The joule is the SI derived unit of energy.
Joules to Kilojoules Converter — J to kJ
Convert Joule (J) to Kilojoule (kJ) using the exact conversion factor (1 joule = 0.001 kilojoule). See the formula, worked examples, conversion table, history, and common uses.
Joules to Kilojoules converter
Converter
Energy Converter
Convert joules, calories, kilowatt-hours, BTU, electronvolts, therms, and mechanical work units.
Context values
Some unit families need one extra value for a physically accurate result.
Kilojoule is 1000 joule units.
Result = input x 1 / 1000.
Energy uses the joule as the base unit.
Reserved above the conversion cards and below the converter.
About Converting Joules to Kilojoules
Joule and Kilojoule both measure energy, and the conversion is especially useful for comparing nutrition labels (kilocalories) against scientific energy figures (joules). Joule is most often used for physics and engineering energy calculations.
Formula
kilojoules = joules × 0.001
This factor comes from each unit's defined relationship to the category's base unit: 1 joule equals 1 base unit, and 1 kilojoule equals 1000 base units, so dividing one by the other gives the direct joule-to-kilojoule factor of 0.001.
Simple example
1 J × 0.001 = 0.001 kJ
1 joule = 0.001 kilojoules.
Real-world example
100 J × 0.001 = 0.1 kJ
100 joules = 0.1 kilojoules.
Conversion table
| Joule (J) | Kilojoule (kJ) |
|---|---|
| 1 J | 0.001 kJ |
| 10 J | 0.01 kJ |
| 100 J | 0.1 kJ |
| 500 J | 0.5 kJ |
| 1,000 J | 1 kJ |
| 2,000 J | 2 kJ |
Reverse conversion: Kilojoule to Joule
0.001 kJ × 1000 = 1 J
0.001 kilojoules = 1 joule.
joules = kilojoules × 1000
For a page dedicated to this direction, see Kilojoule to Joule.
Understanding the Joule
Joule (J) measures energy. The joule is the SI derived unit of energy, defined as the work done by one newton acting over one meter. It is classified as a SI derived unit.
Where it's used: Standard scientific and engineering energy unit worldwide
Uses of the Joule today
- Physics and engineering energy calculations
- Electrical energy in scientific contexts (though kWh is used for billing)
- Nutrition labeling in some countries (alongside or instead of calories)
History of the Joule
The joule is named after English physicist James Prescott Joule, whose experiments in the 1840s established the mechanical equivalent of heat — demonstrating that mechanical work and heat are interchangeable forms of energy, foundational to the first law of thermodynamics.
The unit itself was formally adopted as the SI derived unit of energy by international agreement as part of the broader SI system, defined coherently from the base units (kg, m, s) rather than as an independent physical standard.
Historical origin. Joule 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.
Joule — sources
- BIPM SI Brochure — Coherent derived units — Defines the joule as the SI unit of energy.
Reserved between the cards and the FAQ so the page stays balanced.
Frequently asked questions
How many kilojoules are in 1 joule?
1 joule equals 0.001 kilojoules, using the exact defined conversion factor rather than an estimate.
How do I convert joule to kilojoule?
Multiply the joule value by 0.001. The converter above does this instantly to whatever precision you set.
How do I convert kilojoule back to joule?
Use the reverse factor: 1 kilojoule equals 1,000 joules. You can also use the swap control in the converter above to flip the direction instantly.
What is a joule?
Joule (J) measures energy. The joule is the SI derived unit of energy, defined as the work done by one newton acting over one meter. It is classified as a SI derived unit.
What is a kilojoule?
Kilojoule (kJ) is a unit of energy.
Is the joule to kilojoule conversion exact?
Yes. Both joule and kilojoule are defined by fixed standards rather than physical artifacts, so the factor of 0.001 used above is exact to as many digits as you choose to display.
Where is the joule still used today?
Standard scientific and engineering energy unit worldwide