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The correct way to measure tortoises for Jackon Ratio assessment (safe hibernation weight)

The Jackson Ratio is a useful tool for assessing the fitness of tortoises (Testudo graeca and Testudo hermanni only) to hibernate. Unfortunately it is often misunderstood and misused. As we have pointed out previously, it cannot be used with species other than that which it was developed for: see our separate article on this issue.

In addition, following the depiction of a recent television program of a veterinary surgeon taking the Jackson Ratio measurement incorrectly, there is also some confusion about how to do it.


There is only one way to take meaningful measurements for the purpose of computing the Jackson Ratio, and that is in a straight line. Any measurements taken 'over the curve' of the carapace will invariably produce a false result which will indicate that the tortoise's weight/length ratio is lower than it actually should be.

This is the WRONG way to measure the length:

This is the RIGHT way to do it:

It is essential that measurements (and weights) are taken accurately, otherwise the result of the exercise will be meaningless, and may well endanger the tortoise.

More useful hibernation information on this site:

Safer Hibernation and Your Tortoise

Refrigerator Hibernation

Hibernation Hints

Post-Hibernation Problems

Incorrect Use of Jackson Ratio

Hibernating Juvenile Tortoises