Jackson Ratio Calculator

How to use the Jackson Ratio

  • Measure the weight of your tortoise in grams
  • Measure the SCL (Straight Carapace Length) in mm
  • The Jackson Ratio is only suitable for tortoise with an average body morphology (Testudo hermanni & Testudo graeca). Using it for any other tortoise will give an incorrect result,

Note: our calculator will convert this to cm for you. It’s easier and more accurate for most people to measure in mm.


Use the following as a guide to see what your results mean and whether or not your tortoise is at a healthy weight for its size. It will also allow you to see if it is safe to hibernate your tortoise this year of it will require over wintering.

  • 0.16 or less: An underweight tortoise requiring special care.
  • 0.17: Too light for hibernation, unless an active male or a tortoise with unusually flared scutes.
  • 0.19: Normal weight to length ratio. Safe to hibernate.
  • 0.21: A very good weight, ideal condition. Safe to hibernate.
  • 0.23 or more: Possibly too heavy, check for water retention (puffiness) consider seeking veterinary advice.

Points to take into consideration

When using the Jackson Ratio, there are some points to take into consideration that may change the results that you get. They are as follows:-

  • A female that is carrying eggs will be much heavier
  • An overweight tortoise may have a lot of body fat or excess fluid, and is not necessarily healthy.
  • A tortoise that has just emptied its bladder of 30ml will be 30grams lighter
  • A tortoise that has just had a drink will be heavier

If there doesn’t appear to be any problems with your tortoise and its weight falls within the healthy range in the ratio then you can safely assume that it will be ok to hibernate.

If it falls into the dangerously low category, you should seek veterinary advice. It is also advisable that that the tortoise should be over wintered until it’s weight is restored to a safe / healthy level. Many tortoises which die in hibernation unfortunately do not have enough body reserves to see them through the winter.

If your tortoise is at the opposite end of the scale and is ‘obese’ you should also seek veterinary advice and a check up should be performed to ivestigate the possibility of abnormalities.

It is crucial to keep records of your tortoises weight and Jackson Ratio over time, as it allows you to see any recursive patterns and can also be a great help to vets when investigating any potential problems with your tortoise health.

It also allows you to see the weight of your tortoise when it hibernated last year and it’s ratio. As well as the ratio it came out of hibernation. Ideally the ratio should be consistent from year to year with growth and weight gain and be within the range of 0.19 – 0.21 on the Jackson Ratio.

Record keeping of weight and length on a regular basis will give you an early indication of any adverse weight loss or gain and alert you to seek advice from a vet.

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