Calories are a form of energy. To calculate the energy content, or calories, of a given food, scientists burn it. When it burns, they can measure its energy content by putting it in water and seeing how much it increases the water temperature. 1 calorie will raise 1 gram of water by 1 degree Celsius. To calculate the calories, the scientist must know the mass of water, the initial temperature of the water, and the final temperature of the water. They then plug in their values into this equation:

Heat = (The mass of the water in grams) X (the specific heat of water, which is 4.18 Joules/gram C) X (The change in water in C)

If you look at this equation, you can see your answer of "heat" will be in Joules. Joules is another measurement of energy and it can be converted to calories. The conversion is 1 calorie = 4.184 Joules.