mass = energy. can you determine the absolute value of energy in mass, in one electron?

a simple question, (probably a big problem) : they say energy equivalent of the mass of an electron is

0,511 MeV = 1,2356* 10²⁰ Hz

quantity of mass is an absolute value, this equivalent should be an absolute value, how do you determine an absolute value?, how do you get 0,511 MeV?


Edit: math is obvious. I mean practically: the concrete experimental measure that validates theoretical formula

To partially answer your questions,the absolute value is calculated using rest mass.hope this helps:)