Saitama is so strong because, in the story, he’s portrayed as a gag-level character who “broke his limits” and became absurdly, immeasurably powerful—far beyond normal power-scaling logic in anime and manga.

In-story explanation

  • Saitama says he trained with a simple routine: 100 push-ups, 100 sit-ups, 100 squats, and a 10 km run every day, with no air conditioning or breaks, for about three years, until his hair fell out.
  • A scientist in the series (Dr. Genus) believes this training made him “remove his limiter,” meaning the natural cap on a living being’s growth was shattered, so his power can rise with no real ceiling.
  • After this, his physical abilities—strength, speed, durability, and growth—became so extreme that normal concepts like “training gains” or “power levels” stop making sense for him.

How strong is he, roughly?

  • His punches casually one-shot almost any monster or villain; many literally explode from a single “normal punch.”
  • When he uses a “Serious Punch” against Boros, the shockwave is strong enough to part clouds on a global scale, implying planet-level or higher force.
  • He has shown feats like destroying a massive meteor, overpowering an attack drawing power from the Earth’s core, and even attacks in space that blow away the gases around Jupiter in later fights.
  • No enemy so far has been able to seriously injure him, and he rarely needs to get serious, which is why he’s constantly bored in battle.

Meta / narrative reason

  • One-Punch Man is written as a parody of typical shonen power creep: instead of slowly grinding his way up, Saitama starts already at the “final boss” level and stays there.
  • His “training” is intentionally simple and unrealistic; it’s a joke about how typical anime heroes get stronger with training arcs, while here, the same idea is pushed to a ridiculous extreme.
  • The real focus of the story is his boredom, loneliness, and search for meaning when nothing can challenge him, not the detailed mechanics of his power.

Why nobody else can copy him

  • Other characters don’t believe his training explanation because it sounds too basic; they assume there must be some secret technique or unknown power.
  • The idea is that Saitama’s insane effort, pain tolerance, and sheer stubbornness—combined with the fictional “limiter” concept in that universe—produced a one-of-a-kind result.
  • So even if someone tried the same routine, the story treats Saitama as a unique, almost “bug in the system” existence rather than a reproducible build.

In short, Saitama is so strong because he broke his limiter through extreme training in-universe, and because, on a meta level, he’s written as a parody character whose power is intentionally beyond normal logic.

TL;DR: He trained so hard he shattered his natural limits in the story, and as a character, he’s designed to be comically, absolutely unbeatable—“one punch” is the whole point.

Information gathered from public forums or data available on the internet and portrayed here.