The most famous equation in all of science, E = mc2, tells us that mass 'm' (measured in kg) can be converted into energy 'E' (measured in J).
The 'exchange rate' between the two is the constant 'c', the speed of light (3×108 m/s). As this value is very large, we see that even a small amount of mass will convert into a huge amount of energy; this is observed in nuclear fusion.
Figure 1. Einstein's mass-energy equivalence equation.