Although animal vocalizations and human speech are known to communicate physical formidability, no previous study has examined whether human listeners can assess the strength or body size of vocalizers relative to their own, either from speech or from nonverbal vocalizations. Here, although men tended to underestimate women's formidability, and women to overestimate men's, listeners judged relative strength and height from aggressive roars and aggressive speech accurately. For example, when judging roars, male listeners accurately identified vocalizers who were substantially stronger than themselves in 88% of trials, and never as weaker. For male vocalizers only, roars functioned to exaggerate the expression of threat compared to aggressive speech, as men were rated as relatively stronger when producing roars. These results indicate that, like other mammals, the acoustic structure of human aggressive vocal signals (and in particular roars) may have been selected to communicate functional information relevant to listeners' survival.