Calories are a measure of energy. They refer to the amount of energy in foods and beverages, or the amount of energy you burn exercising.

Depending on where you live, energy may also be measured in kilocalories (kcal) and kilojoules (kJ).

This can create confusion, especially if you’re counting calories or comparing the calorie contents of various foods and drinks.

This article explains what calories are and how they compare with kcal and kJ, as well as how to convert between the two.

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When it comes to calories, they may be “small” or “large.”

If the “c” in calories is uppercase, it indicates a large calorie, and if it’s lowercase, it indicates a small calorie.

A large calorie estimates the amount of energy required to raise the temperature of 1 kg (2.2 pounds) of water by 1°C (34°F) (1).

On the other hand, a small calorie estimates the amount of energy it takes to raise the temperature of 1 gram (0.035 ounces) of water by 1°C (34°F) (1).

By these definitions, 1 large calorie equals 1,000 small calories, as 1 kg equals 1,000 grams.

To avoid confusion between large and small calories, it’s thought that the term kilocalorie — the prefix “kilo” meaning 1,000 — was created to refer to a large calorie (1).

However, the term small calorie is rarely used today outside of physics and chemistry research.

Instead, the terms calories — capitalized or not — and kcal are used interchangeably and refer to the same amount of energy in relation to food or energy burned with exercise.

Therefore, you don’t need to convert them, as 1 kilocalorie equals 1 calorie in nutrition.

Calories may also be expressed as kilojoules (kJ).

One calorie (kcal) equals 4.18 kJ or 4,184 joules (J) (1).

To convert from calories to kJ, multiple calories by 4.18. Conversely, to convert from kJ to calories, divide kJ by 4.18.

For example, a medium-sized banana (118 grams) provides 105 calories (kcal) or 439 kJ (2).

Summary

When it comes to nutrition and exercise, kilocalories (kcal) and calories equal the same amount of energy. Calories may also be expressed as kilojoules (kJ), with one calorie or kcal equaling 4.18 kJ.

Regulations require that food and beverage manufacturers display a nutrition facts label on their products. It lists, among other information, the amount of energy the item contains per serving or weight (3).

The nutrition facts label serves to help inform you about the healthfulness of packaged foods and beverages, as well as whether they contain ingredients that you may need to avoid due to an allergy, intolerance, or personal preference.

Depending on where you live, the nutrition facts panel may express the energy value of a food or beverage in calories, kcal, kJ, or a combination of them.

Below is a list of countries and which label they use for energy (4, 5, 6, 7, 8):

  • United States: Calories
  • Canada: Calories
  • European Union (EU): kJ and kcal
  • Australia and New Zealand: kJ or both kJ and kcal
  • China: kJ

Manufacturers determine the number of calories a food or beverage contains based on the amount of energy-supplying nutrients it contains.

The three main energy-supplying nutrients are protein, carbs, and fats.

Protein and carbs each provide about 4 calories (16.7 kJ) per gram, while fat provides 9 calories per gram (37.6 kJ) (9).

Alcohol also supplies 7 calories (29.3 kJ) per gram.

Manufacturers round to the nearest 1-gram increment, so if you were to quantify the number of calories or kJ from each of the macronutrients, they may add up to be slightly different than the number listed on the nutrition label (4).

Moreover, the nutrition labels of foods that contain fiber, which is classified as a carbohydrate, may contain fewer calories than what you calculate.

This is because fiber, depending on the type, is either indigestible or poorly digested, thus contributing zero or few calories (10).

Summary

Most packaged foods and beverages contain a nutrition label that lists the amount of energy the item contains in calories, kcal, kJ, or some combination of these.

Calories are a measure of energy in nutrition and exercise.

Calories and kcal are used interchangeably and refer to the same amount of energy.

Calories or kcal may also be measured in kJ, with one calorie (kcal) equaling 4.18 kJ.