What fraction of body mass do you actually lift when doing a normal push-up?
I won’t answer your question precisely, but as long as you’re satisfied with a “close enough”, we’re good to go.
Stay with me while I will attempt to reach back through the vast spaces of time and rust to a very questionable command of a little high school trigonometryand Newtonian mechanics, nearly forgotten. Specifically Newton’s 2nd Law (Dear Mrs. Beeson, I do regret deeply all those wasted opportunities spent ditching your high school Latin & Physics classes. Yep, I’m paying dearly for the hard-learned lessons of my youthful crassness then, with my dumbness in dotage now).
I am also making a decision to spare myself the embarrassment and everyone else here the pain of calculation and proof.
So, assuming all things being equal (which they, in the real world, are not) and that we are doing a standard push-up at an even pace (1–2 count up, 1–2 count down) similar to the illustration above, unmodified in any other way: about 56% of total body mass is lifted by your upper body muscles. This will leave the remaining 44% landing on or around your feet and toes.
Well, let’s now assume that you’re a strapping 100kg lad.
If you are, you’d then be doing roughly the equivalent of 56 kg bench press, along with a reasonable amount of core work involved, which would be required to preserve good form, thrown in for good measure.
If you are also the distrusting DIY type, which I recommend, and want to check this, get yourself a scale and do your push up with that. You don’t even have to do the push part, just hold the up and set your scale down on a level floor, position yourself in the standard pushup position, with your hands on the scale.
Then, just read the scale.