# Rest Mass Energy Calculator

E₀ = mc² for electron, proton, neutron, or custom mass: energy in J and eV-scale units, mass in kg and u — the collider mass table.

> Canonical page: https://elysiatools.com/en/tools/rest-mass-energy

- **Category:** Science & Education

- **Keywords:** rest energy, rest mass, electron mass energy, proton mass energy, mc squared, particle physics

## Overview

The Rest Mass Energy Calculator converts mass into equivalent rest energy using Einstein's mass-energy equivalence formula, E₀ = mc². It provides instantaneous calculations for fundamental subatomic particles—electrons, protons, and neutrons—as well as arbitrary custom masses, outputting values in Joules, mega-electronvolts (MeV), kilograms, and atomic mass units (u).

## Inputs

- **Particle** (select)
- **Custom mass (kg)** (number): e.g. 9.109e-31

## When to use

- Analyzing collision kinematics, decay thresholds, or mass defects in modern physics problems.
- Converting particle masses between kilograms, atomic mass units (u), and MeV/c² equivalents.
- Calculating total rest energy release for particle-antiparticle annihilation reactions.

## How it works

- Select a standard particle (proton, electron, neutron) or choose the custom mass option.
- Enter the mass in kilograms if calculating for a custom particle or arbitrary object.
- The tool multiplies mass by the speed of light squared (c²) to obtain rest energy in Joules.
- Outputs are converted into MeV and atomic mass units (u) alongside the base SI values.

## Use cases

- Determining gamma-ray photon energies produced during electron-positron pair annihilation.
- Verifying mass-energy equivalence values in undergraduate and advanced physics coursework.
- Looking up precise rest mass benchmarks for particle physics research and collider data analysis.

## Frequently asked questions

### What formula is used to calculate rest mass energy?

The calculator uses Einstein's mass-energy equivalence equation, E₀ = mc², where c is the speed of light in vacuum.

### Which standard particles are included?

The tool includes preset values for the electron, proton, and neutron, alongside a custom mass input field.

### Why is rest mass energy frequently quoted in MeV?

In particle physics, MeV (or MeV/c²) is a convenient scale that avoids unwieldy scientific notation for subatomic particles.

### Can I enter non-subatomic custom masses?

Yes, you can enter any positive mass in kilograms using standard or scientific notation (e.g., 9.109e-31 or 1.0).

### What units are provided in the calculation output?

Results display mass in kilograms (kg) and atomic mass units (u), and rest energy in Joules (J) and mega-electronvolts (MeV).

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