Convert Joules to Kilojoules
Joules to kilojoules is the conversion you reach for when the numbers get big — the energy in food, a chemical reaction, or a battery is often thousands of joules, and kilojoules keep the figure short. One joule is exactly 0.001 kilojoules, because 'kilo' means a thousand, so you just shift the decimal point three places.
Joules to Kilojoules conversion table
| Joules (J) | Kilojoules (kJ) |
|---|---|
| 1 | 0.001 |
| 2 | 0.002 |
| 3 | 0.003 |
| 5 | 0.005 |
| 10 | 0.01 |
| 20 | 0.02 |
| 50 | 0.05 |
| 100 | 0.1 |
| 500 | 0.5 |
| 1,000 | 1 |
The formula
To convert joules to kilojoules, multiply the number of joules by 0.001 (or just divide by 1,000):
kilojoules = joules × 0.001
A worked example
Say you have 5,000 joules and want it in kilojoules:
5000 × 0.001 = 5 kJ
So 5,000 joules is exactly 5 kilojoules.
Why the number is exact
The joule and the kilojoule are the same unit — the only difference is the "kilo" prefix, which is defined to mean exactly a thousand. There's no measurement or rounding involved, so one joule is exactly 0.001 kilojoules, or one thousandth. Converting between them is just moving the decimal point.
When you'll use it
Reading energy figures on food labels or in physics and chemistry problems, where values in the thousands of joules are easier to handle as kilojoules, and comparing energy amounts that would otherwise carry a lot of digits.
Frequently asked questions
How many joules are in a kilojoule?
Exactly 1,000 joules. The 'kilo' prefix means a thousand, so a kilojoule is a thousand joules with nothing rounded.
How many kilojoules are in a joule?
Exactly 0.001 kilojoules, which is one thousandth. Since a kilojoule is 1,000 joules, one joule is 1/1000 of a kilojoule.
Is the joules to kilojoules conversion exact?
Yes. The 'kilo' prefix is defined as exactly a thousand, so one joule is exactly 0.001 kilojoules — an exact figure, not a rounded one.