I see heats of combustion given in kg [dot] calories per gram molecular weight (Handbook of Chemistry and Physics, 1969-1970). I don't know how that makes sense, as I don't know what a kg cal might be. I would have expected units of energy per mass (calories per gram) or energy per mole. What is a kg cal, and what is wrong in my thinking?

  • $\begingroup$ "gram" in the title of this thread should be "gram molecular weight". I previously misread the units in the table in the Handbook. $\endgroup$ Jun 22, 2015 at 22:15

1 Answer 1


$\mathrm{kg~cal}$ probably represents the kilogram calorie (especially given that it is an old book). Nowadays it is usually just known as the Calorie, usually abbreviated $\mathrm{kcal}$. It is roughly the amount of energy required to raise the temperature of $1~\mathrm{kg}$ of water by $1~\mathrm{^\circ C}$. Since this is temperature and pressure dependent, there are slight variations in the actual definition depending on the context in which it is used.

  • $\begingroup$ Okay, thanks. Then a kg cal is the same as a "food calorie". Back in the day, we just called a calorie a calorie and a kilocalorie a kilocalorie. BTW I find that paraffin wax has about the same energy content as food fat. $\endgroup$ Jun 21, 2015 at 20:49
  • $\begingroup$ I sincerely doubt that kilogram-calorie is "nowadays known as the Calorie"... the two don't have the same dimensions. Perhaps you meant to say that the kilogram-calorie-per-gram is now known as the Calorie? $\endgroup$
    – user541686
    Jun 22, 2015 at 0:48
  • $\begingroup$ The kilogram calorie is a unit of energy, not energy times mass, despite the name. It is named 'kilogram' to distinguish it from the 'normal' calorie which is the energy required to heat $1~\mathrm{g}$ of water by $1~^\circ C$. However, the kilogram calorie ($\mathrm{kcal}$) has become the 'normal' calorie due to its extensive usage in food labelling and is now colloquially referred to just as the calorie. $\endgroup$
    – bon
    Jun 22, 2015 at 20:19
  • $\begingroup$ Presumably the "kilogram" in the name "kilogram calorie" refers to heating a kilogram of water, rather than heating a gram of water. $\endgroup$ Jun 22, 2015 at 22:13
  • $\begingroup$ @JohnSurname That is correct. $\endgroup$
    – bon
    Jun 24, 2015 at 13:56

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.