Within a healthy, balanced diet, a man needs around 10,500kJ (2,500kcal) a day to maintain his weight. For a woman, that figure is around 8,400kJ (2,000kcal) a day. These values can vary depending on age, metabolism and levels of physical activity, among other things.

What are calories? Calories are a measure of how much energy food or drink contains. The amount of energy you need will depend on: your age – for example, growing children and teenagers may need more energy

your lifestyle – for example, how active you are

your size – your height and weight can affect how quickly you use energy Other factors can also affect how much energy you burn. For example: some hormones (chemicals produced by the body) – such as thyroid hormones

some medications – such as glucocorticoids, a type of steroid used to treat inflammation

being unwell

Calories and kilocalories The term calorie is commonly used as shorthand for kilocalorie. You will find this written as kcal on food packets. Kilojoules (kJ) are the equivalent of kilocalories within the International System of Units, and you'll see both kJ and kcal on nutrition labels – 4.2kJ is equivalent to approximately 1kcal.