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 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?

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 = 0.23900574 kilocalorie (dietary)

1 kilocalorie (dietary) = 4,184 joule

How to convert kilojoule to kilocalorie (dietary)

To convert kilojoule to kilocalorie (dietary), multiply the value by 0.23900574.

To convert kilocalorie (dietary) back to kilojoule, 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: kilojoule to kilocalorie (dietary)

Common values

Available Energy units

• joule conversions
• kilojoule conversions
• kilowatt hour conversions
• calorie (thermochemical) conversions
• kilocalorie (dietary) conversions
• watt hour conversions
• megajoule conversions
• electronvolt conversions
• British thermal unit conversions
• therm (US) conversions

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.