Calories vs ジュール
Two Ways to Measure Energy
Learn the DifferenceA food label shows 200 Calories. A physics textbook uses joules. Your fitness tracker might display either—or both. Calories and joules both measure energy, but they come from different traditions and are used in different contexts. 理解する both helps you navigate nutrition labels, science, and energy calculations.
Quick Comparison
| Attribute | Calorie | Joule |
|---|---|---|
| Definition | Energy to heat 1g water by 1°C | Work done by 1 newton over 1 meter |
| System | CGS (older) | SI (international standard) |
| Primary use | Nutrition, chemistry | Physics, engineering |
| Symbol | cal (small), Cal/kcal (food) | J |
| Food labels | US, some countries | EU, Australia, most countries |
理解する Calories
The Small Calorie (cal)
The energy needed to raise 1 gram of water by 1 degree 摂氏. This is the thermochemical calorie used in chemistry and physics.
The Food Calorie (Cal, kcal)
What nutrition labels call a "Calorie" is actually a kilocalorie—1,000 small calories. A 2,000-Calorie diet is really 2,000,000 small calories, or about 8.4 megajoules.
- Food Calorie (kcal): Heats 1 kilogram of water by 1°C
- Small calorie (cal): Heats 1 gram of water by 1°C
理解する ジュール
The joule is the SI (International System) unit of energy, named after physicist James Prescott Joule. One joule equals:
- The work done by a force of 1 newton moving 1 meter
- The energy of 1 watt of power for 1 second
- The kinetic energy of a 2 kg mass moving at 1 m/s
- Approximately 0.239 calories
Common Joule Prefixes
- Kilojoule (kJ): 1,000 J—used for food energy outside US
- Megajoule (MJ): 1,000,000 J—fuel energy
- Gigajoule (GJ): 1,000,000,000 J—industrial energy
Conversion Table
| Calories (kcal) | Kilojoules (kJ) | Context |
|---|---|---|
| 50 kcal | 209 kJ | Apple |
| 100 kcal | 418 kJ | Small snack |
| 250 kcal | 1,046 kJ | Granola bar |
| 500 kcal | 2,092 kJ | Large meal |
| 1,000 kcal | 4,184 kJ | Half daily intake |
| 2,000 kcal | 8,368 kJ | Typical daily diet |
Where Each Unit Is Used
Calories (kcal) Predominate In:
- US food labels
- Some Asian countries (Japan, China)
- Diet and fitness discussions (US)
- Chemistry thermodynamics (cal)
ジュール/Kilojoules Predominate In:
- European Union food labels
- Australia, New Zealand
- Physics and engineering worldwide
- Most scientific publications
Both Shown:
Many food products show both units (e.g., "250 kcal / 1046 kJ") to serve international markets.
Energy Context Examples
| Activity/Item | Calories (kcal) | Kilojoules (kJ) |
|---|---|---|
| 1 hour walking | ~200 kcal burned | ~840 kJ |
| 1 hour running | ~600 kcal burned | ~2,500 kJ |
| Banana | ~100 kcal | ~420 kJ |
| Cheeseburger | ~300-500 kcal | ~1,250-2,090 kJ |
| Daily basal metabolism | ~1,400-1,800 kcal | ~5,860-7,530 kJ |
| 1 gallon gasoline | ~31,000 kcal | ~130,000 kJ |
Why Two Units Persist
Science has standardized on the joule, yet calories persist in nutrition for several reasons:
- Historical momentum: Calorie counting began in the 1800s
- Intuitive size: Food portions are 100-1,000 kcal; in joules, that's 400-4,000 kJ—larger numbers
- Industry investment: Decades of nutrition research, databases, and education use calories
- Regional preferences: US resistance to metric adoption extends to energy units
まとめ
Calories and joules measure the same thing—energy—but come from different measurement traditions. The joule is the scientific standard; the food Calorie (actually a kilocalorie) persists in nutrition. Converting between them is straightforward: multiply kcal by 4.184 to get kJ, or divide kJ by 4.184 to get kcal.