Calorie (cal)

The heat required to raise one gram of water by one degree Celsius, introduced by French chemists in the early nineteenth century.

The calorie is the source of one of the most persistent confusions in everyday measurement. The scientific calorie is a very small quantity of heat, while the Calorie printed on food packaging, conventionally capitalised, is a kilocalorie a thousand times larger. British labelling law requires both kilojoules and kilocalories to be shown, which is why nutritional panels carry two numbers that differ by a factor of roughly 4.2.