How to Convert Calories to Joules
A complete guide to converting between calories and joules, covering both small calories and food Calories, with formulas, examples, and practical applications.
Last updated: 2025-03-15
Introduction: Two Units of Energy
Calories and joules both measure energy, but they come from different traditions. The calorie was the standard energy unit in chemistry and nutrition before the SI system made the joule the official unit. Today, the calorie is still widely used in food labeling (especially in the US), while the joule and kilojoule dominate in science and in countries like Australia that label food energy in kilojoules. This guide shows you how to convert between them accurately and explains why both units persist.
Understanding the Two Types of Calories
Before converting, you must know which “calorie” you are dealing with:
- Small calorie (cal): Also called the gram calorie. The energy needed to raise 1 gram of water by 1°C. Used in chemistry.
- Food Calorie (Cal or kcal): Also called the kilocalorie. Equal to 1,000 small calories. This is what appears on nutrition labels. A food label saying “200 Calories” means 200 kcal.
The Conversion Formulas
- Small calories to joules: J = cal × 4.184
- Joules to small calories: cal = J ÷ 4.184
- Food Calories (kcal) to kilojoules: kJ = kcal × 4.184
- Kilojoules to food Calories: kcal = kJ ÷ 4.184
The factor 4.184 is exact for the thermochemical calorie, which is the standard definition used in modern science and nutrition.
Worked Examples
Example 1: Converting 500 food Calories to kilojoules
500 kcal × 4.184 = 2,092 kJ. A meal containing 500 Calories provides about 2,092 kilojoules of energy.
Example 2: Converting 8,700 kJ to food Calories
8,700 ÷ 4.184 = 2,079.3 kcal. The Australian recommended daily intake of 8,700 kJ is roughly equivalent to the US recommendation of about 2,000 Calories.
Example 3: Converting 100 small calories to joules
100 × 4.184 = 418.4 joules. This is the energy released in a small chemical reaction or the energy to heat 100 grams of water by 1°C.
Quick Reference Table
| Food Calories (kcal) | Kilojoules (kJ) | Example Food |
|---|---|---|
| 50 | 209 | Medium apple |
| 100 | 418 | Banana |
| 250 | 1,046 | Bagel with cream cheese |
| 500 | 2,092 | Fast food burger |
| 700 | 2,929 | Large restaurant entree |
| 2,000 | 8,368 | Typical daily intake (US) |
| 2,500 | 10,460 | Active adult daily intake |
Real-World Applications
- Nutrition labels: US labels show Calories (kcal); Australian, New Zealand, and EU labels show kilojoules. Knowing the 4.184 factor lets you compare.
- Fitness tracking: Exercise equipment may display energy burned in Calories or kilojoules depending on the manufacturer and region.
- Chemistry: Thermodynamic calculations in chemistry use joules (SI), but older textbooks and some calorimetry data use calories.
- Diet planning: When following an international diet plan, you may need to convert between kcal and kJ to match your local food labels.
Use our energy converter for instant conversions between calories, joules, and other energy units.
Frequently Asked Questions
How many joules are in one calorie?
One thermochemical calorie equals exactly 4.184 joules. One food Calorie (kcal) equals 4,184 joules or 4.184 kilojoules.
What is the difference between a calorie and a Calorie?
A lowercase calorie (cal) is the small or gram calorie — the energy to heat 1 gram of water by 1°C. An uppercase Calorie (Cal) is the food Calorie or kilocalorie (kcal), equal to 1,000 small calories. Nutrition labels use food Calories.
How do I convert food Calories to kilojoules?
Multiply the number of food Calories (kcal) by 4.184 to get kilojoules. For example, a 2,000 Calorie diet equals 8,368 kJ.