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In my world, I have relatively robust humanoids as the focal human sub-species, and a number of other human subspecies that are more gracile in build. I wanted to work out roughly how much they would realistically weigh given their builds and heights.

In my research, I've used prehistoric humans as baselines. Neanderthals were shorter, but more robust than equivalent Sapiens. Homo floresiensis added in as an additional data point. These are the figures:

  • Neanderthal average: 164cm tall (5'5"), 77kg
  • Upper paleolithic Sapiens avg.: 179cm tall (5'9"), 67kg
  • Late upper paleolithic Sapiens avg.: 166cm tall (5'4"), 62kg
  • Floresiensis est.: 109cm tall (3'7"), 25kg

This works out to Neanderthals having 0.49kg/cm, Sapiens having 0.37kg/cm for both, and H. Floresiensis 0.23kg/cm. However, square-cube law being what it is, scaling these up to make, say, a 6-foot Neanderthal gives a weight of 89kg which is not too far from the expected weight for a real-life gracile Sapiens according to BMI calculators. Not very realistic.

Perhaps it's just been too long since I took a maths class, but I can't work out what sort of formula to use to be able to plug height and a 'robustness ratio' into to be able to get a realistic weight for scaling humanoids up and down.

Criteria for a strong answer

Ideally, any calculation would be viable for calculating a realistic weight for anything from goblins up to giants. Another option for a successful answer would be a BMI chart that is scaled far enough to encompass goblins or giants.

A stellar answer would propose a method of calculation, and provide an example of it in practice that matches the figures for prehistoric humanoids above.

Edit: Played around a bit with the NHS BMI calculator.

Seems to be that Neanderthals had an average BMI of 28.6, Sapiens 20.9/22.4 respectively, and Floresiensis 21. Playing around with the calculator gave me a 6' Neanderthal as weighing 95kg with the same BMI, whereas a 6' late upper paleolithic human would weigh 75kg.

I believe this is relatively realistic for humanoids that do not differ too greatly in size to require physiological adaptations to large weight. However, it doesn't let you plug number in for 3m tall people so doesn't quite cut it. Could be useful for checking answers though.

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    $\begingroup$ Why not just use individual modern humans, humans come in a wide variety of heights and builds, If the hominid is supposed ot have a human sized brain this will give you better measurements than any extinct hominids will. Humans already produce variation as strong as 272 cm - 57cm $\endgroup$ – John Mar 21 at 13:47
  • $\begingroup$ @John I had considered that, but it's surprisingly difficult to find information on the web of good enough quality to build a model for it. Oddly, there seems to be more information about average weight for a given height (and an assessment of relative robustness) for prehistoric hominins than for living humans. Part of the issue is that modern figures are very often for people in a modern lifestyle, which doesn't map very well to a subsistence lifestyle. I've started looking at modern hunter-gatherers like the Hadza which might help add another data point. $\endgroup$ – Ynneadwraith Mar 21 at 13:59
  • $\begingroup$ The formula for BMI is really easy it is BMI=kg/m2, kg being the weight and m being height. as you pointed out the BMI for humans is around 21 and around 29 for robust neanderthal. Not that BMI actually means much $\endgroup$ – John Mar 21 at 14:00
  • $\begingroup$ @John That's part of the issue really. As there's so much variation in modern humans, a lot of the figures (like BMI) don't mean an awful lot. Thanks for the BMI formula though. Useful to know it's that simplistic! $\endgroup$ – Ynneadwraith Mar 21 at 14:05
  • $\begingroup$ Bmi does not mean much for health I mean, but works as a decent ball park calculation. just keep a range in mind a human children has a BMI of around 15 while a human weightlifter tends to have a BMI in 30 range. and an NBA player tends towards a BMI of around 25, so their is a pretty strong increase with size but it is not as strong as the change due to build. $\endgroup$ – John Mar 21 at 14:20
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The general rule here is that, if proportions are kept the same, weight is proportional to volume, which goes as the cube of a length dimension. Thus, if you double the height, you multiply volume/weight by two cubed = eight.

Therefore, if you have a man who weighs 100 kg, and another who has the same proportions (same waist to height ratio, same fraction of leg length to height, etc.), he'll weigh 800 kg.

Complicating this is the fact that, in real creatures, proportions generally change as height changes. This isn't always the case -- island dwarfism, for instance (sometimes blamed for the small size of the floresiensis "hobbit" fossils) typically changes size much faster than conventional selection can respond to the advantage of not growing unneeded bone and muscle. Otherwise, however, if you have a species that's much larger than an otherwise similar one, it will have different proportions -- thicker legs and body, thicker bones, etc. in order to support and move its greater weight.

In practice, it probably isn't far wrong to add an additional square on top of the cube of the height -- that is, instead of double height weighing eight times as much, it wouldn't be unreasonable for it to be enough thicker to weigh twelve times as much. On this basis, if you start with a real human (myself) who weighs 91 kg at 1.55 m, and "expand" him to 3m, you'd expect him to weigh:

(3/1.55)^3 * 91 kg + (3/1.55)^2 * 91 kg = about 1000 kg (simply 8x would be 728 kg)

Clearly at this scale the "thickening" is needed -- if you asked legs of merely human proportion to support that kind of weight, you'd have knees that last days, or at best weeks, bones in danger of snapping with the slightest stumble, and so forth.

This factor fits well with insects, which are the most common creatures we see in a very wide range of sizes. The biggest ones, certain beetles as big as a large mouse, are much heavier than simple cube-law from smaller beetles would suggest.

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  • $\begingroup$ Good answer explaining the principles of calculating weight. I've edited in some criteria for an answer to being accepted that includes a worked example of any proposed calculation/method that matches the observed figures. I'll see if I can put together a calculation based on that for you to include (unless you're happy to yourself). Once that's in there, it will be a very strong contender. Especially thanks to the insect-weight point :) $\endgroup$ – Ynneadwraith Mar 21 at 11:56
  • $\begingroup$ So, as a start: (new height / old height)² * old weight = new weight. Plugging in the Neanderthal numbers to that gives a 6' Neanderthal as weighing 103.4kg which is pretty much bang on the BMI figures. If we can factor in a 'robustness ratio' or something like that which can account for Neanderthal vs Sapiens then it's perfect :) $\endgroup$ – Ynneadwraith Mar 21 at 12:03
  • $\begingroup$ Managed to get my neanderthal figures wrong. We'd expect a 6' Neanderthal to weigh ~95kg. Formula still works out though... $\endgroup$ – Ynneadwraith Mar 21 at 12:54
  • $\begingroup$ Okay, added worked example with real-world (not "nice") figures. Also corrected how to arrive at the correction factor. $\endgroup$ – Zeiss Ikon Mar 21 at 14:26
  • $\begingroup$ I like it :) on its own, the cubed height difference * weight scales pretty well with BMI charts for both little people, and measured figures for proportionate giants like Angus MacAskill. Perhaps splitting it out into two formulae for 'proportionate' humanoids, and humanoids that will require adaptation for increased weight :) $\endgroup$ – Ynneadwraith Mar 21 at 14:58
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You can actually use BMI just be aware BMI changes with size, the formula is BMI = kg/m^2

BMI has to be increased with height because of the square cube law, which so simple formula can completely account for.

for a human of around 100cm with a gracile build a BMI of 15 (goblin), for normal human height 22 is fine for a human, over 2 meters you want something closer to 24 (slender giants). For very robust builds (football players weight lifters) you want something like 24 on the robust short end (dwarves), and 30 for normal human height, and for something over 2m you want to go to around 35 (beefy giants).

You will have a margin of error no matter what you do but a population will have a range of around +/-5 anyway, more if they have agriculture and thus fat individuals.

I used human athletes, children, pygmies, and a little estimation to get these numbers.

pygmy humans

athletes

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    $\begingroup$ BMI scales poorly with size. Even across the normal range of human sizes, it gives bad numbers once you get away from the middle. For example, the typical professional basketball player is considered "obese" under the BMI definition. $\endgroup$ – Mark Mar 21 at 22:42
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    $\begingroup$ @Mark Its not be used to assess health here, its being used to estimate mass, and you will notice the BMI I give are well outside the pseudoscience "healthy" range. $\endgroup$ – John Mar 22 at 12:24
  • $\begingroup$ Interesting idea, and I've upvoted as it's quite easy to play around with. Would benefit from adding the formula for BMI (I think you've put it in one of your comments on the main question). $\endgroup$ – Ynneadwraith Mar 22 at 12:29
  • $\begingroup$ @Ynneadwraith the formula is in the first line... $\endgroup$ – John Mar 22 at 12:58
  • $\begingroup$ @John That'll teach me to read things properly won't it ;) $\endgroup$ – Ynneadwraith Mar 22 at 12:59

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