I've been looking at the various Planck units, and I'm wondering how they are constructed from the constants involved. Like is there any physical reasoning behind those equations? I've looked all over the place for derivations of the equations or what the actual physical meaning of the units are, but it seems to be that Planck just found that you could combine the constants to give the correct dimensions for the units. Is that all there is to it? Or is there an actual reason for assuming that a fundamental unit of length will involve G, c and h? If someone could explain I would be very grateful.