Joules to Kilojoules Converter
Convert joules to kilojoules (J to kJ) and back, live. 1 kJ = 1000 J, always. Includes the full energy unit ladder, quick tables, and real-world context.
J to kJ converter
Type in either box. Converting J to kJ divides by 1000; converting kJ to J multiplies by 1000.
Energy unit ladder
Your value, expressed in every energy unit at once. Based on 2500 J.
Calorie values use the thermochemical calorie (1 cal = 4.184 J exactly). The electronvolt uses the exact 2019 SI value 1 eV = 1.602176634e-19 J.
Formulas
kilojoules = joules / 1000
joules = kilojoules x 1000
The SI prefix kilo always means 10^3, so the J to kJ conversion is an exact decimal shift. No rounding is involved unless you choose fewer display decimals.
Where you meet joules and kilojoules
Nutrition labels
Labels in Australia, New Zealand, the EU and many other countries list food energy in kJ. A 2000 kcal reference diet is 8368 kJ. Divide kJ by 4.184 to estimate food calories.
Physics
The joule is the SI unit of energy: lifting an apple (~100 g) one metre takes about 1 J. A lightning strike releases roughly 1,000,000 kJ.
Electricity bills
Meters count kWh, not kJ: 1 kWh = 3600 kJ = 3,600,000 J. A 5000 mAh phone battery holds about 66.6 kJ.
Quick conversion tables
Joules to kilojoules
| Joules (J) | Kilojoules (kJ) |
|---|
Reference values
| Reference | Value |
|---|---|
| 1 food kcal | 4.184 kJ |
| 2000 kcal (daily reference) | 8368 kJ |
| 1 kWh (electricity) | 3600 kJ |
| 1 Wh | 3.6 kJ |
| 1 MJ | 1000 kJ |
| Phone battery 5000 mAh at 3.7 V | 66.6 kJ |
| Lightning strike (approx) | 1,000,000 kJ |
Which tool for which question
- This page: fast J to kJ and kJ to J conversion plus the common energy unit ladder.
- energy-unit-converter: 12 energy units at once (BTU, therm, toe, fuel equivalents).
- kw-to-joules: power x time questions, e.g. how many joules does a 2 kW heater use in an hour.
- energy-cost-calculator: electricity bill math: kWh x tariff.
Frequently asked questions
How do I convert joules to kilojoules?
Divide the number of joules by 1000. Formula: kilojoules = joules / 1000. Example: 2500 J / 1000 = 2.5 kJ.
How many kilojoules are in 1000 joules?
Exactly 1 kilojoule. 1000 J / 1000 = 1 kJ.
How do I convert kilojoules back to joules?
Multiply the number of kilojoules by 1000. Formula: joules = kilojoules x 1000. Example: 2.5 kJ x 1000 = 2500 J.
What is the formula for joules to kJ?
kJ = J / 1000 and J = kJ x 1000. The kilo prefix always means a factor of 1000 in the SI system.
How many joules are in one kilojoule?
1000 joules. 1 kJ = 1000 J exactly.
How many kJ is 5000 joules?
5 kJ. 5000 J / 1000 = 5 kJ.
How many kJ is 2500 joules?
2.5 kJ. 2500 J / 1000 = 2.5 kJ.
How many kilojoules are in 1 food calorie (kcal)?
1 kilocalorie (thermochemical) = 4.184 kJ exactly. So 2000 kcal = 8368 kJ.
How many kJ are in a 2000 calorie diet?
8368 kJ. 2000 kcal x 4.184 = 8368 kJ. This is the reference daily intake printed on many nutrition labels.
How many kilojoules are in 1 kWh?
3600 kJ. 1 kWh = 3600000 J = 3600 kJ.
What is a joule in physics?
The joule is the SI derived unit of energy: 1 J = 1 kg m2/s2. It is the work done when a force of 1 newton moves an object 1 metre.
Why do nutrition labels use kJ instead of calories?
Food labelling rules in Australia, New Zealand, the EU and many other countries require energy in kilojoules, with calories optional. To estimate food calories from kJ, divide by 4.184.
Which is bigger, a joule or a kilojoule?
A kilojoule is bigger: 1 kJ = 1000 J.
What is 1 electronvolt in kilojoules?
1 eV = 1.602176634e-22 kJ, exact by the 2019 SI definition of the elementary charge.
Is a megajoule bigger than a kilojoule?
Yes: 1 MJ = 1000 kJ = 1000000 J.
Which tool should I use for watts, time, or electricity bills?
This converter is for energy amounts (J to kJ). For power x time use kw-to-joules, for electricity cost use energy-cost-calculator, and for 12 energy units at once use energy-unit-converter.
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