The Jackson Ratio

Weighing and measuring your tortoise, and keeping accurate records provides a great deal of information about your tortoise and its growth pattern.  However, while this will tell you whether the tortoise is putting on or losing weight, it won't tell you whether it is over or underweight, which is vital to know, especially if you are considering hibernation.

In 1976, Dr. Oliphant Jackson recorded weights and measurements of a large number of healthy and sick T. graeca and T. hermanni tortoises.  The data he gathered showed that there is an optimum body weight for a healthy tortoise, and this is a vital piece of information when assessing the tortoise’s general health.

A formula was devised to compare the weight-to-length ratio, which is known as ‘The Jackson Ratio’.  However, this ratio can only be applied to the Testudo Graeca (Mediterranean Spur-Thighed Tortoise) and Testudo Hermanni (Hermanns Tortoise) and CANNOT BE USED FOR ANY OTHER SPECIES due to the different body shapes.

Calculating the Jackson Ratio

Weigh the tortoise in grammes.

Measure the straight carapace length (SCL) in centimetres (for information on how to weigh and measure your tortoise correctly CLICK HERE).

Calculate the weight in grammes, divided by the (length in cms cubed) 


Or alternatively, please use our Jackson Ratio Calculator

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.

For ease of use, Dr Jackson devised a graph, which shows the upper and lower limits.  Once you have weighed and measured your tortoise, draw a vertical line on the graph corresponding to the length, and a horizontal one for the weight, and at the point where they cross, you can see whether your tortoise falls into the underweight, healthy or overweight category.

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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