It depends on factors such as their age, gender, and how physically active they are. Children aged 7-10 need between around 1,500 and 2,000 calories a day, according to the NHS, with the amount increasing slightly each year. Boys tend to need more calories than girls. Teenagers need more energy still, with calorie requirements ranging from around 2,200 for a 13-year-old girl to over 3,000 for an 18-year-old boy (depending on how much exercise they are doing).