| Calories (cal) | Joules (J) |
|---|---|
| 1 Calorie | 4.184 J |
| 2 Calories | 8.368 J |
| 3 Calories | 12.552 J |
| 4 Calories | 16.736 J |
| 5 Calories | 20.92 J |
| 10 Calories | 41.84 J |
| 20 Calories | 83.68 J |
| 25 Calories | 104.6 J |
| 50 Calories | 209.2 J |
| 100 Calories | 418.4 J |
| Reference | Calories (cal) | Joules (J) |
|---|---|---|
| One food calorie (kcal) | 1000 cal | 4184 J |
| An AA alkaline battery | 2390.06 cal | 10000 J |
| Daily adult food intake | 2000000 cal | 8368000 J |
| One unit on an electricity bill | 860421 cal | 3600000 J |
| A lightning strike | 239005736 cal | 1 × 109 J |
The calorie is a unit of energy equal to 4.184 joules. Its symbol is cal. It was originally defined as the heat needed to raise the temperature of one gram of water by one degree Celsius, and it is not an SI unit, though it survives almost everywhere that food and heat are discussed.
The original definition was awkward because the answer depends on the starting temperature. Water takes slightly more heat to warm from 14.5 to 15.5 degrees than from 19.5 to 20.5, so several competing calories existed: the fifteen-degree calorie, the mean calorie and the International Steam Table calorie among them. The thermochemical calorie now used is simply defined as exactly 4.184 joules, cutting the link to water entirely and ending the ambiguity.
The greater confusion is one of scale. The calorie printed on food packaging in the United States is not this unit at all but the kilocalorie, a thousand times larger, sometimes distinguished by a capital C. A biscuit described as containing sixty calories in fact contains sixty thousand of the unit described here. The convention is entrenched and unlikely to change, so nutritional figures and physical ones must be read in different registers.
Chemistry retained the calorie long after physics moved on. Enthalpies of reaction, bond energies and heats of formation were tabulated in kilocalories per mole for most of the twentieth century, and older literature and some current American textbooks still are. The equivalent SI figure in kilojoules per mole is larger by a factor of 4.184, and misreading which is meant produces errors that look plausible.
Nutrition science uses it because human energy balance happens to sit at a convenient scale in kilocalories, with a daily requirement of around two thousand. In joules the same figure is roughly eight and a half million, which is accurate but harder to reason with.
Air conditioning and refrigeration in some countries still rate capacity in kilocalories per hour, and heating values of fuels appear in calories per gram in older engineering tables.
One calorie equals exactly 4.184 joules, 0.001 kilocalories, or about 0.00116 watt-hours.
The joule is the SI unit of energy, work and heat. Its symbol is J. One joule is the work done when a force of one newton moves its point of application through one metre, and equally the energy dissipated when one watt of power flows for one second.
That equivalence is the reason the unit exists. Before the nineteenth century, heat and mechanical work were treated as different things measured in different units, heat in calories and work in foot-pounds. James Prescott Joule spent years demonstrating that a fixed amount of mechanical work always produces the same amount of heat, most famously by stirring water with a paddle wheel driven by falling weights and measuring the temperature rise. Establishing that exchange rate, the mechanical equivalent of heat, unified mechanics and thermodynamics into one subject.
Because it is defined through the base units, the joule connects everything. It is a newton metre, a watt second, a coulomb volt and a kilogram metre squared per second squared, and every one of those forms appears in ordinary use depending on which branch of physics is speaking. Electrical, mechanical, thermal and chemical energy are all counted in the same unit, which is what makes energy accounting across a system possible at all.
The scale is human-sized in a useful way. Lifting an apple one metre takes about one joule. A heartbeat uses roughly one joule. Dropping a textbook from a desk releases about ten. Below that, joules become clumsy and the prefixed units take over; above it, kilojoules and megajoules do the same.
Food energy is properly measured in joules, and labels in Australia, New Zealand and the European Union give kilojoules alongside or instead of calories. The calorie survives in general use mainly through habit, and the conversion, one calorie equal to 4.184 joules, is fixed by definition rather than measured.
Nutrition, ballistics, battery capacity, laser output, explosive yield and the energy released by an earthquake are all quoted in joules or its multiples, which allows quantities from completely different domains to be compared directly.
One joule equals one newton metre, one watt second, about 0.239 calories, or about 0.000278 watt-hours.