A thermochemical calorie is 4.184 joules.
Calories to Kilojoules Converter — cal to kJ
Convert Calorie (cal) to Kilojoule (kJ) using the exact conversion factor (1 calorie = 0.004184 kilojoule). See the formula, worked examples, conversion table, history, and common uses.
Calories to Kilojoules converter
Converter
Engineering Converter
Convert common stress, force, torque, power, and energy units used in engineering.
Context values
Some unit families need one extra value for a physically accurate result.
Kilojoule is 1000 joule units.
Result = input x 4.184 / 1000.
Energy units are compatible.
Reserved above the conversion cards and below the converter.
About Converting Calories to Kilojoules
Calorie and Kilojoule both measure a quantity used in structural or mechanical engineering — commonly stress, pressure, force, torque, power, or energy — where converting between the unit conventions used in different regions, industries, or equipment specifications is a routine part of design work. Calorie is most often used for food energy labeling (as kilocalories, often written 'calories').
Formula
kilojoules = calories × 0.004184
This factor comes from each unit's defined relationship to the category's base unit: 1 calorie equals 4.184 base units, and 1 kilojoule equals 1000 base units, so dividing one by the other gives the direct calorie-to-kilojoule factor of 0.004184.
Simple example
1 cal × 0.004184 = 0.004184 kJ
1 calorie = 0.004184 kilojoules.
Real-world example
100 cal × 0.004184 = 0.4184 kJ
100 calories = 0.4184 kilojoules.
Conversion table
| Calorie (cal) | Kilojoule (kJ) |
|---|---|
| 1 cal | 0.004184 kJ |
| 10 cal | 0.04184 kJ |
| 50 cal | 0.2092 kJ |
| 100 cal | 0.4184 kJ |
| 500 cal | 2.092 kJ |
| 1,000 cal | 4.184 kJ |
Reverse conversion: Kilojoule to Calorie
0.004184 kJ × 239.0057361 = 1 cal
0.004184 kilojoules = 1 calorie.
calories = kilojoules × 239.005736138
For a page dedicated to this direction, see Kilojoule to Calorie.
Understanding the Calorie
Calorie (cal) measures energy. The thermochemical calorie used in this converter is defined as exactly 4.184 joules. It is classified as a Non-SI unit (limited use with SI).
Where it's used: Nutrition labeling in the US and several other countries still commonly uses the kilocalorie (as 'Calorie')
Calorie vs. Calorie (kilocalorie). Food labels' capitalized 'Calorie' is actually one kilocalorie (1,000 small calories) — a long-standing labeling convention that causes frequent confusion.
Uses of the Calorie today
- Food energy labeling (as kilocalories, often written 'Calories')
- Historical chemistry and thermodynamics contexts
History of the Calorie
The calorie was originally defined in the early 19th century as the heat needed to raise one gram of water by one degree Celsius, but different scientific traditions defined it at slightly different reference temperatures, producing several close but non-identical 'calorie' values historically (15 °C calorie, mean calorie, thermochemical calorie).
To resolve this ambiguity, the thermochemical calorie was fixed by convention at exactly 4.184 joules; the very similar International Table calorie is fixed at exactly 4.1868 joules. The nutritional 'Calorie' seen on food labels is actually a kilocalorie (1,000 calories).
Historical origin. Calorie was not the invention of a single named person; it developed through the process described above.
Understanding the Kilojoule (kJ)
Kilojoule is 1000 joule units.
Calorie — sources
- NIST Special Publication 811 — Documents the thermochemical calorie's exact joule equivalence.
Reserved between the cards and the FAQ so the page stays balanced.
Frequently asked questions
How many kilojoules are in 1 calorie?
1 calorie equals 0.004184 kilojoules, using the exact defined conversion factor rather than an estimate.
How do I convert calorie to kilojoule?
Multiply the calorie value by 0.004184. The converter above does this instantly to whatever precision you set.
How do I convert kilojoule back to calorie?
Use the reverse factor: 1 kilojoule equals 239.0057361 calories. You can also use the swap control in the converter above to flip the direction instantly.
What is a calorie?
Calorie (cal) measures energy. The thermochemical calorie used in this converter is defined as exactly 4.184 joules. It is classified as a Non-SI unit (limited use with SI).
What is a kilojoule?
Kilojoule (kJ) is a unit of engineering.
Is the calorie to kilojoule conversion exact?
Yes. The thermochemical calorie used in this converter is defined as exactly 4.184 joules. That fixed definition is what the converter above uses, so any rounding you see is a display choice, not a limitation of the math.
What is the difference between calorie vs. calorie (kilocalorie)?
Food labels' capitalized 'Calorie' is actually one kilocalorie (1,000 small calories) — a long-standing labeling convention that causes frequent confusion.
Where is the calorie still used today?
Nutrition labeling in the US and several other countries still commonly uses the kilocalorie (as 'Calorie')