Just as we use different units to measure length or distance (i.e. metres, kilometres, light years), different units are used for measuring the amount of energy in food and drink.
Kilojoules and calories
The most common unit of energy is the kilojoule (kJ). As we know, the prefix 'kilo' means one thousand, so $1$1 kilojoule (kJ) is equal to $1000$1000 joules (J).
Calories are an older unit of energy in food. The term 'calorie' can be confusing, because $1$1 Calorie (Cal), spelt with an uppercase C, is equal to 1 kilocalorie (kcal).
Although kilojoules (kJ) are the standard unit of energy for food and drink in Australia, kilocalories (kcal) still appear on some nutritional labels, and the term 'calories' is often used to describe them.
A kilocalorie (kcal) is defined as the amount of energy required to raise the temperature of $1$1 kg of water, by $1$1 degree. One kilocalorie (kcal) is equal to $4.184$4.184 kilojoules (kJ), or approximately $4.2$4.2 kJ.
While calories and joules are both metric units, only the joule is part of the International System (SI) of Units.
| $1$1 kilojoule (kJ) | $=$= | $1000$1000 joules (J) |
| $1$1 Calorie (Cal) | $=$= | $1$1 kilocalorie (kcal) |
| $1$1 kilocalorie (kcal) | $=$= | $4.184$4.184 kilojoules (kJ) |
Practice Questions
Question 1
Convert $35$35 kilocalories into kilojoules, using $1$1 kilocalorie$=$=$4.184$4.184 kilojoules.
QUESTION 2
Convert $2363.96$2363.96 kilojoules into kilocalories, using $1$1 kilocalorie$=$=$4.184$4.184 kilojoules.