Do You Really Know What a Calorie Is?

Maxim Bogaert

Nov 17, 2023

2 min read

Everyone knows what a calorie is, right?

Background

Well, a calorie is a unit of energy, just like the kilograms (kg) displayed on your weight scale is a unit of mass, or the centimeters on your tape measure a unit of length. In nutrition sciences, although we typically speak of calories, we technically refer to kilocalories. For clarification, we may also write it down as Calorie with a capital 'C', although this is quite uncommon. If you remember my previous blog post, this might be the perfect opportunity to also consider swapping calories for kilocalories in your speech and writing. For example, saying that you eat 3000 calories, instead of kilocalories, per day is exactly as logical as saying that your body weight is 75 grams, instead of kilograms. In scientific writing, however, the energy unit joule is preferred, in accordance with the International System of Units (SI) guidelines.

A brief summarization:

1 calorie (cal) = 4.184 joules (J)

1000 cal = 1 kilocalorie (kcal) = 1 Calorie (Cal) = 4.184 kilojoules (kJ)

1000 kcal = 4184 kJ

Visualization

We can all imagine what 1 kg looks and feels like: just think of a standard package of sugar or a 1 liter (L) bottle of water. But, what does 1 kcal represent?

Simply stated, 1 kcal is approximately the amount of energy required to heat 1 kg of water by 1 degree Celsius (°C), or 1 degree Kelvin (°K), for that matter.

Considering that 1 kg of water is nearly equal to 1 L of water, and considering an average female who consumes and expends 2000 kcal per day, it follows that with 2000 kcal, ~25 L of water can be brought from room temperature (20 °C) to the boiling point (100 °C)!

Calculation

1 kcal is required to heat 1 L of water by 1 °C

80 kcal are required to heat 1 L of water by 80 °C (100 °C − 20 °C)

--> 2000 kcal/(80 kcal/L) = 25 L