Jackson Ratio Calculator

Weight in grams (g):

SCL in centimetres: 


This ratio can only be applied to the Testudo Graeca (Mediterranean Spur-Thighed Tortoise) and Testudo Hermanni (Hermanns tortoise)


the resulting figure is the tortoise’s Jackson Ratio.

  • Jackson Ratio of less than 0.16 indicates an underweight tortoise needing special care.
  • Jackson Ratio of 0.17 indicates an animal which is light for hibernation.  However, a healthy, but active male or a tortoise with very flared scutes could also give this result.
  • Jackson Ratio of 0.19 is normal and safe to hibernate.
  • Jackson Ratio of 0.21 is a good weight, and ideal condition for hibernation.
  • Jackson Ratio of 0.23 is possibly too heavy; you should check for any puffiness, or if a female, she could be carrying eggs.

By kind permission of the British Chelonia Group


If the tortoise has a very empty or very full bladder, you may not get an accurate weight reading.  If you get an unexpected result, it is worth repeating the exercise a couple of days later.  If you have a female tortoise which is not eating, but still seems heavy, she may be carrying eggs.  A tortoise which appears overweight should be checked by a vet for any underlying problems.

Jackson Ratio and Hibernation

The Jackson Ratio is an excellent way of assessing whether your T. graeca or T. hermanni tortoise is heavy enough to hibernate.  However, even if your tortoise falls well within the ‘safe to hibernate’ range on the graph, you still need to do all of the other health checks (link) before deciding whether to go ahead.

If the tortoise falls into the underweight category DO NOT ATTEMPT TO HIBERNATE it.  Seek your vet’s advice and overwinter it.  If the tortoise has insufficient body fat reserves, it may not survive hibernation.

If you check the tortoise in August, and find that it falls into the borderline category, you will need to be prepared to overwinter it.  A tortoise which is too light at the end of the summer will certainly not put on sufficient weight before hibernation time in October/November.


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