Quick answer
A calorie is a unit of energy. The small calorie (cal) equals 4.184 joules. The food calorie (kcal, or Calorie) is 1,000 small calories, which equals 4.184 kJ.
Calorie definition
The thermochemical calorie is defined as exactly 4.184 joules. Historically it was the heat needed to raise one gram of water by one degree Celsius. The kilocalorie (kcal) is 1,000 of these, or 4,184 joules, and roughly equals the heat needed to warm one kilogram of water by one degree Celsius.
Cal vs kcal vs Calorie
| Name | Symbol | Energy |
|---|---|---|
| Small calorie | cal | 4.184 J |
| Kilocalorie (food calorie) | kcal or Cal | 4,184 J = 4.184 kJ |
| Joule | J | 0.239 cal |
When a food label or a diet app says "calories", it means kilocalories. Some sources write Calories with a capital C to show this. When a chemistry textbook says calories, check whether it means cal or kcal.
A short history
The calorie began as a heat unit in 19th-century French physics. The chemist and physician Nicolas Clément introduced it in lectures in the 1820s. In the 1890s the American chemist Wilbur Olin Atwater used it to measure the energy in foods and developed the energy factors that most nutrition labelling still builds on: about 4 kcal per gram of carbohydrate and protein, and 9 kcal per gram of fat.
Where do the calories in food come from?
Originally, food was burned in a device called a bomb calorimeter and the heat released was measured. Because the body does not absorb everything it eats, scientists later adjusted these values for digestion. Today, labels usually add up energy from the nutrients: 4 kcal per gram for carbohydrate and protein, 9 for fat, 7 for alcohol and 2 for dietary fibre.
What calories do and do not tell you
A calorie count measures energy and nothing else. 100 kcal of nuts and 100 kcal of soft drink contain the same energy, yet they differ in protein, fibre, vitamins and how filling they are. Calories are useful for comparing amounts of energy, but they are not a measure of how healthy a food is.
Converting calories to kJ
Multiply calories (kcal) by 4.184 to get kJ: 500 kcal is 2,092 kJ, and 2,000 kcal is 8,368 kJ. Use the calories to kJ converter or read how many kJ are in a calorie.
Frequently asked questions
What is a calorie in simple terms?
A calorie is a measure of energy. In food, one calorie (kcal) equals 4.184 kilojoules.
Is a calorie the same as a kilocalorie?
On food labels, yes. Strictly speaking, a small calorie (cal) is one thousandth of a kilocalorie (kcal).
Who invented the calorie?
Nicolas Clément introduced the calorie as a unit of heat in the 1820s. Wilbur Atwater later applied it to the energy content of food.
How many calories should I eat a day?
It depends on your age, sex, size and activity. See our guide on how many kJ you should eat per day.
Last updated 11 Oct 2026. How we calculate.