 In Hartree atomic units ( and ), the fine structure constant is

The 2018 CODATA recommended value of is
This has a relative standard uncertainty of 0.15 parts per billion.This value for gives , 3.6 standard deviations away from its old defined value, but with the mean differing from the old value by only 0.54 parts per billion.For reasons of convenience, historically the value of the reciprocal of the fine-structure constant is often specified.