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For over a thousand years, the dreaded dragon Lohebrand has roamed the plains of the east. His breath melts mountains, his shadow blots out the sun, the earth trembles beneath his talons – or so the singers day. He has claimed the land as his own, driven almost all humans from it and feast on the herds of wild cattle that roam the plains. Even the sight of him flying on the horizon, leagues away, makes warriors quake with fear. But one question remains unanswered:

Just how big is he, exactly?

Fluff aside: I am working on a Medieval Fantasy Setting, and I have no idea how to scale this dragon. Lohebrand is supposed to be ancient and wet-your-pants-don’t-even-think-about-fighting-scary.

And, of course, as gigantic as realistically possible. (At least, realistic as far as dragons go).

Some facts:

  • Territory: Lohebrand claims about 25,000 $\text {miles}^2$ as his own, mostly plains that turn mountainous towards the North and West. The land is fertile, but mostly devoid of humans.
  • Diet: Horses, wild cattle, goats, sheep and humans foolish enough to enter his territory and be discovered.

  • Age: About 2000 years old.

My idea was that a dragon will continue to grow (not necessary consistently) as long as his environment supports this.

Assuming Lohebrand does not care for husbandry, but is smart enough to change up his hunting patters to try and avoid over-hunting—just how big could he possibly have grown? And how would this relate to proportions (i.e. wingspan, size of his head)?

The bigger, the better, of course…

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    $\begingroup$ Related: Can you simply scale up animals?, What basic principles should I keep in mind when scaling things up and down? and more or less the entire [scaling] tag. $\endgroup$ – a CVn Apr 25 '16 at 12:09
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    $\begingroup$ Aren't dragons magical creatures? Can't they just ignore natural laws because "magic"? If you wanted to make dragon realistic, you wouldn't really make a dragon(since they aren't realistic...), and as a result - dragon would be ancient and look-we-actually-might-kill-him-scary. $\endgroup$ – MatthewRock Apr 25 '16 at 14:42
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    $\begingroup$ Well, in the end, it always comes down to 'suspension of disbelief'. Your audience knows you are telling a story and will accept things like a flying, fire-breathing lizard at face value. But if you get to much 'wrong' and visible details (like in this case, size) don't add up, suspension of disbelief collapses and your audience just goes "The size of Pluto? Reaaaally?". You can skip some realism - but not all of it. (That's my personal opinion, at least.) $\endgroup$ – HideAndSeek Apr 25 '16 at 15:00
  • $\begingroup$ Wouldn't it be better for ur dragon if he actually had slaves cultivating the land for his purposes, rather than letting the land flourish idly? $\endgroup$ – gaazkam Apr 25 '16 at 15:27
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    $\begingroup$ I had similar ponderings from the other direction - given a dragon of a given size, how much does s/he eat? In the answers are good information about how to properly size the dragon. Check it out: worldbuilding.stackexchange.com/q/10098/294 $\endgroup$ – corsiKa Apr 25 '16 at 15:39
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I assume that your dragon is not limited by biological issues like the square-cube law of muscles (scaling up a muscle means scaling up its strength in a quadratic fashion; however the weight scales up in a cubic fashion. Meaning: the bigger the dragon gets, the more muscle it has to invest in simple things like walking let alone flying). Also, temperature issues follow the same square-cube law: a body's heat production follows the cubic law, the cooling surface follows the square law (= the bigger the dragon gets, the more likely it is to have overheating issues). Not to mention that at a certain point the body might just break down from its own weight (bone strength, blood pressure, etc.)

So basically all you limit your dragon by is its food source and the energy it can gain from it.

For one, you definitely need to minimize the energy requirements of your dragon. Your dragon should be coldblooded so that it doesn't need as much energy for its base metabolic rate. Also, the more you can outsource to magic instead of conventional energy (e.g. generating the energy for breathing fire) the better for your dragon size. That is, assuming that magic doesn't require conventional energy to regenerate.

The ecological pyramid says that approximately 10kg prey are needed to sustain 1kg predator. When optimizing your dragon, you might be able to go so far that 1kg dragon mass requires only 5kg prey. So, a very simplified calculation might give you the following: Assuming your 25000 sqmi can sustain 10,000 horses/cattle/other big herbivores with an average mass of 300kg per animal, you've got a total prey mass of 3000t - or a predator mass of 600t.

So your dragon weighs 600t. How big should it be? Well, an airbus A380 is sized for a maximum takeoff weight of 580t with a wingspan of 78m and a total length of 73m. I think that is the minimum size requirements to get your 600t dragon flyable. However, the A380 weighs only about 280t when empty -- which might be a better comparison to your dragon. On the other hand, jet wings are a lot smaller than wings needed for gliding, so at least triple the wing span. So, all in all, I'd say a wing span of 200m with a head-to-rump length (no tail) of 80m is a good estimate for your dragon size.

EDIT: Of course, this link here is a more realistic view on dragon sizes when creating a physically possible dragon. Meaning: any dragon above 70-200kg is just about impossible thanks to biology and physics...

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    $\begingroup$ Related to the point on the ecological pyramid, we have What efficiencies make a realistic food chain? and to a lesser extent How much land area do my land-based animals (herbivores) need for food? which can help determining the ecological footprint of the dragon. $\endgroup$ – a CVn Apr 25 '16 at 12:21
  • $\begingroup$ I am aware that a dragon this size is not biologically possible, thanks to the Square/Cube Law - sadly. As for overheating.. maybe "overheating + magic = fire-breathing"? But the size sounds really about right - truly frightening but not completely breaking suspension of disbelief. Thank you! $\endgroup$ – HideAndSeek Apr 25 '16 at 13:10
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    $\begingroup$ Dragons have a very good system for venting excess heat from their bodies, no overheating issues there. $\endgroup$ – Separatrix Apr 25 '16 at 13:24
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    $\begingroup$ @Separatrix Assuming you are talking about fire, i cant imagine any situation where breathing fire would stop a biological organism overheating. $\endgroup$ – Trotski94 Apr 25 '16 at 13:48
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    $\begingroup$ @JamesTrotter What about an endothermic chemical process that goes on inside the dragon that produces a flammable substance which can be excreted out of the dragon's mouth? $\endgroup$ – Shufflepants Apr 25 '16 at 14:44
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As you've said this beast is capable of flight, it's important to note that the bigger it is the more force needed to fly as noted by the Square/Cube Law. You've already mentioned that this dragon is capable of keeping enough prey alive to sustain itself but even then, following the usual lore of Dragons, they would eat until they filled their stomachs and sleep for long amounts of time (even centuries).

Assuming the dragons internal structure was more focused on big-bones, muscle and wings, than I would suggest it's largest size equal that of a Spinosaurus. Based on this Whatif page your dragon (if it were around the size of a Spinosaurus) would need to consume around 1 human every day, therefore in a Medieval setting it's possible for the dragon to each as much as 300 - 500 humans/animals and sleep for the year without disrupting the balance of the local area.

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    $\begingroup$ It could perhaps eat more without disturbing the balance, if it mostly concentrates on eating (male) heroes. They'd be a good source of iron too. $\endgroup$ – T.E.D. Apr 25 '16 at 14:05
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This is not my usual advice, but... absolutely ignore all the physics and biology advice. Pratchett had the right idea. You simply cannot justify a dragon using any kind of common sense. They merrily violate any kind of sense.

Dragons are thaumic creatures. Sure, they'll roast you and eat you but that's as a snack. Your princess is candy to a dragon. The wild animals it hunts for fun are just bacon sprinkles.

Dragons cross the planes into the Dungeon Dimensions and what they catch and eat there has so much magical power that they then can lie dormant for hundreds of years just digesting and lying on their hoard.

Our dimension is comfy. Warm. And has candy.

A long-lived dragon can be any size you feel it to be narratively relevant. Those old legends of his size may be out of date. The adventurers go there expecting something the size of perhaps a sailing ship in the cave under a mountain, then as they crest the top of the last hill and look up at it, they realize the mountain on the other side is, very slowly, breathing.

Physics and realism is great, but don't ever let it get in the way of being cooler than people expect. The Rule of Cool is there to back you up if you waver.

Unless you are actually out to make the anticlimactic downplay part of the tale, in which case, the dragon should probably be a little old guy with an old leather hang glider and a bag of levitation and flame runes, who retired form his dragonning days years ago.

[Edit: that said, don't make a dragon fat. Think lithe, lissom, and massive. At worst, a tiger. Not an elephant. That's the trap that physics-respecters fall into. A dragon mostly covering a mountain that he is sprawled over, is scarier than a dragon who is a mountain.]

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    $\begingroup$ We do have quite a few users who seem to disagree with your statement that a dragon simply cannot be justified using common sense. In fact, we have a whole question on how to explain dragons without invoking magic. $\endgroup$ – a CVn Aug 10 '16 at 5:03
  • $\begingroup$ @aCVn True: you can justify dragon physiology logically. The result is as sad as it is absurd. So I stand by my assertion: you can't scientifically justify a dragon if you want it to have strength in your tale. It becomes a greyhound or elephant with wings, a non-scary batlike animal with flame-breath. For it to be a real dragon, worthy of the name, it needs a certain element of the awe-inspiring and mythic, which cannot be granted merely by adhering to the laws of physics. No mere flying, flaming armored whale-bat can remain scary or impressive once you've harpooned a few. $\endgroup$ – Dewi Morgan Dec 4 '18 at 21:10
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If you're saying that their biggest prey is a horse then a dragon should be 3 to 12 feet long (without tail) because the general size ratio between predators and prey are between .5 to 2x that of the predator.

In the abstract of this paper are those findings https://www.jstor.org/stable/20143171?seq=1#page_scan_tab_contents

The tail of a dragon would add between 50 and 100% more to his body length... So 18 to 24 feet on the large size. Wing spans are usually about about 30 to 50% more of the body length so I'd go with at least 18 foot wingspan up to 36 feet.

This is ignoring that dragons would need a ton of coalories to produce fire, flight, and be intelligent. And that fire is a necessity to intelligent dragons btw because cooked food is the only way humans can eat enough to have energy to spare on thinking. We're talking a diet of something like 100,000+ calories a day. A big horse may feed a dragon for 7 to 9 days. So whereever it hunts would need to produce at least 50 full grown large well fed horses...

I'd bet on needing more though as these are really crudely gotten numbers

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