An isolated electron has an angular momentum and a magnetic moment resulting from its spin. While an electrons spin is sometimes visualized as a literal rotation about an axis, it cannot be attributed to mass distributed identically to the charge. The above classical relation does not hold, giving the wrong result by the absolute value of the electrons -factor, which is denoted :
where  is the Bohr magneton.
The gyromagnetic ratio due to electron spin is twice that due to the orbiting of an electron.
In the framework of relativistic quantum mechanics,
where  is the fine-structure constant. Here the small corrections to the relativistic result  come from the quantum field theory calculations of the anomalous magnetic dipole moment. The electron -factor is known to twelve decimal places by measuring the electron magnetic moment in a one-electron cyclotron:
The electron gyromagnetic ratio is
The electron -factor and  are in excellent agreement with theory; see Precision tests of QED for details.