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According to a very short google search, it takes somewhere between 150 kg and 500 kg to crush a humanoid skull. The safest option is however to just go with a bulldozer. Find authorized novels in , faster updates, better experience, Please click #%!d(string=13012365806403505)/hard-head_%!d(string=51587382167784593) for visiting.

On a less important side note, you should never have to worry about your skull getting fractured from a barefisted punch. The human hand would definitely break first. Falling down after getting knocked out is however another story.