Calorie vs. Kilocalorie

To a scientist, a calorie is the quantity of heat (or energy) needed to boost the temperature of 1 ml of water by 1°C. A kilocalorie (kcal, often pronounced kay-cal) is the amount of heat required to raise the temperature of 1000 ml of water by 1°C. A kilocalorie is 1000 calories. Even though there is 1000-fold difference between these two values, calorie and kilocalorie are often used synonymously when discussing food and eating. Though it is commonplace, it is technically incorrect. When in doubt, use the word energy, as in: butter is rich in energy.

Calorie and kilocalorie will not be used to mean the same thing in this text. When you see food labels say that a serving of food is 100 calories, the real meaning is that the food has 100 kcals. Likewise, when we say that running a race burned 300 calories, we mean 300 kcals.