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So, you're trying to come up with your own Kaiju, and need to know a reasonable weight for it. You can go look up canonical numbers, which is fine as far as they go, but what if you're not making "Godzilla, but with horns and the scales are blue"? This file is intended to help you work out at least a ballpark number.
First off, don't worry about the square-cube law, or little things like "how does something that big remain standing?" It's a convention of kaiju movies to just ignore that issue. If you must, posit stuff like internal antigravity organs (especially on the flying ones) or skeletons made of exotic matter. We're just here to get some idea of the bulk involved.
The 1954 Godzilla's height is 50m, with a mass of 20,000 tons, so I'm going to assume he has an "average" density for kaiju. (There are taller and bulkier versions, of course, but let's keep it simple for now.) Furthermore, since there's a guy in that suit, we can use human "mass to height" proportions as a starting point. Let's go with a particularly bulky man, 2m tall and 100kg. Tack on 100kg for tail, scales, and general bulkiness. For now, just assume everything has a density of 1 g/cm3. So our man-sized Standard Kaiju would mass 200kg, give or take.
If you keep all proportions the same, increasing height by a factor of N will increase mass by a factor of N3. So a 2m tall Standard Kaiju scaled up to 50m gets 25 times taller and therefore 253 or 15,625 times more massive. That gives us an effective mass of 3125 tons. But a 50m tall Godzilla weighs six times that much, so he must have a density of about 6 g/cm3. Even if I missed the mass of a man-sized Godzilla shape at flesh density, Godzilla is still significantly denser than just about any non-metal (and some metals). This means he'd literally sink like a stone, but that's one of those niggling matters we don't worry about. Anyway, taking density into account, our standard bipedal bulky Kaiju is 2m tall and 1200kg, give or take. Keep in mind, with Kaiju like Mothra, you'd only worry about the mass and size of the main body, not the wings.
With this as a starting point, we can lay out Standard Kaijus for various body types:
|Athletic||1000kg||GINO/Zilla, Jet Jaguar|
|Husky||1500kg||Space Godzilla, M.O.G.U.E.R.A., Gamera|
|Mecha-||+200kg||Modifier to body type above|
So, M.O.G.U.E.R.A. is Husky, but also Mecha-, so his base mass is 1700kg for a 2m tall Kaiju. Mecha nature doesn't add as much as you'd think, since baseline Kaiju density is already in the metal range. Keep in mind, though, the examples listed probably don't come out "correct" from my formula, as it's only a starting point (and the people who came up with the official numbers were probably pulling numbers out of their nether regions half the time anyway).
Once you have your base mass, divide your intended "long" dimension (height for bipedal, body length for insects, overall length for quadrupeds) by 2m and cube the result. This will be how much you multiply the base mass by. For instance, if I want a 30m tall Husky Mecha-Kaiju, I take 1700kg (1.7 tons) for my base mass, then multiply it by (30m/2m)3 to get 5,738 tons. That gives me a starting point, I can always round up to 6,000 tons, or decide there's more empty spaces inside and drop it to 5,000 tons, etc.
Sample - Laelapis
For an example, let's take Laelapis, specifically the Millennium verion. He's Athletic, so his base mass is 1000kg. His height is listed at 40m, with 80m long. Since he's proportioned as a guy in a suit, I'll stick with the 40m. I'd only want to use the 80m figure if he were particularly beefy in the tail. So, 1000kg times (40m/2m)3 gives me a starting point of 8000 tons. That feels about right, so I keep it.