How does nuclear fission compare with burning coal in energy released per gram?

Nuclear fission releases significantly more energy per gram of fuel than the combustion of coal.

In a nuclear fission event, such as the splitting of uranium-235, the energy released is derived from the difference in rest mass between the reactants and the products, as described by the Einstein relation, $E=m{c}^2$. When completely fissioned, 1 kilogram of uranium-235 releases energy equivalent to approximately 17,000 tons of TNT.12

In contrast, the energy released by coal is chemical energy stored within the material, which is released as thermal energy upon combustion. The calorific value of coal—typically measured in megajoules per kilogram—is orders of magnitude lower than the energy density provided by nuclear fuel. While coal remains a common fuel source, the physics of nuclear reactions allows for a vastly higher energy yield per unit of mass compared to the chemical processes involved in burning coal.312