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
 SCLcm^{3} x 0.191 = tWTgr (Straightline 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 overweight the tortoise may be.
 cWT/tWT = tBMI (current weight divided by target weight from the Donoghue Ratio equals the tortoise BMI)
Examples
A tortoise with a SCL of 12.3 centimeters and a current weight of 349 grams.

12.3^{3 }(length in centimeters, cubed) x 0.191 = 355.4 (the target weight, rounded off).
 Current weight divided by target weight is 349/355.4 = 0.982
 tBMI is 0.982 a little underweight.
Google do all the work for you if you type in:
 weight in
grams/(SCL in centimeters^3 x 0.191), or
 weight in ounces/(SCL in inches^3 x 0.113) for 'standard' measurements.
 Replace the words in italics with your actual measurements.
Results
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
appropriate.
 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.
Resources
 Barthel, Tom. “The Hydration Equation.” Reptiles, July 2007.
 Donoghue, Susan,1997. "Nutritional status of tortoises using morphometrics to assess body condition". Vivarium Magazine, Volume 8 Number 2
 Mader, Douglas R., DVM, ed. Reptile Medicine and Surgery. Saunders Elsevier, 2nd Edition 2006. ISBN 072169327X
 Pingleton, Mike. The Redfoot Manual: A Beginner's Guide To The Redfoot Tortoise
. Art Gecko Press, 2009. ISBN 1441494030.
Appendix
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.
4172012 (C) Mark Adkins
