According to Emc2-explained, the equation E=mc^2 describes the conversion between matter and energy. This is a constant relationship, and it applies to the conversions observed in atomic fission and nuclear fusion as well as the release of energy obtained from matter or antimatter reactions. In each case, E=mc^2 is a way of predicting energy release. The equation also predicts the amount of energy required to synthesize matter from energy.
The American Museum of Natural History explains the simple equation: The potential energy (E), expressed in joules, is equal to the mass of the matter (m), expressed in kilograms, multiplied by the speed of light squared (c^2) in meters per second.
According to Wikipedia, when an atom splits, two daughter atoms are formed. The atomic weight of these two atoms together is somewhat less than the atomic weight of the original atom. That difference in mass is m. Multiplied by the speed of light squared, c^2, or around 90 trillion meters per second squared, the product is the energy released, in joules, of the missing matter.
Wikipedia further explains that the same is true of fusion reactions. Two light atoms fuse to form another, heavier atom. The heavier atom is lighter than the atomic weight of the two predecessor atoms put together with the difference having been converted to energy.
For antimatter interactions, 100 percent of the mass of the two particles is converted. Applying E=mc^2, m is equal to the total mass of both particles together.