The Planck scale is the scale at which quantum-gravity effects are expected to become important.
It is built from three fundamental constants: the speed of light c, Newton’s gravitational constant G, and the reduced Planck constant ħ. Their combination gives the Planck length, approximately 1.616 × 10⁻³⁵ meters, and the Planck time, approximately 5.391 × 10⁻⁴⁴ seconds. The corresponding Planck energy is about 1.22 × 10¹⁹ gigaelectronvolts.
At this regime, quantum mechanics and general relativity should both be essential. Existing theories describe quantum fields in spacetime and gravity as spacetime geometry, but a complete experimentally confirmed theory unifying them is still lacking. Candidate approaches include string theory and loop quantum gravity.
The Planck scale is not simply the smallest length already measured. It is a natural dimensional scale formed from constants, and its extreme energy makes direct experiments difficult. Calling it a scale does not by itself prove that spacetime is granular.