An unit of heat energy is called what?

A unit of heat energy is called a calorie. In its traditional scientific definition, one calorie is the amount of heat needed to raise the temperature of one gram of water by one degree Celsius under specified conditions.

The calorie comes from the history of caloric theory, an old explanation of heat that treated it as a fluid-like substance. Although that theory was eventually replaced by modern thermodynamics, the calorie remained useful as a measurement of energy. The International System of Units now uses the joule instead.

Food labels create a common source of confusion. The “calorie” listed in nutrition is technically a kilocalorie, equal to 1,000 small scientific calories. That is why a food label showing 200 Calories represents 200 kilocalories, or about 837,000 joules.

Calories measure energy, not weight or nutritional quality. The body obtains energy from carbohydrates, proteins, fats, and alcohol, while physical activity and metabolism use that energy. The calorie is therefore relevant to both physics and nutrition, but the two uses differ in scale.