 The 2018 CODATA recommended value is given by

While the value of can be "estimated" from the values of the constants appearing in any of its definitions, the theory of quantum electrodynamics (QED) provides a way to measure directly using the quantum Hall effect or the anomalous magnetic moment of the electron.Other methods include the AC Josephson effect and photon recoil in atom interferometry.There is general agreement for the value of , as measured by these different methods.