E = mc^2 E is Energy m is mass c is the speed of light in a vacuum This formula shows us that matter and energy are related. The more massive something is, the more energy content it possesses. The formula is most useful in determining how efficient an energy-producing process is. Given that we know that the total energy content of the universe is unchanging and knowing that the input of a system is equal to its output, we can figure out how much energy is released through burning something, through fission, or through fusion. For instance, when hydrogen atoms fuse into Helium atoms, there is a loss of mass. It's a small amount, but it is measurable. We take this amount of mass, multiply it by c^2 and that tells us how much energy was released in the process.
Energy= mass * the speed of light squared