It may not be obvious, but a blazing campfire relies on the principles contained in Einstein's famous equation, E = mc^2. This equation demonstrates the relationship between mass and energy. These two properties are interchangeable; if a system loses mass it must gain energy, and vice-versa. In the campfire example, the flames consume some of the mass of the wood, resulting in energy in the form of heat. Calculating E = mc^2 for any object tells you how much energy would result if the entire object disappeared.
Weigh the object you will perform the calculation for with a mass balance scale. Record its mass.
Convert the mass to kg, if necessary. To convert from g to kg, for example, divide by 1,000.
Square the speed of light. The speed of light is approximately, 300,000,000 m per second; (300,000,000 m/s)^2 equals 90,000,000,000,000,000 meters squared per second squared, or 9 x 10^16 m^2/s^2.
Multiply the result by the mass of the object in kilograms. If the mass is 0.1 kg, for example, (0.1 kg)_(9 x 10^16 m^2/s^2) = 9 x 10^15 kg_m^2/s^2.
Record the result in Joules, the standard metric unit for energy. One Joule equals 1 kg_m^2/s^2, so 9 x 10^15 kg_m^2/s^2 equals 9 x 10^15 J.
Convert the result to kilojoules -- abbreviated as kJ -- by dividing by 1,000. The accurate speed of light is 299.792.458 m per second.