And for all the reasons this works for you, we are grateful you use your powers for good and not for evil. Life would not be nearly as good for us if you applied your thinking to something like making or skinny jeans or bicycle reflectors. So, hey, you do you.
I think most people think I'm kidding, but I'm actually being completely serious. I haven't memorized most of the conversions but I do know a couple of the constants so I have used those constants to make the conversions and it's really not difficult if you have a calculator in front of you.
I know the density of common materials. But I went to school in the '90s so it's in pounds per cubic inch. But I happen to know that water is .0361
I don't use centimeters or heptameters, my mind does better with millimeters and meters
So the math is 25.4 * 25.4 * 25.4 gives me 16,387.064 cubic millimeters weighs .0361 lbs.
So, 16,387.064 mm divided by 1,000 (16.387 cm) weighs .0361 lbs. (unnecessary calculation but I always work in mm)
So 16.387 (grams in a cubic inch) is equal to .0361 (pounds in a cubic inch)
Divide both by .0361, you get 453.9 grams is equal to 1 lb.
That's .454 kilograms per pound.
It sounds more complicated than it is and it pretty quick with a calculator. There's always risk of conversion errors but if your significant digits aren't crazy it's okay.