Healthy Weight and Size

Susan Donoghue, DVM wrote an article about tortoise weights in which she offers a helpful formula, the Donoghue Ratio, that gives us a target weight for most tortoises. The data is based on 76 tortoises and box turtles representing 11 species, and works fairly well even for young tortoises, but does not seem to apply to hatchlings.

The Donoghue Ratio
The basic formula gives us the ‘ideal’ target weight-

  • SCLcm3 x 0.191 = tWTgr (Straight-line Carapace Length, in centimeters, cubed, times 0.191 equals the target weight in grams.
  • SCLin3 x 0.113 = tWToz (Less accurate variation for inches and ounces.)

Tortoise BMI (tBMI) Formula
This formula gives us a number that helps us better understand how under– or over-weight the tortoise may be.

  • cWT/tWT = tBMI (current weight divided by target weight from the Donoghue Ratio equals the tortoise BMI)

A tortoise with a SCL of 12.3 centimeters and a current weight of 349 grams.

  1. 12.33 (length in centimeters, cubed) x 0.191 = 355.4 (the target weight, rounded off).
  2. Current weight divided by target weight is 349/355.4 = 0.982
  3. tBMI is 0.982- a little underweight.

Google do all the work for you if you type in:

  • weight in
    /(SCL in centimeters^3 x 0.191), or
  • weight in ounces/(SCL in inches^3 x 0.113) for ‘standardmeasurements.
    • Replace the words in italics with your actual measurements.


You can determine if your tortoise is normal, dehydrated, or obese based on these results. Note- the figures used below are extrapolated from several sources and should be used as guidelines only!

  • 0.66 or lower:   Very dehydrated. Needs immediate cares.

  • 0.66 to 0.83:      Dehydrated and/or underfed. Needs appropriate care.

  • 0.83 to 0.99:      A little underweight. Review and correct cares and diet as
  • 1.00:                 ‘Ideal’ target weight.Note that few tortoises will hit this exactly, even with the best of care.

  • 1.01 to 1.16:      A little overweight. Review and correct cares and diet as appropriate.
  • 1.16 to 1.33:      Overweight. Adjustment cares and diet.

  • 1.33 or over:     Obese. Needs appropriate care.



I. Using the formulas in other ways.

Inches and ounces rather than metric. If you are using ‘standard’ measurements instead of metric,
– a. Learn and metric- it is more accurate and easier for this sort of thing.
– b. The formulas above should still work, just substitute ‘ounces’ for ‘grams’, ‘inches’ for ‘centimeters’, and 0.113 instead of the 0.191. It is not as accurate as the metric version is, but is close.

Using weight to predict length. You can switch the Donoghue formula a bit to determine the target length of a given weight of tortoise- SCLcm= cube root of (WTgm/0.119)… I think.

II. Getting a good SCL reading.

The easiest way to get an accurate SCL without tools like calipers is to…

1. Place a metric ruler on the floor or table, ‘0’ against the wall or some sort of block.

2. Set the tortoise on the ruler, head towards the wall, positioned so the shell touches the wall.

3. Place a block, box, etc. behind the tortoise, on the ruler, and move it so it touches the back of the tortoise’s shell firmly.

4. Move the tortoise and read the ruler at the block.

4-17-2012 (C) Mark Adkins