What is the speed of light value used?

The exact value is c = 299,792,458 m/s (meters per second), as defined by the International System of Units (SI). Therefore c² ≈ 8.98755 × 10¹⁶ J/kg.

Why is the energy so large for small masses?

Because c² is an enormous number (roughly 9 × 10¹⁶). Even 1 gram of matter, fully converted, releases about 9 × 10¹³ joules — equivalent to roughly 21 kilotons of TNT.

Does this apply to all matter?

Yes. E=mc² applies to all matter at rest. This is the rest-mass energy. In practice, converting mass entirely to energy (as in matter-antimatter annihilation) is theoretical for most substances; nuclear reactions convert only a tiny fraction of mass.

What is an atomic mass unit (u)?

One atomic mass unit (u or Da) equals approximately 1.66054 × 10⁻²⁷ kg. It is used in atomic and nuclear physics to express the mass of atoms and subatomic particles.

How accurate are the results?

Results use JavaScript's double-precision floating-point arithmetic (IEEE 754), which gives about 15–17 significant digits of precision — sufficient for all practical scientific calculations.

Can I calculate the mass equivalent of a given energy?

This tool calculates energy from mass (E=mc²). To find mass from energy, simply divide the energy in joules by c² (≈ 8.9876 × 10¹⁶ J/kg).

Mass Energy Equivalence Calculator

Enter a mass value and instantly get its equivalent energy using E=mc², displayed in joules, kilojoules, megajoules, kilowatt-hours, and electron-volts.

Enter Mass

Mass Value

Unit of Mass

Formula: E = mc²
c = 299,792,458 m/s | c² ≈ 8.9876 × 10¹⁶ J/kg

Energy Equivalents

Enter a mass value and click Calculate

Results will appear here in multiple energy units

Mass in kilograms:

Joules

SI base unit of energy

Kilojoules (kJ)

10³ joules

Megajoules (MJ)

10⁶ joules

Kilowatt-hours (kWh)

Common electricity billing unit (1 kWh = 3.6 MJ)

Electron-volts (eV)

Used in atomic and particle physics

Mega electron-volts (MeV)

Nuclear physics unit (1 MeV = 10⁶ eV)

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Summary

Einstein's mass-energy equivalence principle, expressed as E=mc², states that mass and energy are interchangeable. Even a tiny amount of mass contains an enormous amount of energy when multiplied by the square of the speed of light (c ≈ 299,792,458 m/s). This calculator converts any mass value into its equivalent energy across multiple units for scientific, educational, and engineering reference.

How it works

Enter a mass value in the input field.
Select the unit of mass (kilograms, grams, milligrams, pounds, or atomic mass units).
The calculator multiplies the mass in kilograms by c² (≈ 8.9875 × 10¹⁶ J/kg).
Results are displayed simultaneously in joules, kilojoules, megajoules, kilowatt-hours, and electron-volts.
Use the copy button next to any result to copy that value to your clipboard.

Use cases

Physics students verifying E=mc² homework problems.
Understanding the energy released in nuclear reactions.
Comparing the energy density of matter to conventional fuels.
Teaching and demonstrating the concept of mass-energy equivalence.
Engineering calculations involving relativistic energy.
Exploring the implications of antimatter annihilation.
Scientific research and reference conversions.
Estimating the theoretical yield of nuclear devices.

Frequently Asked Questions

Last updated: 2026-09-30 · Reviewed by Nham Vu