What do calories measure?
The 'calorie' is a unit of thermal energy.
A calorie is the amount of energy required to raise 1 g of water by 1 degree Celsius. It is equal to 4.184 Joules
The "calorie" used in food values is really a kilocalorie (kcal). It is the amount of energy, or heat, it takes to raise the temperature of 1 kilogram of water 1 degree Celsius (1.8 degrees Fahrenheit). One kcal is equal to 4,184 Joules, or 4.184 kJ.