We normally think of calories as being something to avoid. However, a calorie is simply a measurement of energy. Too many calories can be bad for your health as any unused energy is stored in the body as fat. Without any energy though, your body would not be able to function. Think about your smartphone, it's not much use to you when it doesn't have any energy! Also keep in mind when you buy an energy drink, you could also be thinking about it as a calorie drink! Probably doesn't seem as appealing now, does it?
Just like we can measure length in lots of different ways (inches, centimetres and so forth) there are different ways to measure the amount of energy in food. Two of the most common ways to measure energy are in Calories (Cal) or in kilojoules (kJ). If you remember from the other metric units, the prefix "kilo" means one thousand of the unit of measurement, so kilojoules means $1000$1000 joules.
A calorie and a Calorie are two different measurements, so look out for the upper case 'C'.
When people say "I'm counting Calories" they mean the kcal numbers that are found on food labelling.
People usually eat somewhere between $2000$2000 to $2500$2500 Calories per day, which is the same as $2$2 to $2.5$2.5 million calories (lower case 'c')!
Out of the two, Calorie is the more commonly used measurement but is in fact $1000$1000 calories.
That is, $1$1 Calorie is equivalent to $1000$1000 calories (lower case 'c'), which is also called $1$1 kilocalorie (kcal).
On some nutrition labels you may see kilocalories or Calories used as well as kilojoules.
$1$1 Calorie (Cal) $=4.184$=4.184 kilojoules (kJ)
$1$1 kilojoule (kJ) $=1000$=1000 joules (J)
$1$1 Calorie (Cal) $=1$=1 kilocalorie (kcal) $=1000$=1000 calorie (cal)
Here is a conversion diagram to help:
The most common conversion required is the central conversion between kilojoules and Calories. Calories are the larger unit, thus to convert from Calories to kilojoules we need to multiply by $4.184$4.184. The conversion factor is often approximated to $4.2$4.2 or even $4$4 to get a quick estimate.
Practice questions
Question 1
Convert $25$25 kilocalories into kilojoules, using $1$1 kilocalorie$=$=$4.2$4.2 kilojoules.
QUESTION 2
Convert $3108$3108 kilojoules into kilocalories, using $1$1 kilocalorie$=$=$4.2$4.2 kilojoules.