Simply put, “calorie” is a unit of energy. There are two types of calories:

Small calorie (symbol: cal), aka the Scientific calorie; and,

Large calorie (symbol: Cal, kcal), aka the Nutritional calorie

Mathematically, 1 large calorie (1 Cal, or 1 kcal) = 1,000 small calories (1,000 cal).

When talking about food, you will always encounter the large Calorie, i.e., Cal. An average adult requires around 2,000 Cal per day to fuel all body functions. (By comparison, running one mile burns just about 100 Cal.)

Everything you consume has Calories. A gram of fat has 9 Cal. A gram of carbs has 4 Cal, same as a gram of protein. So it is quite easy to calculate which food has how many Calories, and how much of that stuff you should be eating. A regular pizza slice has 270 Cal. A 350ml can of Coke Classic has 140 Cal. An apple has 80 Cal. Ingesting too few Calories can make you lethargic, and ingesting more than you require can fatten you up.

So, how much actual energy does 1 Cal represent, in tangible terms? It’s really simple:

1 Cal is the amount of energy that can heat up one liter of water by one degree Celsius.

So, 50 Cal can heat up one liter of water by 50 degrees Celsius.

And, 100 Cal would be enough to boil a liter of ice water!

Wait, does that mean you could actually burn Calories by drinking ice water? Absolutely yes! Let’s see how many:

Ice water is at zero degrees Celsius. Suppose you drink up a liter of it. Your body heats it up to your body temperature, i.e., 37 degrees Celsius. Congratulations, you just burned 37 Cal in this process!