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Mass-Energy Equivalence Calculator

· Last updated：2026-08-29

E = mc² both ways: mass to rest energy (J, kWh, eV-scale, TNT) and energy to mass (kg, g, u, electron masses).

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Written by the Elysia Tools team

Tool usage guide

Learn when to use this tool, what it supports, and how real users apply it.

Key facts

Category
Science & Education
Input types
select, number
Output type
text
Sample coverage
4
API ready
Yes

Overview

The Mass-Energy Equivalence Calculator converts between mass and rest energy using Einstein's equation E = mc² in both directions. Whether translating microscopic atomic mass units into electron-volts or macro-scale kilowatt-hours into grams, it provides instant rest-energy breakdowns, particle counts, and TNT equivalents.

When to use

  • Calculating the energy released by nuclear mass defects during fission, fusion, or radioactive decay.
  • Finding the equivalent mass of high-energy particle physics phenomena in electron masses or atomic mass units.
  • Converting macroscopic energy quantities (such as kWh or Joules) into equivalent relativistic mass values.

How it works

  1. 1Select your conversion mode: Mass → energy (m2e) or Energy → mass (e2m).
  2. 2Enter the numeric value and pick the desired input unit from options like kg, g, u, electron masses, J, kWh, or eV-scale units.
  3. 3Select the target display unit for secondary reference if desired.
  4. 4The tool computes the result using the speed of light squared (c² ≈ 8.98755×10¹⁶ m²/s²) and outputs conversions across multiple mass and energy units simultaneously.

Use cases

  • Nuclear physics students verifying binding energy calculations and mass defects in MeV per nucleon.
  • Science educators demonstrating the immense energy density stored in ordinary rest mass compared to chemical explosives.
  • High-energy physics researchers converting particle beam energies in GeV to equivalent electron or nucleon mass multiples.

Examples

1. Converting Atomic Mass Unit to MeV

Nuclear Physics Student

Background

A student needs to find the exact rest-energy equivalent of 1 atomic mass unit (u) for a nuclear binding energy assignment.

Problem

Manual calculations with constants can lead to unit conversion errors between joules, kilograms, and megaelectron-volts.

How to use

Set Conversion mode to Mass → energy, enter 1 into the Mass field, and choose 'u' as the Mass unit.

mode: m2e, mass: 1, massUnit: u, energyUnit: kWh

Outcome

Displays 1.49242×10⁻¹⁰ J, 931.494102 MeV, and 3.56696×10⁻²³ kilotons of TNT equivalent.

2. Finding the Mass Equivalent of One Kilowatt-Hour

Physics Educator

Background

An instructor wants to demonstrate how much mass is added or lost when storing 1 kWh in a battery.

Problem

Needs an accurate relativistic mass conversion expressed across grams, atomic units, and electron masses.

How to use

Set Conversion mode to Energy → mass, enter 1 into the Energy field, and select 'kWh' as the Energy unit.

FAQ

What formula is used for these calculations?

Calculations use Einstein's rest energy formula E = mc², with c² ≈ 8.98755×10¹⁶ m²/s².

Which mass units are supported?

Supported mass units include kilograms (kg), grams (g), milligrams (mg), micrograms (µg), atomic mass units (u), and electron masses (mₑ).

Which energy units can I convert to and from?

Supported energy units include joules (J), kilojoules (kJ), megajoules (MJ), watt-hours (Wh), kilowatt-hours (kWh), and electron-volt units (eV, keV, MeV, GeV).

Does the calculator output TNT equivalent energy?

Yes, mass-to-energy calculations display the equivalent explosive yield in kilotons of TNT.

Can I calculate the mass of stored electricity or heat?

Yes, using the Energy → mass mode allows you to compute the exact relativistic mass corresponding to any energy value in kWh, J, or eV.

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Outcome

Outputs 4.00554×10⁻¹¹ kg (approx. 40 nanograms), equal to 2.41219×10¹⁶ u and 4.39716×10¹⁹ electron masses.