Joules to Kilojoules
Convert joules to kilojoules instantly — one joule is 0.001 kilojoules.
| Joules | Kilojoules |
|---|---|
| 1 J | 0.001 kJ |
| 2 J | 0.002 kJ |
| 3 J | 0.003 kJ |
| 5 J | 0.005 kJ |
| 10 J | 0.01 kJ |
| 25 J | 0.025 kJ |
| 50 J | 0.05 kJ |
| 100 J | 0.1 kJ |
How to use
- Enter a value in the Joules field.
- The Kilojoules figure appears immediately — no button to press.
- Typing in the Kilojoules field converts back to Joules the same way.
- Use the reference table for the values that come up most often.
Joules to Kilojoules formula
To convert joules to kilojoules, multiply by 0.001.
kJ = J × 0.001
Put another way, one joule is exactly 1/1,000 of a kilojoule.
Why this factor: the joule and the kilojoule are both defined against the joule, so dividing one definition by the other gives 0.001 directly. Both definitions are set out in full below.
To go the other way, divide by 0.001 — or use the Kilojoules to Joules converter, which is the same calculation reversed.
Worked example
Start with 100 J. Multiply by 0.001 to get 0.1 kJ.
100 J × 0.001 = 0.1 kJ
The converter above does this for any value you type, in either direction.
What is a joule?
The joule is the SI unit of energy — the work done by one newton acting over one meter.
Definition: Defined as 1 N·m, equivalently one watt-second. Named after James Prescott Joule.
Where it is used: All scientific energy calculations, food energy on European labels, and mechanical work.
What is a kilojoule?
A kilojoule is 1,000 joules, the metric unit of food energy.
Definition: Exactly 1,000 joules.
Where it is used: Nutrition labels in Europe and Australia, and heating and cooling loads.
Joules vs Kilojoules
The joule is an SI unit while the kilojoule is a metric unit. That is why the factor between them is not a round number: the two systems were defined independently and only later tied together by exact agreement.
Frequently asked questions
How many kilojoules are in a joule?
One joule equals 0.001 kilojoules.
What is the formula to convert joules to kilojoules?
Multiply the number of joules by 0.001. To reverse it, divide by 0.001 instead.
How do I convert kilojoules back to joules?
Divide by 0.001, which is the same as multiplying by 1000.
How many kilojoules are in 10 joules?
10 J works out to 0.01 kJ.
How is the joule defined?
Defined as 1 N·m, equivalently one watt-second. Named after James Prescott Joule.