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I'm trying to figure out the cross sectional area of the smallest part of a Red Great Wyrm's leg bones to mathematically determine how much force it has to be able to withstand in order for said Great Wyrm to jump into the air at full speed as a non-magical ability. The Draconomicon provides some wonderful dimensions and anatomy sketches of dragons with all kinds of great measurements... unfortunately, the measurements are all external (nose to tail, wingspan, etc), and so it doesn't directly provide the bone sizes I need.

(I've tagged this without naming an edition as I believe that their size has been pretty consistent throughout the editions, but if there are differences that I'm not aware of, use 3rd edition, as that's the Draconomicon I'm referring to)

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    \$\begingroup\$ While that seems like an interesting topic, do be careful about how you use the result. The saying often goes something like "DnD is not a physics simulator", because many things do not respect the laws of physics at all, and trying to explain them through real world science only brings inconsistencies. Even non-magical things, like a dragon's breath, often follow this rule of thumb. \$\endgroup\$
    – Matthieu
    Commented Jun 8, 2023 at 6:27
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    \$\begingroup\$ Fair point. Part of the purpose of doing these physics calculation is just recreational, but the other part is to be able to tell my players exactly how awesome their high level characters are, maybe using some modern-day weaponry comparisons to get the point across. "You slice right through a tendon, something that a fighter jet mounted minigun wouldn't be able to do" is the sort of narration that can get a player pumped and excited. \$\endgroup\$
    – Richard Winters
    Commented Jun 8, 2023 at 7:28
  • \$\begingroup\$ Use a known measurement to determine the scale of the image, you can then use that scale on any other part of an image. All you need is a ruler and a calculator. \$\endgroup\$
    – John
    Commented Jun 8, 2023 at 20:26
  • \$\begingroup\$ Notably, the square-cube law is the nemesis of massive creatures. When you apply physics it's often the case that the creature cannot stand up, much less fly. \$\endgroup\$
    – Comic Sans Seraphim
    Commented Jun 13, 2023 at 11:59
  • \$\begingroup\$ ---unless you assume material properties non-existant in the real world. Which is what I was going to do. Basically magic, but MEASURABLE magic. \$\endgroup\$
    – Richard Winters
    Commented Jun 14, 2023 at 1:26

1 Answer 1

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A great wyrm leg bone would measure maybe 1.3 x 9 feet

Thank you for your wonderful question. I'll try to answer this using the help of a dracolich.

The Draconomicon gives the dimensions for a red gargantuan dragon with a wingspan of 60–150 ft and an overall body length of 120 ft.1 Using the image of this dragon below, if we assume the body length to be 120 feet, the diameter of the lower fore-leg would be about 3.2 feet, and the length of it about 9 feet.

enter image description here

How much of that is bone? The length of the bone would likewise be about 9 feet, but what counts for jumping force is more likely the cross section. We'll use a dracolich, the skeleton of a former dragon, to look at the thickness of its leg-bones in the official depictions in D&D materials to estimate this. Here is the leg of an adult blue dracolich from 5e, compared to the leg of an ancient blue dragon, scaled to similar size:

enter image description here enter image description here

The bone seems to be a bit less than half the diameter of the leg here, maybe 4/10. Combining this with a leg diameter from the red Great Wyrm, we get a bone diameter of about 1.3 feet, and a cross section of about 1.3 square feet.

For comparison, a 6.5 feet long brontosaurus femur that was found in France appears to be about 0.8 feet thick. The largest of those dinosaurs were estimated to be about 85 feet long, giving you a ratio that is pretty similar. Note however, that D&D is not a physics simulation, and the attributes for its creatures do not need to work in the real world.

enter image description here

1 The nomenclature changes between editions. In 3.5e, a Great Wyrm the last age category of 12. 5e sports only four age categories in the core rules, and a Greatwyrm (spelled in one word) is introduced Fizban's Treasury of Dragons as an ancient dragon (p. 37) even older and larger (p. 168). In all cases these dragons are of Gargantuan size, which means they take up at least 20 by 20 feet, but Gargantuan can be a lot larger than that.

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    \$\begingroup\$ +1 I totally geeked out reading your answer. :) \$\endgroup\$
    – Wolpertinger
    Commented Jun 9, 2023 at 18:34

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