Tortoise Growth Calculator
Check shell length against weight with the Jackson ratio for health.
Enter your values
ⓘ Live results
- Condition
- overweight or gravid — review diet
- Jackson ratio
- 1.36
- Expected weight at this length
- 382 g
- Difference
- +138 g
- Healthy weight range
- 344 - 420 g
- Growth rate
- 2.8 cm per year
Working
- 1expected = 0.191 x 14^2.88 = 382 g
- 2520 / 382 = 1.36
Quick answer
The Jackson ratio compares a tortoise's weight with its straight carapace length. A value far below the curve suggests dehydration or underfeeding, far above suggests overfeeding.
expected g = 0.191 x length cm^2.88 (Mediterranean species)
How to use the Tortoise Growth Calculator
- 1Measure straight carapace length with calipers or a ruler, not over the curve of the shell.
- 2Weigh on a digital kitchen scale in grams and enter the tortoise's age.
- 3Record both monthly — a falling ratio is an early warning long before a tortoise looks unwell.
Worked example
Using the values the calculator loads with:
Inputs
- Straight carapace length14 cm
- Weight520 g
- Age5 years
Results
- Conditionoverweight or gravid — review diet
- Jackson ratio1.36
- Expected weight at this length382 g
- Difference+138 g
- Healthy weight range344 - 420 g
- Growth rate2.8 cm per year
- expected = 0.191 x 14^2.88 = 382 g
- 520 / 382 = 1.36
Frequently asked questions
Which species does the Jackson ratio suit?
It was developed for Mediterranean tortoises such as Hermann's and spur-thighed. Use it as a rough guide only for other species.
What if my tortoise is above the range?
A gravid female or a recently drunk tortoise can read high. Persistent high readings with rapid shell growth usually mean overfeeding on protein-rich food.
About this calculator
- Built and reviewed by James Whitfield · How we build and test calculators
- Report an error
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.