What is kilojoule?
A kilojoule is a unit of energy equal to 1,000 joules. It is commonly used in nutrition to express the energy content of food, as well as in thermodynamics and mechanical engineering.
Real-world uses
Kilojoules are the standard energy unit for food nutrition labelling in Australia, New Zealand, and parts of Europe (alongside kilocalories). A typical adult daily intake is about 8,700 kJ. They are also used in chemistry for reaction enthalpies and combustion energies.
History
The kilojoule became important in nutrition labelling as countries adopted SI-based energy reporting. Australia was among the first to mandate kJ on food labels in the 1970s. The EU requires both kJ and kcal on nutrition labels.
Common mistakes
Confusing kilojoules with kilocalories—1 kcal = 4.184 kJ. Food labels showing "2000 kJ" are about 478 kcal, not 2000 kcal. Also, some people incorrectly assume kJ and kW are the same thing; one is energy, the other is power.
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.
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.
Worked examples
1 kilojoule = 1,000 joule
1 joule = 1 joule
How to convert kilojoule to joule
To convert kilojoule to joule, multiply the value by 1,000.
To convert joule back to kilojoule, multiply by 0.001.
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.
Quick reference: kilojoule to joule
| kilojoule | joule |
|---|---|
| 0.1 | 100 |
| 0.5 | 500 |
| 1 | 1,000 |
| 2 | 2,000 |
| 5 | 5,000 |
| 10 | 10,000 |
| 25 | 25,000 |
| 50 | 50,000 |
| 100 | 100,000 |
| 250 | 250,000 |
| 500 | 500,000 |
| 1,000 | 1,000,000 |
Common values
| kilojoule | joule | |
|---|---|---|
| A food calorie (1 kcal) | 4.184 kilojoule | 4,184 joule |
| A AA battery | 9.36 kilojoule | 9,360 joule |
| Boiling a kettle | 420 kilojoule | 420,000 joule |
| Daily human diet (2000 kcal) | 8,368 kilojoule | 8,368,000 joule |
| 1 gallon of gasoline | 131,760 kilojoule | 131,760,000 joule |
Available Energy units
More kilojoule conversions
- Convert kilojoule to joule
- Convert kilojoule to kilowatt hour
- Convert kilojoule to calorie (thermochemical)
- Convert kilojoule to kilocalorie (dietary)
- Convert kilojoule to watt hour
- Convert kilojoule to megajoule
- Convert kilojoule to electronvolt
- Convert kilojoule to British thermal unit
- Convert kilojoule to therm (US)
Assumption: calorie means thermochemical calorie, and therm means US therm.