In physics, mass–energy equivalence is the concept that the mass of a body is a measure of its energy content. The mass of a body as measured on a scale is always equal to the total energy inside, multiplied by a constant c that changes the units appropriately:

where E is energy, m is mass, and c is the speed of light in a vacuum, which is 299,792,458 meters per second.

Mass–energy equivalence was proposed in Albert Einstein's 1905 paper, "Does the inertia of a body depend upon its energy-content?", one of his Annus Mirabilis ("Miraculous Year") Papers. Einstein was not the first to propose a mass–energy relationship, and various similar formulas appeared before Einstein's theory with incorrect numerical coefficients and an incomplete interpretation. Einstein was the first to propose the simple formula and the first to interpret it correctly: as a general principle which follows from the relativistic symmetries of space and time.

In the formula, c is the conversion factor required to convert from units of mass to units of energy. The formula does not depend on a specific system of units. Using the International System of Units, joules are used to measure energy, kilograms for mass, meters per second for speed. Note that 1 joule equals 1 kg·m/s. In unit-specific terms, E (in joules) = m (in kilograms) multiplied by (299,792,458 m/s). In natural units, the speed of light is set equal to 1, and the formula becomes an identity.