How is heat measured in joules and calories?

In physics, heat is a form of energy transferred between bodies due to a temperature difference. Because heat can be converted into work, it is measured using units of energy, primarily joules and calories.1

Measuring Heat

• The Joule: As the SI unit of energy, the joule is defined as the work done by a force of one newton acting through one metre. In thermodynamics, it is the standard unit for expressing both heat and work to ensure consistency.23
• The Calorie: Historically, the calorie was defined as the amount of heat required to raise the temperature of one gram of water by 1°C at a constant pressure of one atmosphere. Because this value varies slightly with temperature, specific definitions exist, such as the "15° calorie" (4.1855 joules) and the thermochemical calorie (4.184 joules).41

Conversion

There is a direct equivalence between these units, known as the mechanical equivalent of heat. While definitions vary slightly by context, 1 calorie is approximately equal to 4.184 to 4.2 joules. In modern scientific practice, it is common to express heat in joules to maintain a unified system of units.34