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.
What is kilocalorie (dietary)?
A kilocalorie, written as Calorie with a capital C in nutrition contexts, is a unit of energy equal to 1,000 thermochemical calories. It is the standard unit used to express the energy content of food on nutrition labels globally.
Real-world uses
Kilocalories are the standard unit for food energy labelling in the United States (labelled as "Calories" with a capital C), dietary planning, and exercise energy expenditure. A typical adult needs about 2,000 kcal/day. Fitness trackers report calories burned in kcal.
History
Wilbur Atwater pioneered the use of kilocalories for food energy measurement in the 1890s. The confusing convention of using a capital-C "Calorie" to mean kilocalorie originated in American nutritional science and persists on US food labels today.
Common mistakes
The most common mistake is confusing Calories (kcal) with calories (cal)—they differ by a factor of 1,000. When food labels say "100 Calories," they mean 100 kilocalories. Also, assuming all macronutrients have the same caloric density (fat has 9 kcal/g vs 4 kcal/g for protein and carbohydrates).
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 joule = 0.00023901 kilocalorie (dietary)
1 kilocalorie (dietary) = 4,184 joule
How to convert joule to kilocalorie (dietary)
To convert joule to kilocalorie (dietary), multiply the value by 0.00023901.
To convert kilocalorie (dietary) back to joule, multiply by 4,184.
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: joule to kilocalorie (dietary)
| joule | kilocalorie (dietary) |
|---|---|
| 0.1 | 0.0000239 |
| 0.5 | 0.0001195 |
| 1 | 0.00023901 |
| 2 | 0.00047801 |
| 5 | 0.00119503 |
| 10 | 0.00239006 |
| 25 | 0.00597514 |
| 50 | 0.01195029 |
| 100 | 0.02390057 |
| 250 | 0.05975143 |
| 500 | 0.11950287 |
| 1,000 | 0.23900574 |
Common values
| joule | kilocalorie (dietary) | |
|---|---|---|
| A food calorie (1 kcal) | 4,184 joule | 1 kilocalorie (dietary) |
| A AA battery | 9,360 joule | 2.23709369 kilocalorie (dietary) |
| Boiling a kettle | 420,000 joule | 100.38240918 kilocalorie (dietary) |
| Daily human diet (2000 kcal) | 8,368,000 joule | 2,000 kilocalorie (dietary) |
| 1 gallon of gasoline | 131,760,000 joule | 31,491.3957935 kilocalorie (dietary) |
Available Energy units
More joule conversions
- Convert joule to kilojoule
- Convert joule to kilowatt hour
- Convert joule to calorie (thermochemical)
- Convert joule to kilocalorie (dietary)
- Convert joule to watt hour
- Convert joule to megajoule
- Convert joule to electronvolt
- Convert joule to British thermal unit
- Convert joule to therm (US)
Assumption: calorie means thermochemical calorie, and therm means US therm.