Conversion from Gigajoules to Calories

Information about the Gigajoule (GJ)

The gigajoule is a unit of energy equal to a billion joules, or 1000 megajoules. Its symbol is GJ. Unlike the smaller prefixed joules it is a working commercial unit: gas and heat are bought and sold in gigajoules, and a household's annual energy use is a number of gigajoules that fits comfortably on one line of a bill.

Heating is where most people meet it. District heating in Denmark and the Netherlands is metered and invoiced in gigajoules, and Canadian natural gas bills are denominated in them. One gigajoule is roughly the heat in 26 cubic metres of natural gas, or in 29 litres of petrol, and a well-insulated house in a cold climate consumes something between 40 and 100 gigajoules a year for space heating and hot water.

The unit also measures what materials cost to make. Producing a tonne of structural steel takes roughly 20 to 25 gigajoules of primary energy, a tonne of cement about 4, and a tonne of primary aluminium close to 200. Those figures are why aluminium is described as stored electricity, and why embodied-energy tables in construction are published in gigajoules per tonne rather than in any smaller unit.

For destructive energy it is the natural scale between the domestic and the catastrophic. A tonne of TNT releases 4.184 gigajoules by definition, and a lightning strike delivers on the order of one to five, though almost all of that goes into heating and shattering the air rather than into whatever it hits. A tonne of hard coal holds about 29 gigajoules of chemical energy.

Human beings appear in the same table. An adult eating about ten megajoules a day consumes roughly 3.7 gigajoules of food energy a year, which is less than a single small car burns in a month. Setting the two figures side by side in one unit is one of the clearest ways to show how much of modern energy use happens outside the body.

Above the gigajoule, national statistics move on to terajoules and petajoules; below it, appliances and vehicles are described in megajoules and kilowatt-hours. The gigajoule sits at the join, which is why energy regulators and utilities converge on it: it is large enough for a building and small enough for a bill.

One gigajoule equals 1,000,000,000 joules, about 277.8 kilowatt-hours, or about 239,000 kilocalories.

Information about the Calorie (cal)

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.