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 mist & vast spaces of time and rust…back to a very questionable command of a little high school trigonometry and Newtonian mechanics, nearly forgotten (unsure whether I ever learned it properly in the first place). 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, never are) and we are likely to be 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% of your body mass landing on or around your feet and toes.
Ok? Well, let’s now assume that you’re a strapping 100 kg. lad (or lass).
At this body mass you’d be doing roughly the equivalent of 56 kg bench press, along with a reasonable amount of associated involved core muscular work required to preserve good form thrown in as a bonus.
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.