Answer: one calorie is equal to approximately 4.2 joules.
Explanation: one calorie is equal to approximately 4.2 joules. The calorie was originally defined as the amount of heat required at a pressure of 1 standard atmosphere to raise the temperature of 1 gram of water 1° Celsius. Since 1925 this calorie has been defined in terms of the joule, the definition since 1948 being that one calorie is equal to approximately 4.2 joules.