The name calorie stands for two units of energy.

You can name it a small calorie or gram calorie (cal). It’s the amount of energy to raise the temperature of one gram of water by one degree Celsius at a pressure of one atmosphere.

While the large calorie, kilogram calorie, dietary calorie, nutritionist’s calorie, nutritional calorie, Calorie (capital C) or food calorie (Cal) is the amount of energy to raise the temperature of one kilogram of water by one degree Celsius. Thus, the large calorie is equal to 1,000 small calories or one kilocalorie (kcal).

So, we hear about calories all the time: How many calories are in this cookie?

How many are burned by doing 100 jumping jacks, or long-distance running, or fidgeting?

But what is a calorie, really? And how many of them do we actually need to lose weight?

TED Series

Emma Bryce explains in that TED video how a few different factors determine the recommended amount for each person.