Naruto is one of the most if not the most popular work to write fanfiction about, but as you guessed from the title of the guide as well as the introduction, it also has the least accurate power scaling in fanfiction. Why is this so? To be honest I'm not really sure, it could be for a multitude of reasons like cognitive dissonance or close-mindedness or simply being ignorant.
Anyways, the first and most important factor in Naruto scaling is speed. Just how fast are Naruto characters? Are they wimpy scrubs that can barely run faster than a car or are they styling on Wally West? Well the answer is somewhere in the middle. At some points they are running barely faster than car(1) and on some others even light is not fast enough to keep up. yes, you heard me. Naruto characters are faster than light. How much faster? well, we'll get into that later.
I know, I know, I've probably lost some of you completely and others are rushing to the comments to try and hurt my feelings by calling me mean names or saying crude remarks about my dear mother but hear me out first before running at me with your machetes and knives like wannabe roadmen.
Now, how do I know that Naruto characters are faster than light? Is it because of Naruto easily dodging Madara's light fang jutsu during the war arc? Well no, my source is from far earlier than that. As for the light fang feat, although the feat supports my claim this is the last time you'll hear of it as I'm sure you've been beaten over the head with it for a decade now and are tired of hearing about it.
No, my source is from the very first proper arc of Naruto, the Land of Waves arc.
"What the Land of Waves Arc? How is that possible!?"
Fear not, my fellow keyboard warrior, I will enlighten you. During said arc, we were all introduced to our first childhood trap, Haku. Now Haku has this jutsu called "Ice Release: Demonic Mirroring Ice Crystals". This jutsu allows Haku to move from mirror to mirror at the speed of light by reflecting his image onto each one(2).
Yes I know, you heard me, Haku is able to travel through each mirror at the speed of light.
"But then how is Sasuke able to dodge and counter Haku? Even if he awakened his sharingan, he's still just a genin and Haku was stated to be stronger than both Kakashi and Zabuza who are both jonin!?"
The narrative explains this by stating that Haku -despite his rough upbringing and being treated as a tool by Zabuza- is by nature a gentle and kind soul, not wanting to kill or harm others. Basically, he was holding back considerably. We also have to keep in mind that just because Haku CAN move at light speed, it doesn't mean he always does.
"Ok Ok, maybe Haku can move at light speed if he wanted to because of his mirrors, how does this prove that Naruto characters are light speed or even in that tier in general!?"
I'm glad you asked! To explain this he just have to examine how Haku dies.
So, Kakashi and Zabuza are doing round 2 of their battle and after a brief back and forth Kakashi summons his ninja dogs who skewer Zabuza with their teeth, holding him in place. Kakashi then runs at Zabuza planning to unleash the fastest method to a person's heart, Chidori.
Haku, seeing his pseudo-adoptive father/master, about to die rushes in at his full speed to tank the blow and die in his place. Doing some calculations, we find out that Kakashi traveled roughly a third of the distance that Haku did.
If Haku's full speed is as fast as light and Kakashi covered a third of the distance that Haku did before turning Haku into a shish kabob, we can conclude that Kakashi's minimum max speed at this point in the story is 1/3 the speed of light(3).
This speed is also consistent with Kakashi's childhood feat of chopping a bolt of lightning in half. If you didn't know, lightning has three stages; Stepped leader: first preparatory step of lightning, this is when a channel of ionized air is formed before the bolt strikes(travels at around 150,000-300,000 meters per second), Electrical Discharge: the actual bolt of lightning itself, it generates the flash and heat(travels at 89.4 million meters per second), The Flash: the flash of light that is generated by the electrical discharge(travels at 299.79 million meters per second).
This means that as a kid, Kakashi was already able to move a third of the speed of light since that's roughly how fast a bolt strikes.
It seems that losing his mother, father, teacher, surrogate mother, best friend, and personally killing his potential love interest with his bare hands really left a mark on our boy Kakashi caused it seems he's stagnated for like 13 years. Or he actually did grow and he's actually much faster than 1/3 the speed of light and just didn't feel like going faster since Zabuza was pinned and he didn't know that Haku was a super prodigy that could reach light speed at 14-years-old. Just speculation.
Anyway, keeping all this info in mind, if we use basic power scaling methodology to upscale characters who surpass this early series Kakashi then logically anyone who is elite jonin or above is at least mid relativistic to light speed.
Kage-level/s-rank characters would easily scale to light speed as using basic reasoning, even the weakest Kage-level ninja would surely be several stronger then this version of Kakashi at the very least. After all if Kakashi were relative to a Kage-level ninja he would've been the Hokage already.
"But a human moving at light speed is impossible! When things approach or surpass light speed their mass increases exponentially, in order to reach light speed you would need infinite energy to sustain the ever increasing mass! If you somehow managed to surpass light speed spacetime would break apart and you would travel forward in time causing a paradox and collapsing the universe! And even if those two things didn't happen, traveling at that speed would cause so much friction that the air molecules would tear apart and cause an ever increasing, exponentially powerful nuclear explosion that would destroy the planet!!!!"
Wow, you sure are smart to have deduced all that in less than 2 seconds ;).
To your question, all I can say is to keep in mind that Naruto is a fictional world with an all-powerful all-purpose mystical energy that can allow flight, teleportation, resurrecting the dead, talking animals, telepathy, elemental manipulation, and creating dimensions/universes. When it comes to Naruto or even most fictional stories, real-world physics can only be used as a reference and nothing more. Author intent and the narrative are more important and reliable, especially when it comes to power scaling.
Anyways, staying on the topic of Kakashi's fights. After getting embarrassed by Orochimaru, Kakashi takes a break from his 13-year-long hiatus from training and gets back to it.
The first time we see him training, he's doing intensive rock climbing and even manages to open the first gate. The first gate boosts someone by 5x meaning that even if he didn't grow stronger thanks to the training, simply opening the gate would allow to be 65% faster than light. (0.33x5=1.65)
However even being 65% faster than light wasn't enough to save him from Itachi who made hand seals so fast that Kakashi couldn't even perceive them. Although Kakashi was able to recover by barely managing to make a Water wall jutsu in time to block, it doesn't change the fact that he was speed blitzed.
In power scaling, being blitzed means that the character doing the blitzing is so fast that you can't even react to them. To blitz someone you need to be a bare minimum 5x faster, meaning that Itachi is at least 8x faster than light at a bare minimum. And remember, this is Itachi going easy on him and barely trying since he's a spy placed in the Akatsuki by the Third Hokage and doesn't even want to kill Kakashi. (1.65x8=8.25)
He even allowed Kakashi to defend with a water wall to save him some face. What kind, upstanding man!
"The Uchiha would beg to differ!"
Speaking of water did you know that in the data book, Itachi's random water jutsu that he threw at Kakashi was a light speed attack?(4)
"No way, You're kidding!"
Yes way, It's what the data books say.(5)
Taking this into account, Kakashi being able to block the jutsu with his water wall would put his base reaction time at 2.5x the speed of light since there was no indication that he was using the first gate. This mean's Itachi would be faster would be 12.5x faster than light. (2.5x5=12.5)
Kakashi's theoretical top speed would also be around 12.5x if he opened the first gate. if the first gate was opened during this fight this would put Itachi at 65x faster than light. However I'm going to disregard this and go the lower interpretation for the sake of argument.
"Man, my brain is breaking! how am I supposed to keep my world view that the Naruto Verse is full of wall-level-barely-faster-than-a-car fodder now! The arguments are airtight!"
How indeed.
"Aha! I've figure it out! when Itachi and Sasuke were fighting their climatic duel didn't Zetsu brag that Sasuke's Kirin jutsu was only a thousandth of a second? That's no where near light speed! He bragged that such a speed was undodgeable!!!"
Wow, I didn't know you were capable of arguing in bad faith like that! For shame! We both know that Zetsu said it was undodgeable because it covered the entire range of the hideout not because of its speed!
"Fine you caught me! But what about the first part of my argument!"
I was getting to that.
If you read the passage in context, you would see that the reason Zetsu said that was because he thought that both Sasuke and Itachi were nearly completely out of chakra and were thus vastly weaker and slower.
He thought Sasuke would use the last embers of his chakra to merely redirect natural lightning, he had no idea Sasuke had enough chakra to actually bring forth an s-rank jutsu like Kirin. He also had no idea that Itachi had enough chakra to react using his Susanoo.
Basically, Zetsu was making an assumption based on the limited information that he had. Zetsu isn't omniscient after all, he may be the info guy but he isn't them and thus can't make an accurate predication of their states.
In short, Zetsu is an unreliable narrator. To further prove this, all we have to do is also realize that the speed of lightning is 1/3 the speed of light anyway, meaning that he pulled that 1/1000 of a second out of his ass.
1/1000 of a second isn't even comparable to the preparatory stage of lightning, the stepped leader stage. Shows that you cant trust that mother-loving cocksucker anyway.
"Don't you mean mother-fucking?"
No.
".."
Anyway, if that isn't enough the satisfy you, the data books back me up and state that Kirin is a light speed attack(6).
"Dammit, you could of just said that and been done with it! Did you like seeing me squirm you bastard!"
Take it eas-
"NO DAMMIT! Don't you understand that I want things that Confirm my bias not make me consider other options or alternatives! I want an echo chamber not a challenge-my-weirdly-strongly-held-beliefs-chamber!"
Here take a chill pill.
"I feel much better."
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"Damn what's in this shit? is it laced"
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bonus feats cause why not?:
Fourth Raikage's lariat is light speed. He can then increase his speed further with the lightning cloak.
Minato speed blitz the Raikage.
Naruto Speed blitz the Raikage.
Darui's Laser Circus jutsu is refracted light.
Pain's shinra tensei and bansho tenin use gravity waves. Gravity waves are radiation and are therefore light speed.
Kakashi Retsudan novel states that special jonin can fire attacks that hit light speed
The Raikage and Tsunade travel using heavenly transfer jutsu during the war arc. This jutsu is stated to be light speed in the manga and data book. They are able to react to Madara and Mu's attacks before even the jutsu is even complete thereby proving they are faster than light.
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(1) Ninjas seem to have severe stamina issues, just look at Kakashi during the pain fight, a few jutsu and he's out of chakra. Traveling at full speed would likely leave them tired and surrounded by enemies due to the commotion of traveling the world at relativistic to light speed. There is also the concern of having to navigate all the varied terrain of the world. This is most likely why ninja's cruise at car-like speeds and don't run at full speed across the world, to conserve energy and not attract attention. They likely only use their true speed when in short range combat.
(2) proof that Haku travels through mirrors at light speed. Both the data books and the anime dub support it.
(3)proof for Kakashi.
(4) proof for Itachi's light speed water jutsu
(5) Addressing potential concerns about the data books: The data books are written by Kishimoto and his team and are official works licensed by Shonen Jump and published by Shueisha. Yes, the data books frequently use hyperbole and exaggerate certain claims but as long as you read between the lines, look at each example case-by-case, and use the source material as a reference its not a big deal. If being hyperbolic were an issue you would also have to discard the manga too since it also uses hyperbole to exaggerate certain feats or statements to support the narrative and make it more engaging(Strongest Hokage Hiruzen anyone?).
(6) proof Kirin is light speed