Electron mass energy equivalent

m_{\mathrm{e}} c^{2} = 8.187105788 \times 10^{-14}\ \text{J}

Value	8.187105788e-14 J
Status	Measured: ± 2.60e-23 J (3.2e-10 relative)
Source	CODATA 2022
Categories	Universal & Atomic physics nuclear

m_e c² in every energy unit

joule	8.1871058e-14 J
kilojoule	8.1871058e-17 kJ
megajoule	8.1871058e-20 MJ
calorie	1.9567652e-14 cal
kilocalorie	1.9567652e-17 kcal
watt-hour	2.2741961e-17 Wh
kilowatt-hour	2.2741961e-20 kWh
British thermal unit	7.7598790e-17 BTU
foot-pound	6.0384993e-14 ft⋅lb
electron volt	510,998.95 eV
kiloelectron volt	510.99895 keV
megaelectron volt	0.51099895 MeV

Learning zone

511 keV is one of the most recognisable numbers in physics. When a positron meets an electron the pair annihilates into two photons of exactly this energy flying back-to-back, which is what a PET scanner's detector ring times to milimetre precision, and what gamma-ray telescopes see glowing from the centre of the Milky Way.

It also sets thresholds. Pair production from a single photon needs at least 2m_e c² = 1.022 MeV plus a nucleus to absorb the recoil, so no gamma below 1.022 MeV can make an electron-positron pair. And it marks the boundary of relativity for electrons: a 511 keV kinetic energy means γ = 2, so anything in a linac or a CRT beyond a few tens of keV already needs relativistic kinematics, while thermal electrons at 0.025 eV are hopelessly non-relativistic.