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. Calories are a measure of how much energy food or drink contains. The amount of energy you need will depend on:Other factors can also affect how much energy you burn.