Total length, snout-vent length, and body mass are tightly correlated with length and other dimensions of the femur in Alligator mississippiensis. American alligator-based equations relating total length and body mass to femoral dimensions predict the sizes of other extant crocodylian species reasonably well, suggesting that alligator-based relationships should also work for extinct crocodylians and their close relatives. Size estimates from different femoral dimensions are most consistent when extinct forms have femora similar in size and shape to those of alligators. For many extinct mesoeucrocodylians, alligator femur-based total length estimates are close to length estimates based on reasonably complete skeletons or estimates made from osteological parameters other than femoral dimensions. However, the total length estimated for Pristichampsus is much larger than the length of the skeleton, indicating a relatively longer femur in this form, perhaps reflecting a more terrestrial lifestyle. In contrast, femur-based size estimates of a large individual of Deinosuchus are considerably less than an estimate based on mandible length. Femur size may be reduced relative to body size in Deinosuchus, which may be associated with more strictly aquatic habits than in alligators.