 Planck mass

The Planck mass combines the reduced Planck constant, speed of light and gravitational constant into a mass scale, $m_P=\sqrt{\hslash c/G}$. Its associated Compton wavelength, using the reduced convention $\hslash /(m_{P}c)$, equals the Planck length ${\ell }_P=\sqrt{\hslash G/c^3}$. It therefore identifies a scale at which these particular quantum and gravitational length estimates coincide.

This dimensional comparison motivates the use of Planck units in quantum gravity; it is not an experimental observation of a particle with that mass. The reduced Planck mass instead uses an additional factor $1/\sqrt{8\pi }$, so a gravitational equation written with either convention must state which one is meant.