single linear correlation between mitochondrial volume and flagellar length for all mammals was found. These allometric relationships were used along with basic flagellar hydrodynamic theories to establish a unifying equation that predicted flagellar frequencies for any mammalian sperm between 40 microns and 200 microns in length. These findings imply that, at least in mammals, the mechanisms for energy production and dissipation in sperm flagella are highly conserved.