Calorie (nutritional) to Gigajoule

In the late 19th century, nutritionists adopted the kilocalorie as the practical unit for food energy but dropped the "kilo" prefix in everyday speech. A banana labelled "90 calories" actually contains 90 kilocalories (90,000 small calories). Some labels use a capital "C" (Calorie) to distinguish it from the small calorie, but this convention is inconsistently applied and remains a source of confusion worldwide.

One kcal (kilocalorie) equals 1,000 cal (calories). European and Australian labels typically show energy in both kJ and kcal explicitly. US labels use "Calories" (capital C), which actually means kcal. If a label says 200 Calories, it means 200 kcal = 200,000 small calories = 836.8 kJ. The small calorie (4.1868 J) is rarely seen outside laboratory contexts.

Adults typically need 1,600–2,500 kcal per day depending on sex, age, weight, and activity level. Sedentary women average about 1,800 kcal; active men about 2,500 kcal. Endurance athletes during competition can burn 4,000–8,000 kcal/day. These figures are based on the International Table calorie (4.1868 J), though the thermochemical calorie gives near-identical results in practice.

Australia, New Zealand, and EU member states mandate SI-based labeling, so they use kilojoules (kJ) as the primary energy unit. The US and Canada use kilocalories (labelled as "Calories"). To convert, multiply kcal by 4.1868 to get kJ, or divide kJ by 4.1868 for kcal. A 2,000 kcal daily diet equals 8,374 kJ.

Wilbur Atwater and colleagues in the 1890s used bomb calorimeters to burn food samples and measure heat released. They established that carbohydrates yield ~4 kcal/g, protein ~4 kcal/g, and fat ~9 kcal/g — the Atwater factors still printed on food labels today. Modern methods use chemical analysis and Atwater factors rather than direct calorimetry for every product.

In cold-climate countries, 30–60 GJ per year is common for heating and hot water. A well-insulated modern home in Germany might use 20 GJ; a drafty older home in Canada might use 100+ GJ. Australians use about 40 GJ/year on average. Each gigajoule costs roughly $8–$15 depending on local gas prices.

One tonne of coal holds roughly 24–30 GJ depending on grade. One tonne of crude oil contains about 42–44 GJ. One tonne of LNG holds roughly 54 GJ. One tonne of dry firewood stores about 16 GJ. These figures explain why oil and gas are preferred for transport — they pack more gigajoules per kilogram than solid fuels.

One gigajoule equals 277.78 kWh. At an average electricity price of $0.15/kWh, one gigajoule of electrical energy costs about $42. The same gigajoule from natural gas costs $8–15. This price gap is the main reason gas boilers remain popular for heating in countries with cheap pipeline gas.

A single-aisle jet like the Boeing 737-800 burns about 10–12 GJ per flight hour. A six-hour transatlantic flight on a wide-body aircraft can consume 300–400 GJ of jet fuel. The entire global aviation industry uses roughly 12 billion gigajoules of fuel per year — about 3% of total world energy consumption.

At 2,000 kcal/day (8.4 MJ/day), a person consumes about 3.07 GJ of food energy per year. Over 80 years, that is roughly 245 GJ — equivalent to about 6,000 liters of petrol. Your entire lifetime food energy would fit in a medium-sized fuel tanker, which is a humbling thought.