Km is 10^3m cm is 10^-2m so the difference is 10^5 i.e. 100000 (one with 5 zeroes)

Or more intuitively one centi-meter is a 1/100 of a meter and Kilo-Meter is 1000 meter therefore 100*1000 100000

It is quite intuitive, once you start using it

What’s weird is why we don’t use Megameters and megagrams (i.e. one metric ton)