Calorie (nutritional) to Erg

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.

Lowercase cal denotes a small calorie; a kilocalorie is 1,000 small calories of the same variant. A food Calorie (Cal), however, already means one kilocalorie. In this converter, 200 food Calories = 200 nutritional kcal = 836.8 kJ. The separate small thermochemical calorie selection uses 4.184 J per cal th, while each nutritional selection uses 4,184 J per Cal or kcal.

This converter does not determine a daily energy requirement; it only converts a supplied energy quantity. For example, 2,000 food Calories means 2,000 nutritional kcal, or 8,368 kJ, using the selected convention. That conversion example is not a personal intake recommendation.

Kilojoules are an SI energy unit, while kilocalories remain common for food-energy quantities. For the food-energy convention used here, multiply kcal by 4.184 to get kJ, or divide kJ by 4.184 to get kcal. For example, 2,000 kcal = 8,368 kJ. The food Calorie (Cal) and nutritional kcal selections use the same conversion factor.

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.

Astrophysics literature built decades of reference data in CGS units before SI became dominant. Key constants like solar luminosity (3.828 × 10³³ erg/s) and supernova energy (10⁵¹ erg, called a "foe") are baked into textbooks and databases. Switching to SI would require rewriting thousands of reference values, so the field maintains CGS by convention.

A core-collapse supernova releases roughly 3 × 10⁵³ ergs total, of which about 99% escapes as neutrinos. The visible light and kinetic energy of the ejected shell account for about 10⁵¹ ergs — a unit so common in astrophysics it has its own name: one "foe" (ten to the Fifty-One Ergs). In joules, that is 10⁴⁴ J, or the Sun's total output over 10 billion years.

An erg per second is the CGS unit of power, equivalent to 10⁻⁷ watts. Astronomers quote stellar luminosities in ergs per second because the numbers align well with astrophysical scales: the Sun emits 3.846 × 10³³ erg/s, and supernovae peak at ~10⁴³ erg/s. Using watts would give the same exponents minus seven — less tidy for a field that already juggles 40-digit numbers daily.

CGS (centimeter-gram-second) is a metric system that predates SI, formalised in the 1870s. It derives mechanical units from cm, g, and s: force in dynes (g·cm/s²) and energy in ergs (dyne·cm). CGS was standard in physics until the mid-20th century, and its Gaussian variant remains preferred in electromagnetism and astrophysics because Maxwell's equations take a simpler form.

One erg is 10⁻⁷ joules — roughly the kinetic energy of a mosquito in flight or the energy of a single grain of sand falling one centimeter. You would need about 10 million ergs to equal one joule, or 42 billion ergs to match the energy in a single dietary Calorie. The erg is useful precisely because atomic and astronomical quantities span so many orders of magnitude.