E = mc² Calculator

E = mc² is Einstein's mass-energy equivalence equation from his 1905 Special Theory of Relativity. It states that energy (E) equals mass (m) multiplied by the speed of light (c) squared. This reveals that mass and energy are two forms of the same thing and can be converted into each other.

'c' stands for the speed of light in a vacuum, approximately 299,792,458 meters per second (roughly 3 × 10⁸ m/s). Because c is squared in the equation, even a tiny amount of mass corresponds to an enormous amount of energy — this is why nuclear reactions release such tremendous power.

The equation means that mass and energy are equivalent and interchangeable. Any object with mass has a corresponding rest energy, and that energy is proportional to its mass multiplied by the square of the speed of light. It unified the previously separate laws of conservation of mass and conservation of energy into a single principle.

Multiply the mass in kilograms by the square of the speed of light (approximately 8.9876 × 10¹⁶ J/kg). For example, 1 kg of mass is equivalent to about 89,875,517,873,681,764 joules (roughly 9 × 10¹⁶ J). Our calculator handles the unit conversion automatically — just enter your mass and select your units.

Rearranging E = mc² gives m = E ÷ c². Divide the energy (in joules) by the square of the speed of light (≈ 8.9876 × 10¹⁶). The result is the mass equivalent in kilograms. Select 'Calculate Mass given Energy' in our calculator and enter your energy value to get the result automatically.

One gram (0.001 kg) of mass is equivalent to approximately 8.99 × 10¹³ joules, or about 89.9 terajoules. This is roughly equivalent to the energy released by 21.5 kilotons of TNT — comparable to the yield of an early nuclear weapon.

E = mc² underpins nuclear power generation and nuclear weapons, where small amounts of mass are converted to enormous energy. It also explains how the Sun produces energy through nuclear fusion, guides particle physics experiments, and is applied in PET scan technology in medical imaging.

This calculator supports mass inputs in kilograms (kg), grams (g), pounds (lb), and ounces (oz). Energy outputs are available in joules (J), kilojoules (kJ), megajoules (MJ), gigajoules (GJ), and British Thermal Units (BTU). Results are shown simultaneously across all energy units for easy comparison.