The grand unification energy, or the GUT scale, is the energy level above which, it is believed, the electromagnetic force, weak force, and strong force become indistinguishable from each other. The electromagnetic and weak forces become indistinguishable at a lower energy level, forming the electroweak force, and gravitational force requires an even higher energy level to unify with the other three. Grand unification theories (GUTs) attempt to make predictions regarding the grand unification energy and the characteristics of the three unified forces.

According to Stephen Hawking in A Brief History of Time, "The value of the grand unification energy is not very well known, but it would probably have to be at least a thousand million million (10¹⁵) GeV." The most powerful colliders ever designed can go up to a few thousand GeV. It is interesting to note that this scale is only a few orders of magnitude below the Planck scale at which gravitational effects become important.