How much force would your fist need to strike with to punch through a human body?

Their fist would be going as fast as a bullet.

Your question is about how you'd go about hitting someone's fast enough that the thing you're hitting them with is able to punch a hole clean through their body.

Fortunately, we have these in real life: they're called "bullets". A bullet is simply a small, dense mass that moves at a high enough velocity that it will punch a hole clean through the body of a human it strikes. There are a number of images on the Internet of the damage that a bullet can cause, using ballistic gel as a proxy for human flesh.

So, in order for your murderous superhuman to punch a hole through a human body, their fist will need to go approximately as fast as a bullet.

A quick ballpark

I found one source that states the average shear strength for flesh is about 0.4 MPa or 60 psi. Seems somewhat reasonable based on the known danger of compressed air.

Looking at my fist, I guess the area at about 8 sq inches. The means your punch would have to be about 480 lbs force - sustained through the punching depth from front to spine of about 8 inches.

In reality, punching a person this hard would not be enough because the shear strength will be offset by compression of the targets flesh and momentum transfer. At a guess, I figure 1000 or even 2000 pounds of force is more realistic - but quite possibly not very accurate. I'll assume 1500 pounds.

Force says nothing about speed, and in reality shear strength varies with speed (slow speeds gives more time for material to tear). These question then becomes how do expect to have 1500 pounds of force at the end of your punch.

If you can supply 1500 pounds force continuously during a punch, you won't have to slow down during penetration, otherwise you need to be fast enough that that kinetic energy carries you through the punch as you can't supply enough force to maintain penetration based on continuous muscle force.

This answer assumes the super strong person has material properties much stronger than the target, otherwise you have to compensate for the compression, etc. of the arm delivering the blow.

Im not sure if it would be possible but, in order to give this punch the best possible chance of punching through someone, here are some things to consider.

Firstly, you will want your ‘target’ to be braced against something hard. The reason being is that, as you punched them, they would move and absorb a lot of the force. However, if they are braced against the floor or a tree trunk or a concrete wall, they arent going to move backwards as much, reducing the forces they can absorb. I will assume they are against the floor so that way, gravity is working with you, not against you.

Next, we have to pick where about we are going to hit. This is pretty simple as we are trying to punch through to the spine so we know it is the front of the torso. Your best bet is likely through the lower half of the torso, if you aimed for the upper half, you’d need to also break through the rib cage.

Now, you’d need to overcome the shearing strength of human skin and muscle (though i can’t find any source that tells you what this could be) but, assuming you could apply enough PSI to an area the size of a human fist (roughly 0.45 meters squared in this case) you might be able to do it. For the best effect, i would suggest your humanoid jumps and lands on the target’s soft tissue fist first, applying all of their bodyweight behind the target which is backed against a solid concrete floor.

Alternatively, if you’re just trying to punch someone in their spine by punching through them, that would require a lot less force than punching a hole clean through them.

You can get really complicated about this, but if we're really keeping in the spirit of the question you're asking, the answer is that "you can't punch someone hard enough to put a hole in them." Not unless the Puncher and the Punchee are made out of Different Stuff.

The problem is that a fist and a chest (for example) have roughly the same density and the same structural integrity, more or less, so something that's made out of the same kind of materials as a human body is a: isn't physically CAPABLE of moving a fist that fast; b: if they could, the fist and the arm would take almost as much damage as the person getting punched would; and c: the punchee would be getting pushed backward by the blow, dissipating the force more rapidly than you can open up a hole through to their spine.

Now, if your Punching hand is significantly harder and/or denser than a normal human body, then it doesn't have to move very fast at all in order to be able to go through someone. this is really the answer to your question tbh. If you want to punch a hole in someone with some science behind it, your puncher has to be punching with something a lot harder and stronger than what a human is made out of.