Calories to joules – and joules to calories

Formula

Multiply the value in calories by 4.184 to get joules.
J = cal × 4.184

Multiply the value in joules by 0.23900574 to get calories.
cal = J × 0.23900574

How to convert calories to joules

Type a value in the field above and the result is calculated instantly. Use the arrow button to swap direction, or pick other units from the lists.

Doing it in your head

A rough conversion for when the calculator is not to hand. The error comes last, and it holds in both directions at that order of magnitude.

• Calories to joules: × 4 + 5%. That gives 4.2 instead of 4.184, an error of 0.38%.
• Joules to calories: ÷ 4 − 5%. That gives 0.2375 instead of 0.239006, an error of 0.63%.

About the units

Calories. A calorie is not one unit. The thermochemical calorie is exactly 4.184 joules and the international table calorie 4.1868 joules; the conversion here uses the thermochemical one. The calorie is absent from the EU units directive and so is not a legal unit of measurement, while the kilocalorie may appear on a pack alongside the kilojoule. The real trap is the name: food “calories” are in fact kilocalories, a thousand times larger. A banana of 100 calories holds 100,000 calories in the strict sense.

Joules. The joule is the SI coherent unit of energy, work and heat, fixed by the General Conference on Weights and Measures: one joule is a kilogram times a square metre per second squared, the work done by a force of one newton over one metre. It is a legal unit throughout the EU and the natural one in physics, chemistry and engineering, while food labelling counts in kilojoules. An electronvolt is 1.602176634 × 10⁻¹⁹ joules. The trap is the newton metre: same dimension, but torque is not energy.