Before scientists realized that heat is transferred energy, heat was measured in terms of its ability to raise the temperature of water. Thus, the calorie (cal) was defined as the amount of heat that would raise the temperature of 1 g of water from 14.50C to 15.50C.
In 1948, the scientific community decided that since heat (like work) is transferred energy, the SI unit for heat should be the one we use for energy, namely, the joule. The calorie is now defined to be 4.1860 J (exactly). The “calorie” used in nutrition is really is kilocalorie.
1. Calorie (cal): It is the amount of heat required to increase the temperature of 1 g of water from 14.5oC to 15.5oC. (1 cal=4.186 J)
2. Kilocalorie (kcal): It is the amount of heat required to raise the temperature of 1 kg of pure water through 1oC.
3. British Thermal Units (BTU): It is defined as the quantity of heat required to raise the temperature of 1 pound (lb) of pure water through 1oF. It is also referred to as pound-degree Fahrenheit unit.
Conversion: 1 B.T.U = 251.996 Cal ( or 252 Cal)
1 calorie = 4.186 Joule ( or 4.2 joule)
1 calorie = 4.186 Joule ( or 4.2 joule)