Rest energy E₀ = mc² is the energy equivalent of a particle's mass measured when it is at rest. It excludes kinetic energy. A proton at rest has 938.272 MeV (1.503 × 10⁻¹⁰ J) of rest energy.

What is the rest energy of an electron?

The CODATA 2022 value is 0.51099895069 MeV, or 8.1871 × 10⁻¹⁴ J, from the electron mass 9.1093837139 × 10⁻³¹ kg. When an electron and positron annihilate at rest, each of the two photons carries this 511 keV.

What are the rest energies of the proton and neutron?

The proton is 938.27208943 MeV and the neutron 939.56542194 MeV (CODATA 2022). The neutron is 1.293 MeV heavier, which is why a free neutron can decay into a proton, an electron and an antineutrino.

How do I convert MeV/c² to kg?

Multiply by 1.602176634 × 10⁻¹³ J/MeV and divide by c² = 8.987551787 × 10¹⁶ m²/s²: 1 MeV/c² = 1.78266192 × 10⁻³⁰ kg. One atomic mass unit is 931.49410372 MeV/c².

Why do physicists use eV/c² as a mass unit?

Dividing an energy by c² gives a mass, so a particle of mass 938.27 MeV/c² has a rest energy of exactly 938.27 MeV. In natural units, where c = 1, the c² is dropped and masses are quoted simply in MeV or GeV.

What is the Compton wavelength?

λ = h ÷ (mc), the wavelength of a photon whose energy equals the particle's rest energy. For the electron it is 2.42631 × 10⁻¹² m; for the proton 1.32141 × 10⁻¹⁵ m.

How much energy is in one gram of matter?

E = 0.001 kg × c² = 8.988 × 10¹³ J, about 2.5 × 10⁷ kWh or 21.5 kilotons of TNT. Only a small fraction of rest energy is released in fission or fusion; complete conversion requires matter–antimatter annihilation.

How is total energy related to rest energy?

Total energy E = γmc² = K + mc², and E² = (pc)² + (mc²)². An electron with 1 MeV kinetic energy has E = 1.511 MeV, γ = 2.957 and a speed of 0.941c.

Particle Rest Energy Calculator

E₀ = mc² with c = 299,792,458 m/s. Electron 0.51099895069 MeV, proton 938.27208943 MeV, neutron 939.56542194 MeV, muon 105.6583755 MeV, alpha 3727.3794118 MeV (CODATA 2022); 1 u = 931.49410372 MeV/c² and 1 MeV/c² = 1.78266192 × 10⁻³⁰ kg.

Mass → rest energy

Particle

Mass unit

Mass value

Enter mass in kilograms (e.g. 9.1093837139e-31).

Kinetic energy (MeV, optional)

Energy → mass

Mass (kg): —
Mass (u): —
Mass (MeV/c²): —

m = E ÷ c². 1 GeV corresponds to 1.78266 × 10⁻²⁷ kg, slightly more than one proton.

Select a particle or enter a mass, then click Calculate.

Formula

E₀ = m × c²

Input mass

—

Rest Energy E₀ = mc²

Joules —

Electron-volts —

Kiloelectron-volts —

Megaelectron-volts —

Gigaelectron-volts —

Teraelectron-volts —

Kilowatt-hours —

Tons of TNT (4.184 GJ/t) —

Mass and derived quantities

Kilograms (kg) —

Atomic mass units (u) —

MeV/c² —

GeV/c² —

Compton wavelength h/(mc) —

Pair-production threshold 2mc² —

Mass ÷ electron mass —

Mass ÷ proton mass —

Rest energies of common particles

CODATA 2022 recommended values (NIST) and PDG 2024 averages. kg and u for PDG entries are derived from the rest energy.

Particle	Rest energy (MeV)	Rest energy (J)	Mass (kg)	Mass (u)	Source
Electron (e⁻)	0.51099895069	8.18711 × 10⁻¹⁴	9.10938 × 10⁻³¹	5.48580 × 10⁻⁴	CODATA 2022
Muon (μ⁻)	105.6583755	1.69283 × 10⁻¹¹	1.88353 × 10⁻²⁸	0.113429	CODATA 2022
Tau (τ⁻)	1,776.86	2.84684 × 10⁻¹⁰	3.16754 × 10⁻²⁷	1.907538	CODATA 2022
Proton (p)	938.27208943	1.50328 × 10⁻¹⁰	1.67262 × 10⁻²⁷	1.007276	CODATA 2022
Neutron (n)	939.56542194	1.50535 × 10⁻¹⁰	1.67493 × 10⁻²⁷	1.008665	CODATA 2022
Charged pion (π±)	139.57039	2.23616 × 10⁻¹¹	2.48807 × 10⁻²⁸	0.149835	PDG 2024
Neutral pion (π⁰)	134.9768	2.16257 × 10⁻¹¹	2.40618 × 10⁻²⁸	0.144904	PDG 2024
Charged kaon (K±)	493.677	7.90958 × 10⁻¹¹	8.80059 × 10⁻²⁸	0.529984	PDG 2024
Deuteron (²H nucleus)	1,875.612945	3.00506 × 10⁻¹⁰	3.34358 × 10⁻²⁷	2.013553	CODATA 2022
Alpha particle (⁴He nucleus)	3,727.3794118	5.97192 × 10⁻¹⁰	6.64466 × 10⁻²⁷	4.001506	CODATA 2022
W boson	80,369.2	1.28766 × 10⁻⁸	1.43271 × 10⁻²⁵	86.279881	PDG 2024
Z boson	91,188	1.46099 × 10⁻⁸	1.62557 × 10⁻²⁵	97.894339	PDG 2024
Higgs boson (H)	125,200	2.00593 × 10⁻⁸	2.23189 × 10⁻²⁵	134.407721	PDG 2024
1 atomic mass unit (u)	931.49410372	1.49242 × 10⁻¹⁰	1.66054 × 10⁻²⁷	1.000000	CODATA 2022

Constants and worked examples

c = 299,792,458 m/s (exact), so c² = 8.987551787 × 10¹⁶ m²/s².
1 eV = 1.602176634 × 10⁻¹⁹ J (exact); h = 6.62607015 × 10⁻³⁴ J·s (exact).
1 u = 1.66053906892 × 10⁻²⁷ kg = 931.49410372 MeV/c² (CODATA 2022).
1 MeV/c² = 1.78266192 × 10⁻³⁰ kg; 1 kWh = 3.6 × 10⁶ J; 1 ton of TNT = 4.184 × 10⁹ J.

Electron: E₀ = 9.1093837139 × 10⁻³¹ kg × 8.987551787 × 10¹⁶ m²/s² = 8.18710 × 10⁻¹⁴ J ÷ 1.602176634 × 10⁻¹³ J/MeV = 0.510999 MeV. Its Compton wavelength is h ÷ (mₑc) = 2.42631 × 10⁻¹² m and the photon energy needed to create an electron–positron pair is 2mₑc² = 1.022 MeV.

One gram: E₀ = 0.001 kg × c² = 8.98755 × 10¹³ J = 2.49654 × 10⁷ kWh, or about 21,481 tons (21.5 kilotons) of TNT.

Moving electron: with K = 1 MeV, E = 1 + 0.511 = 1.511 MeV, γ = 1.511 ÷ 0.511 = 2.957, β = √(1 − 1/γ²) = 0.9411 and pc = √(1.511² − 0.511²) = 1.422 MeV.

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Summary

This calculator applies the mass-energy equivalence E₀ = mc² to find the rest energy of a particle or any mass. Enter mass in kg, grams, unified atomic mass units (u) or eV/c², MeV/c² or GeV/c², or work backwards from an energy in J, eV, keV, MeV or GeV to the mass. Results appear in joules, electron-volts (eV to TeV), kWh and tons of TNT, with the Compton wavelength, the pair-production threshold and the mass ratio to the electron and proton. Presets and a reference table use CODATA 2022 values for the electron, muon, tau, proton, neutron, deuteron and alpha particle and PDG 2024 values for pions, the charged kaon and the W, Z and Higgs bosons. An optional kinetic energy gives total energy, Lorentz factor, speed and momentum.

How it works

Pick a particle from the preset list or choose Custom and select the mass unit: kg, g, u, eV/c², MeV/c² or GeV/c².
The mass is converted to kilograms (1 u = 1.66053906892 × 10⁻²⁷ kg; 1 eV/c² = 1.602176634 × 10⁻¹⁹ J ÷ c²) and E₀ = m × c² is computed with the exact c = 299,792,458 m/s.
Rest energy is shown in J, eV, keV, MeV, GeV, TeV, kWh and tons of TNT; mass is shown in kg, u, MeV/c² and GeV/c².
The Compton wavelength λ = h ÷ (mc), the photon pair-production threshold 2mc² and the ratios m/mₑ and m/mₚ are also given.
Enter a kinetic energy in MeV to get total energy E = K + mc², the Lorentz factor γ = E ÷ mc², the speed β = √(1 − 1/γ²) and momentum pc = √(E² − (mc²)²).
Use the Energy → mass card to convert an energy back to the equivalent mass m = E ÷ c².

Use cases

Checking textbook E = mc² problems for electrons, protons and nuclei.
Converting particle masses between kg, u and MeV/c² with current CODATA values.
Finding the 511 keV annihilation photon energy and the 1.022 MeV pair-production threshold.
Getting γ, speed and momentum of a particle with a known kinetic energy.
Converting an energy such as 1 GeV or 1 J into its mass equivalent.
Showing how much energy one gram of matter represents (about 21.5 kilotons of TNT).

Frequently Asked Questions

Last updated: 2026-10-10 · Reviewed by Nham Vu