joule (J)
1 joule = 0.001 kilojoules
What is joule?
The joule is the SI base unit of energy. It is the standard unit for work, heat, and electrical energy in physics, engineering, and scientific measurement worldwide.
Notes: Base unit for energy
Real-world uses
The joule is the SI unit of energy used in physics, chemistry, and nutrition (food energy in some countries is labelled in kilojoules). Electrical energy, mechanical work, and heat transfer are all measured in joules. One joule is the energy to lift a small apple about 1 metre.
History
Named after James Prescott Joule, the English physicist who demonstrated the mechanical equivalent of heat in the 1840s, establishing that heat and mechanical work are interchangeable forms of energy. The joule was adopted as the SI energy unit in 1960.
Common mistakes
Confusing joules with watts—a joule is a unit of energy, while a watt is a unit of power (1 W = 1 J/s). Also, assuming "calorie" and "joule" are equivalent without conversion: 1 cal = 4.184 J.
When is this conversion used?
Electricity bills use kilowatt-hours while physics calculations use joules. Converting between them is essential for energy audits, comparing appliance efficiency, and understanding power consumption. Nutritional information is labelled in kilocalories in some countries and kilojoules in others, making this conversion important for dietary planning and food science.
How to convert joule to kilojoule
To convert joule to kilojoule, multiply the value by 0.001.
Conversion formula (from base unit)
x * 1.0
Measurement standards
The joule is the SI derived unit of energy, defined as one kilogram-metre-squared per second-squared (kg·m²/s²). The kilowatt-hour, equal to exactly 3.6 megajoules, is accepted for use with the SI by the BIPM for practical electricity metering.
Did you know?
A single bolt of lightning releases roughly 1 to 5 gigajoules of energy, yet only a small fraction reaches the ground as electrical current. Most of the energy dissipates as light, heat, and the thunder shockwave.
| joules | kilojoules |
|---|---|
| 0.1 | 0.0001 |
| 0.5 | 0.0005 |
| 1 | 0.001 |
| 2 | 0.002 |
| 5 | 0.005 |
| 10 | 0.01 |
| 25 | 0.025 |
| 50 | 0.05 |
| 100 | 0.1 |
| 250 | 0.25 |
| 500 | 0.5 |
| 1,000 | 1 |
Common values
| joules | kilojoules | |
|---|---|---|
| A food calorie (1 kcal) | 4,184 joules | 4.184 kilojoules |
| A AA battery | 9,360 joules | 9.36 kilojoules |
| Boiling a kettle | 420,000 joules | 420 kilojoules |
| Daily human diet (2000 kcal) | 8,368,000 joules | 8,368 kilojoules |
| 1 gallon of gasoline | 131,760,000 joules | 131,760 kilojoules |
Full joule conversion table
| Unit | Value | Factor |
|---|---|---|
| kilojoule | 1 J = 0.001 kJ | × 0.001 |
| kilowatt hour | 1 J = 2.777778e-07 kWh | × 2.777778e-07 |
| calorie (thermochemical) | 1 J = 0.23900574 cal | × 0.23900574 |
| kilocalorie (dietary) | 1 J = 0.00023901 kcal | × 0.00023901 |
| watt hour | 1 J = 0.00027778 Wh | × 0.00027778 |
| megajoule | 1 J = 0.000001 MJ | × 0.000001 |
| electronvolt | 1 J = 6.241509e+18 eV | × 6.241509e+18 |
| British thermal unit | 1 J = 0.00094782 BTU | × 0.00094782 |
| therm (US) | 1 J = 9.480434e-09 therm | × 9.480434e-09 |