What is the difference between calories and kilocalories?

In the U.S., energy in foods is expressed in kilocalories (kcal). The scientific definition of a kilocalorie is the amount of energy needed to raise the temperature of 1 kilogram of water one degree Celsius from 15° to 16° at one atmosphere. The true calorie, sometimes referred to as a "small calorie," is the amount of energy needed to raise the temperature of one gram of water one degree Celsius from 15° to 16° at one atmosphere. A kilocalorie is equal to 1000 calories.

While the term "calorie" technically applies to the "small calorie," in common usage, such as in reference to food energy, the term "calorie" is actually a kilocalorie. Internationally, most countries express food energy in kilojoules (kJ). One kcal equals 4.184 kJ. The USDA Nutrient Database for Standard Reference contains values for both kilocalories and kilojoules.