In physics, the proton-to-electron mass ratio, µ or ß, is simply the rest mass of the proton divided by that of the electron. Because this is a ratio of like-dimensioned physical quantity, it is a dimensionless quantity, a function of the dimensionless physical constants, and has numerical value independent of the system of units, namely:

µ =

The number enclosed in parentheses is the measurement uncertainty on the last two digits. The value of µ is known to about 0.4 parts per billion.