The Serizawa Scale was (and still is) the traditional raiting scale for the Kaiju, monsters sent by an alien race called the Precursors to ultimately exterminate humanity. The Serizawa Scale is based on ambient radioactivity from the Kaiju, the water displacement from the Kaiju exiting their respective breach, and their toxicity, especially the toxicity from their blood (also known as "Kaiju Blue," in most cases).
The Original Serizawa Scale[]
The original Serizawa scale
The original Serizawa Scale, as specified above, is based on raidoactivity, water displacement, and toxicity. It ranked Kaiju on a category scale of I-V, with I being the weakest, and V being the strongest. It was useful for assigning Jaegers to Kaiju, so that the Jaeger(s) deployed would be able to easily dispatch the Kaiju. Category I Kaiju were common at the beginning of the First Kaiju War, as the Precursors found no need to send more powerful Kaiju yet. As the Kaiju that came out of the breach became stronger, Newton Greiszler speculated that types I-IV (and possibly V) were just scouts to test Earth's defences, to pave the way for much stronger "exterminator" Kaiju. Indeed, as more powerful Kaiju emerged from their respective breaches, the feared "exterminator" Kaiju began to surface.
The Need for a New Serizawa Scale[]
As the Kaiju that exited the scale began to reach "exterminator" levels, the need of a new Serizawa Scale arose. Soon, Jaegers were being paired with Kaiju many times stronger than the respective Jaeger, causing a few Jaeger casualties in key cities. Soon, a group of cryptozoologists, and other Kaiju researchers, gathered in Atlanta, GA to create a new scale. The so-called "Atlanta Kaiju Symposium" lead to add ten categories to the Serizawa Scale, as well as to create simulations of ultra-category Kaiju; double, triple, quadruple, quintiple... events; and even battle simulations, Precursor foot troops and "Evil Jaegers." This meeting was a massive success, with plans for stronger Jaeger(s) to be built immediatey followed the meeting.
The New Serizawa Scale[]
The New Serizawa Scale has categories from I-X, with one being the weakest, and X being the strongest. The scale uses the traditional way of rating Kaiju (Water Displacement, Ambient Radioactivity, and Toxicity). It also takes into account how sentient the Kaiju is (i.e. does it learn?). The NSS is also to be used to rate "Rogue" or "Evil" Jaegers, but with disregard to Toxicity and Radioactivity. Instead, it is to use "Relative First-Punch Power," where a Rogue's arm piece is assesed, and an estimation of the power of a Jaeger's first punch.