The Serizawa Scale is the metric used to assign categories to the Kaiju. But how exactly does it work? And does it work at all? Today, on Reaper's Blog 4 Thinkin', I'll attempt to navigate this minefield.
The Serizawa Scale supposedly uses water displacement, ambient radiation levels as they pass through the Breach, and toxicity levels to assign category based on size and threat level.Water displacement is another way of saying "size and shape", which makes sense (bigger Kaiju tend to be more powerful). Toxicity also matters for obvious reasons. Ambient radiation, however, is a trickier one. There doesn't seem to be any particular reason why a Kaiju's radiation levels would affect its threat level. However, there's nothing to suggest that Kaiju are actively radioactive, and even if they were, such a thing would be taken into account as part of toxicity (since a person can't come into contact with one without also coming into contact with the other). This suggests that somehow, the ambient radiation levels of a Kaiju are somehow proportional (or at least related) to its threat level (perhaps they are proportional to the Kaiju's energy consumption, for example).
At first glance, it all seems fairly cut and dried. However, the movie paints a muddier picture. Leatherback and Scunner (both category 4's) have "low" toxicity (compared to the "Medium" toxicity of lower category Kaiju like Onibaba and Knifehead), while Otachi has a toxicity higher than even Slattern. Tales from Year Zero suggests that Trespasser has low toxicity (it poisons Tendo's grandfather instead of causing acid burns), yet he has the size and mass of a category 3 or 4.
Speaking of size confusion, there doesn't seem to be much, if any, difference in size between a category 3 and a category 4. Knifehead is said to be the largest category 3 at the time of its appearance (possibly of all), but it's still a category 3 nonetheless. However, it is larger and more massive than either Mutavore (described as "an enormous category 4" in the movie) and Otachi (note that Otachi and Leatherback were noted as being the largest category 4's in terms of both size and weight at the time of their appearance).
Clearly, it's not all about size, or stats (technically, they don't even come into play), or toxicity. We don't know the ambient radiation levels of any of the Kaiju, so that may well be the most important factor in assigning categories (especially since it only makes sense as a factor at all if it is somehow directly related to a Kaiju's threat potential).
So, how are we to make sense all of this?
There seems to be two potential interpretations. The first is that the category system, is, to borrow a phrase from Chuck Hansen, "a sack of Kaiju [expletive deleted]". The alternative is that the category system, first and foremost, measures threat (even if only roughly), with the factors outlined earlier being used to somehow determine that (with the mysterious ambient radiation playing a major role).
I choose to believe in the latter interpretation for one simple reason: the category system still exists. By the time of the movie, the war with the Kaiju has been going on for 12 years. If the category system were a crock, someone in the movie (besides Newt, who more or less claims that all Kaiju should belong to the same category, and is therefore irrelevant to this discussion, since that is the direct opposite of seeing all Kaiju as completely different) would have expressed a lack of faith in it. Instead, despite the fact that so many Kaiju break one of the three measurements used to inform it, everyone focuses on the Serizawa Scale. The politicians say to Pentecost: "Category 4 Kaiju are beginning to emerge from the Breach. I think you know what this means" (clearly implying that both parties know that category 4 Kaiju are more dangerous than category 3's, despite wildly varying levels of toxicity and a lack of size difference). And when there is a third signature coming from the Breach during Operation: Pitfall, the question on everyone's mind is "what category is it?". In other words, the Serizawa Scale has been put to the test for over a decade, and the people who are best positioned to know of its accuracy (or lack thereof) clearly trust it. If it didn't work, they wouldn't still be using it.
So in a sense, this entire blog post is a tautology. The only way the Serizawa Scale makes sense for measuring the threat level of Kaiju, is if it...well, measures the threat level of Kaiju.