Calories are units that measure the amount of heat a substance can produce. They are commonly found on food labels, and calorie control is a staple part of any weight loss diet. Two units of measure that many people confuse are "Cals" and "Kcals." There is no difference between the two as long as "Cal" has a capital C, but a Kcal is 1,000 times more than a "cal" with a lowercase c.
Large Versus Small
Calories come in two types. Small calories, also known as gram calories, are shown by the symbol "cal" and indicate how much energy is needed to raise the temperature of 1 gram of water by 1 degree Celsius. Large calories are expressed by the symbol "Cal" and approximate how much energy is needed to raise the temperature of 1,000 grams of water by 1 degree Celsius. A large calorie represents about 1,000 times more energy than a small calorie.
Kcal Versus Cal
There is no difference between a Kcal and a Cal. Kcal stands for kilocalories, and 1 Kcal is equal to 1,000 small calories. A Cal is also 1,000 small calories. The differentiation between "Cal" and "cal" is difficult to make when speaking (unless you say "large calorie"), and therefore is typically used in texts where the capitalization of the first letters can be used to make the distinction.
Colloquially, calories have come to mean the same thing as "large calories" in the United States. When you see the calorie count of food in the United States, it indicates the number of large calories, or "Cal," that the food contains. The term "kilocalories" or "Kcal" is used in most other parts of the world, such as Europe. Two thousand standard calories in the United States is the same thing as 2,000 kilocalories in Europe.
Calories are a measure of heat, and therefore the number of calories indicates the amount of energy received from consuming them. Joules are a measurement of energy. It takes about 1 joule of energy to lift something weighing 1 pound by about 9 inches. A standard Kcal or Cal has 4,187 joules of energy. A cal (note the lowercase c) has 4.187 joules of energy. Eating 2,000 Kcals or Cals per day will yield 8.4 megajoules of energy, which is 8.4 million joules. To put that in perspective, around 300,000 joules are needed to jog for six minutes or walk for 15 minutes.