The small calorie or gram calorie (symbol: cal) is the approximate amount of energy needed to raise the temperature of one gram of water by one degree Celsius at a pressure of one atmosphere. The large calorie or kilogram calorie (symbol: Cal), also known as the food calorie and similar names, is defined in terms of the kilogram rather than the gram. It is equal to 1000 small calories, 1 kilocalorie (symbol: kcal).
Although these units are part of the metric system, they have been superseded in the International System of Units by the joule. One small calorie is approximately 4.2 joules (so one large calorie is about 4.2 kilojoules).
The calorie was first defined by Nicolas Clément in 1824 as a unit of heat energy, and entered French and English dictionaries between 1841 and 1867. The word comes from Latin calor meaning “heat”.
For most of us it’s all about food.
We measure the amount of energy contained in an item of food in calories, just as we measure the weight of that item of food in kilograms. When we eat and drink, we’re putting energy (calories) into our bodies. Our bodies then use up that energy, and the more physical activity we do, the more energy (calories) we use.
The calorie content of many foods is stated on the packaging in the nutrition label, which you will often find on the back or side of the packaging. This information will appear under the “Energy” heading. The calorie content is often given in kcals, and also in kilojoules. You can use the calorie information to assess how a particular food fits into your daily calorie intake. As a guide, the average mans needs 2,500kcal (10,500kJ) to maintain his weight, and the average woman needs 2,000kcal (8,400kJ).
That probably doesn’t include wine.
(That bit’s not in wiki or NHS advice though ).