A Jackson ratio for Heosemys grandis

The Jackson ratio

In 1976 dr. Oliphant Jackson devised a ratio to determine the suitability of spur-thighed tortoises and Hermann's tortoises for hibernation. If a tortoise's Jackson-ratio is below a certain level, it is unsuitable for hibernation.
I apply Jackson's method to measurements of Giant Asian pond turtles. To avoid confusion, I will use the phrase 'Body Mass Length Ratio' rather than 'Jackson-ratio'.

Measuring a turtle

The Straight Carapace Length (SCL) of a turtle is measured in a straight line along the (central) keel of the carapace.

Calculating the Body Mass / Length ratio

The Jackson-ratio is calculated by dividing the weight (in g) by the straight carapace length (in cm) to the third power: g / (cm * cm * cm)
If you are using imperial units, you can calculate the ratio by dividing the weight in lbs by the length in inches to the third power, and multiplying the result by 27,68.
metric Jackson ratio calculator (g - cm) - imperial Jackson ratio calculator (lbs - inches)
imperial and metric units converter


My calculations are based on measurements provided by Peter Paul van Dijk, Sylvie Gaarová & Annette Justus, as well as data taken from publications by Jürgen Langmann (1987) and Michael Rudolphi & Ronald Weser (2000).

Results for captive males

The average BML-ratio for captive males is 0.148, with a low boundary of 0.126 and a high boundary of 0.170. See table below.
The low and high boundaries are calculated as average +/- two times standard deviation.

Results for captive females

The average BML-ratio for captive females is 0.171, with a low boundary of 0.118 and a high boundary of 0.224. Females are on average heavier than males. And the range between the high and low values is larger than it is for males. This is because the weight fluctuates with the reproductive cycle: they gain weight when they develop eggs and lose weight when they lay them.


With five measurements of two hatchlings I can only give an indication of the average weight of hatchlings (of up to 1 year old). See table below.

Wild females

Two wild females have been measured by P.P. van Dijk in Thailand. With a BML-ratio of 0.151 they were 12% lighter than an average captive female.
They could have laid eggs just before being caught and measured and that could explain the 12% difference. In any case, no definitive conclusions can be drawn from only two measurements.

Wild males

I have no data on wild males.


15 cm398 g578 g759 g
20 cm944 g1371 g1798 g
25 cm1843 g2678 g3512 g
30 cm3184 g4627 g6069 g
35 cm5057 g7347 g9638 g
40 cm7548 g10968 g14387 g
45 cm10747 g15616 g20484 g
50 cm14743 g21421 g28099 g
15 cm426 g500 g574 g
20 cm1009 g1185 g1360 g
25 cm1971 g2314 g2657 g
30 cm3407 g3999 g4590 g
35 cm5409 g6349 g7289 g
40 cm8075 g9478 g10881 g
45 cm11497 g13495 g15493 g
50 cm15771 g18512 g21252 g
5 cm19 g?
6 cm34 g?
7 cm53 g?
8 cm80 g?
9 cm114 g?
10 cm156 g?
These numbers are purely statistical. They do not constitute a standard for optimal weight.


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