What calculation determined the Kármán line where flight relies on orbital mechanics instead of aerodynamic lift?

The altitude where sustained flight requires speed approaching orbital velocity

The scientific reasoning behind proposing the 100-kilometer mark stemmed from aerodynamic thresholds. Von Kármán calculated the altitude where the atmosphere becomes sufficiently thin that conventional aircraft can no longer sustain flight using aerodynamic lift alone. At this height, the speed required for a hypothetical aircraft to maintain altitude through horizontal speed becomes so high that it essentially equals the velocity needed for orbit. Beyond this point, movement is dictated by inertia and gravity—the principles of orbital mechanics—rather than interaction with the atmospheric cushion that defines conventional flight.