Mexican free-tailed bats are usually 9 cm (3.5 in) long. They weigh around 7–12 g (0.25–0.42 oz). Females tend to be a little bit heavier than males. Their ears are wide, rounded, and large compared to their heads. They use their large ears to help them find prey using echolocation. The wings are long and narrow with pointed tips. The color of their fur ranges from dark brown to gray.