Board Thread:Questions and Answers/@comment-61.6.238.10-20161107022633/@comment-76.126.125.211-20170831055936

Islandboss wrote: Also, he uses barely 1% of his strength when fighting humans, only using his actual strength on monsters. Nope. He uses less than 1% of his strength on monsters. He uses even less on humans. His strength is limitless as far as we know.

He has never "overkilled" a monster (non-human) and he has never directly killed a human.

He holds back enough to kill the monster without obliterating everything else nearby. Not overkilling means he is holding back a lot as his finishing punches cause around the same amount of damage to Wolf level and Dragon level monsters (a large hole through the body). Even for Boros, he held back enough to stop the planet-destroying blast while still keeping most of Boros intact. Boros also noted that Saitama was still holding back considerably during his serious punch. This is what I am basing the less than 1% against monsters part on.

For humans, he holds back enough to injure, but not kill. This is true for humans undergoing monster transformations as well. For the webcomic, he destroys Garou's monster shell while keeping his human body intact. Unfortunately, we do not know where he draws the line for transformations as many of the monsters he has killed were once human.

We have never seen him use his actual strength on anything and we probably never will. If he did, he would definitely destroy the Earth as a side effect. This is again evidenced by Boros's comment. This means that we will probably never see him use anywhere close to his actual strength unless if he is removed from Earth or put inside some sort of magical indestructible barrier.

Even then, given the theme of this manga, he would most likely one punch said indestructible barrier as well.