Researched and devised by Dr. Oliphant Jackson, the Jackson ratio is a method of determining whether a member of the tortoise species Testudo graeca or Testudo hermanni is maintaining its optimum bodyweight, which is necessary for a successful hibernation. This is essentially a value of the density of the tortoise in g/cm<sup>3</sup>. The optimal value for this ratio is 0.21 with a range of 0.18–0.22 being acceptable for hibernation. Values significantly less than this indicate an underweight tortoise and larger values are indicative of overweight tortoises.
The Jackson ratio is expressed as a graph of minimum and optimum weights for a given carapace length. This exhibits the same principle without the maths.