Electron mass energy equivalent
| Value | 510.998950694 keV (8.187105788 × 10⁻¹⁴ J) |
| Status | Measured: ± 1.6 × 10⁻⁷ keV (3.2 × 10⁻¹⁰ relative), standard uncertainty |
| Source | CODATA 2022 |
| Categories | Universal & AtomicNuclear |
| Unit | Value |
|---|---|
| kiloelectron volt | 510.99895 keV |
| hartree | 18,778.865 Eh |
| megaelectron volt | 0.51099895 MeV |
33 more energy units
| electron volt | 510,998.95 eV |
| gigaelectron volt | 0.00051099895 GeV |
| nanojoule | 0.000081871058 nJ |
| erg | 8.1871058e-07 erg |
| teraelectron volt | 5.1099895e-07 TeV |
| microjoule | 8.1871058e-08 µJ |
| millijoule | 8.1871058e-11 mJ |
| inch-pound | 7.2461992e-13 in·lb |
| joule | 8.1871058e-14 J |
| foot-pound | 6.0384993e-14 ft⋅lb |
| calorie | 1.9567652e-14 cal |
| calorie (International Table) | 1.9554566e-14 cal IT |
| kilojoule | 8.1871058e-17 kJ |
| BTU (thermochemical) | 7.7650721e-17 BTU th |
| British thermal unit | 7.7598790e-17 BTU |
| watt-hour | 2.2741961e-17 Wh |
| kilocalorie | 1.9567652e-17 kcal |
| megajoule | 8.1871058e-20 MJ |
| horsepower-hour | 3.0497471e-20 hp·h |
| kilowatt-hour | 2.2741961e-20 kWh |
| therm | 7.7598790e-22 therm |
| gigajoule | 8.1871058e-23 GJ |
| million BTU | 7.7598790e-23 MMBTU |
| megawatt-hour | 2.2741961e-23 MWh |
| ton of TNT | 1.9567652e-23 t TNT |
| tonne of oil equivalent | 1.9554566e-24 toe |
| terajoule | 8.1871058e-26 TJ |
| gigawatt-hour | 2.2741961e-26 GWh |
| kiloton of TNT | 1.9567652e-26 kt TNT |
| petajoule | 8.1871058e-29 PJ |
| terawatt-hour | 2.2741961e-29 TWh |
| megaton of TNT | 1.9567652e-29 Mt TNT |
| quad | 7.7598790e-32 quad |
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.
Cite this page
formula.expert. (2026, October 16). Electron mass energy equivalent. https://formula.expert/constants/electron-mass-energy-equivalent