Okay, this is unrelated to my current world building in any way whatsoever, but...

I have always been a huge fan of the humongous mecha genre (bonus points to anyone who actually recognized my avatar pic), and have done some world building in such a setting before. Unfortunately, it never really gets anywhere.

However, I would like to ask this: Besides bending the laws of physics or the fundamental rules of our current world (i.e. no 'lighter gravity' or 'magic'), what would be the best way for a humongous mecha to bypass the square cube law, while still being able to move respectably? Feel free to make as many assumptions as needed about the setting's scientific level - that is not the issue here.

Some rules:

  1. The mecha must be fundamentally humanoid. It does not need to walk with its feet, but it must have legs.
  2. The mecha must actually be huge. I mean 18 meter minimum. I know there are some mecha shows out there (Code Geass, Kotetsu Jeeg, etc.) that are less than that, but I've always held by 'anything smaller than RX-78 (of Gundam fame) is just too small.
  • $\begingroup$ Can this mecha sit in a giant mecha-sized car and used its mecha-sized feet to push mecha-sized car pedals? $\endgroup$
    – DKNguyen
    Commented Jun 21, 2020 at 7:12
  • $\begingroup$ But after UC 100, minimization of MS becomes a trend--hence the small MS like F90, F91, and heavygun. And in Gundam Wing, those MS are only 15 m tall. So there are still gundam less than 18 meters though $\endgroup$
    – Faito Dayo
    Commented Mar 27, 2022 at 2:14

9 Answers 9


The biggest problem with giant anything, as everyone else has mentioned, is pressure. You can't scale up a human, for instance, because human bones couldn't support the force in play. That means the mech has to be built with strong materials, and the mech has to be lighter than a scaled creature.

Bones and armor

Titanium is a good choice, though it could get expensive for giant mechs; granted, if you're building an enormous machine, cost probably isn't a huge factor. Titanium has a tensile strength of about 434 MPa (63,000 psi) - about the same as steel, but for much lower weight. Titanium alloys have a much higher tensile strength, over 1400 MPa (200000 psi). Regardless, it would make a good armor as well as good support. As a bonus, titanium is very corrosion-resistant, which means you won't have to worry about your robot rusting.

Carbon nanotubes are also a good bet. We don't have the technology to make anything large-scale with carbon nanotubes, but we'll get there eventually. There are many types of carbon nanotubes, depending on how the atoms are arranged; one had the tensile strength tested at 63 GPa (9,100,000 psi). Other varieties are theorized to have a tensile strength of over 160 GPa (23,206,000 psi)! However, as strong as carbon nanotubes can be, they are strongest when pulled, not pushed, and are weakest against forces from the side. They would make great "bones", but probably not outer armor.

Graphene is super strong - a tensile strength of 130 GPa (18,855,000 psi) - but is very brittle. It would make an excellent bone.

Oddly enough, bamboo may actually work quite well. It has a tensile strength of 350-500 MPa (50,760-72,520 psi), and is quite light, with a lower density than even Graphene. Bamboo is also very easy to come by; it grows quickly in natural environments, with no need for mining or creating in a lab.

Other materials may work as well, to varying degrees; I'm sure the future holds much stronger materials, too.

Why not lighter?

The reason titanium is a better choice than steel is weight; but what if we make the mech even lighter? A giant robot that is essentially a thin shell may not stand up well in hand-to-hand combat, but it would satisfy the requirements!

A mech built using thin plastic or aluminium tubing and steel wire as bones and muscles could but placed inside a canvas balloon and inflated; the canvas would be nearly immune to 'crushing' attacks, and the bones wouldn't have to support much weight. If the mech were filled with a mixture of air and a lighter-than-air gas such as helium, it would be able to support itself with very little effort. Because of its low weight, the internal structure would be very thin; a giant mech made of inflated canvas could be built today for less than the price of a used mid-sized car.

Put it together

I suggest the best of both worlds: an air-filled canvas robot, plated with titanium, and with carbon nanotube bones. The internal structure would be filled with a lighter-than-air gas, resulting in a machine that towers above the earth, but can step on a house without crushing it. If it wants to.

As an added bonus, there is a lot of extra room in the mech that can be used to store weapons or snack food, and the bouncy exterior would protect any occupants.

  • 3
    $\begingroup$ So you are suggesting a Humungous Mecha Baymax (2014 ver)? $\endgroup$
    – March Ho
    Commented Jan 29, 2015 at 23:39
  • $\begingroup$ I haven't seen the movie, however, the armored version looks pretty close to what I was thinking, yes :-) $\endgroup$
    – ArmanX
    Commented Jan 29, 2015 at 23:43
  • $\begingroup$ I approve the idea of giant overlord Baymax. Also, thank you for the material ideas! $\endgroup$ Commented Jan 30, 2015 at 1:18

The square cube does not forbid bigger structures; it just means that the structures must be designed accordingly (less dense, with stronger materials, or both).

The square cube means that you cannot just pick up a man or an ant and multiply its size by 10, because it would then weight 1000 times the original while being only 100 times stronger. But if you change the calcium bones with titanium ones, and likewise enhance the muscles, then you can get your giant.

Given that, as long as you justify it, there is no "hard limit".

Possible ways of actions:

  • Strong materials. If they are hard enough, then you may even not need solid beams but may use a grid (think Eiffel tower), helping you save weight.

  • Motors/actuators. These are complicated, specially for arms (since increase of the weight of arms may make your mecha unstable). Also, you would need to modes (maybe two types of motors?), one for rapid movement and other for precision maneuvers (and control software that automatically decides which to use based in the human input).

  • Center of gravity. You have to take into account that the operator is used to stabilize himself with a body quite different from the mecha. Apart from the control software automatically balancing the mecha so it remains stable, you probably want to adjust weight in the top of the mecha so it is not top heavy, and arms must remain light.

  • Also, and based in what I said in the previous points, you do not want payloads (like weapons) in the arms/hands, but in the chest (or even better, in the legs, but that gives them a shorter field of fire). A third leg would be ideal; if it goes against the "humanoid" rule then make the legs as separated and with feet as big as possible.

  • 11
    $\begingroup$ The whole "weapon as third leg" concept would make your mechs look a little bit ...ahem... masculine. $\endgroup$
    – glenatron
    Commented Jan 29, 2015 at 9:43
  • 1
    $\begingroup$ It would certainly give a new meaning to "shooting your (pay)load"! $\endgroup$
    – Pyritie
    Commented Jan 29, 2015 at 17:45

As has already been pointed out, the square-cube law prevents making a human 10x taller. But we're not building a human.

Giant robots need stronger structures, but those are plausible. As I see it, the main limitation would be ground pressure.

If you scale up a human 10x taller, the weight increases 1000x, but the area of the feet increases only 100x. At what point will the giant's feet sink into the ground?

I was thinking about kaiju a while back and looked up this:

https://en.wikipedia.org/wiki/Ground_pressure (unsourced values)

Human male (1.8 meter tall, medium build): 55 kPa (8 psi) M1 Abrams tank: 103 kPa (15 psi) 1993 Toyota 4Runner / Hilux Surf: 170 kPa (25 psi) Adult horse (550 kg, 1250 lb): 170 kPa (25 psi) Passenger car: 205 kPa (30 psi) Wheeled ATV: 240 kPa (35 psi) Adult elephant: 240 kPa (35 psi) Mountain bicycle: 245 kPa (40 psi) Racing bicycle: 620 kPa (90 psi) ... Note: Pressures for Man and Horse are for standing still. A walking human will exert more than double his standing pressure. A galloping horse will exert up to 3.5 MPa (500 psi).

These figures suggest up to 5x human ground pressure is still feasible for walking on non-reinforced surfaces. Beyond that, I don't know.

The big open question is the density of a mech. A human's density is similar to water's 1000kg/m3. I was trying to estimate the density of a tank. http://www.fprado.com/armorsite/abrams.htm I don't have a figure for the volume, but I'm guessing 50m3 or more, with a weight around 60t. So even an armored vehicle isn't much denser than water, surprisingly.

Amusingly, the common anime design style of mecha with large flared calves might be entirely logical - allowing larger feet.

What appears to be an extreme approach to minimizing ground pressure is seen in Basquash. The mecha here are on the small side, but their feet are almost as large as their torsos. I still doubt they could run on ordinary rooftops as they're shown to, though.


you need to contrast gravity anyway. two ideas come to mind:

a "belt" made of thrusters, used to both lower weight and keep balance,

or some sort of huge antigravity core roaring inside the titan's chest, a technological heart that keeps it standing (how poetic)

  • 1
    $\begingroup$ Yeah, that is kinda poetic. I can already picture it quite dramatically. :> $\endgroup$ Commented Jan 29, 2015 at 9:23
  • $\begingroup$ This make me remember that video where you can see people hodling heavy weigth with the little finger. The weigth being rotated on himself really fast $\endgroup$
    – user21263
    Commented Nov 14, 2016 at 15:38

The easiest way to get giant robots that still can move 'easily' is to use strong light materials. Titanium alloys are a great start for lightweight vs. strength. A possible step above that are carbon nanotubes which while currently expensive to make in any quantity would be a light and very strong material to create anything with. It is assumed if we ever create a space elevator it would be made out of this material.


Other answers cover my initial thoughts (anti-grav or things like carbon nanotubes) but I thought I'd throw a more off-the-wall answer into the ring:

A really dense atmosphere

The reason the largest animals exist in water (whales, large squid, etc) is because the water helps support their weight. This significantly reduces the square-cube issue and lets them grow to a bigger size.

A sufficiently dense atmosphere can provide a similar effect - it acts almost like a liquid and will help support the weight of your giant mecha. One theory is that Earth's atmosphere used to be incredibly dense, and that was the reasons dinosaurs were able to grow to such large sizes compared to today's animals and not collapse under their own weight.

There are a couple of potential problems.

  1. Earth's atmosphere isn't dense enough. So you'll probably need to be fighting on alien worlds - maybe an alien race that prefers atmospheres like those, so you'd use these only offensively. Or maybe Venus has stores of Unobtanium and corporations are duking it out with mecha-mercenaries.
  2. Your pilots will be squished by the heavy atmosphere like tiny bugs (well, maybe not. But they might suffocate or get the bends). There's a couple of fixes - you could have a protected control module that's pressurized to human-friendly levels. Or genetically engineered Venus-people who laugh at the pressure. Or non-genetically engineered native Venusians who laugh at the pressure and are willing to fight for you. Or robots. Probably robots.

read CANNON GOD EXAXXION* they handled it great. 1. your mech needs and antigravity/inertia control generator to withstand its own forces 2. it needs force field generator to broaden the "footprint" just keep the feet from sinking into the ground. 3. It needs a nuclear or antimatter reactor inside to power all that. 4. you need a physically enhanced pilot to survive driving it.

And even then just walking through an urban environment destroys said environment.

  • be warned adult content.

There's very few materials capable of being built as a moving object to such a scale. Even if you had tons and tons of titanium. scale it down, it's more plausiable. But none of that really matters; as building a mecha is a notorious waste of money and resources. When Russian armored forces attempted to put down a rebellion in Chechnya from 1994-96 they were slaughtered in urban fighting by light infantry with modern weapons. This was because widespread use of the simple Rocket Propelled Grenade (RPG) has made foot infantry much more powerful than in previous wars. Recall that thousands of illiterate Somalis mauled US Army Rangers in 1993. Unleashing it on the battlefield and your enemy is gonna unload by shooting it's ankles which supports ALL it's weight. With a barrage of RPG's and missiles or tank guns. The Gundam series is by far the silliest emphasis of mechanized warfare. Building an anthropomorphic suit of armor the size of a 10 story building is a target waiting to be shot at.


The solution for a truly humongous mecha in a gravity well is an active support structure, similar to the concept for a space fountain (https://en.wikipedia.org/wiki/Space_fountain). The principle at work here is similar to levitating a plate on a stream of water. It doesn't matter that no realistic material known to man is capable of handling the forces your mechs produce while moving, because you're not relying on intermolecular binding forces here. The "bones" of your mech are simply long cylinders that are under constant internal stress created by a stream of particles pushing the plates apart. This counteracts the force of gravity pushing on the plate from above and allows you to build structures of any height. Including a 50 meter or even kilometer tall mech, if that's what you want!

So far we're totally within the realm of known science and engineering. The caveat? You have to constantly supply energy to push that stream of particles into the plate or your entire structure falls apart. Water isn't a good fit for this because the amount of friction and energy expenditure will be astronomical, unless you're going for a steampunk mech I guess.

What you'll want to use is some kind of superconductor. You fire pellets of magnetic material or streams of charged particles from the bottom of your active support structure at the top, where they get deflected by a magnetic field and fall down again. You do this with enough force behind the stream to hold up the top plate. When the pellets/particle stream fall to the bottom you extract most of the energy you put in (less complicated than it sounds, any magnet falling through a conductive coil induces a current and slows down), collect the stream and shoot it up again. The tube/bone has to be evacuated of air so you don't lose any energy to friction. The superconductors are to make this as efficient as possible.

Now you miniaturize this setup and you have an arbitrarily strong tube, stronger than any old carbon nanotube. As long as you put energy in. Yeah, it will need a lot of energy. How to power this whole contraption is another question entirely. Maybe a nuclear reactor or a giant ball of Polonium-210.


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