Image by cattalin from Pixabay.How many calories do you need a day? RD Natalie Allen explains. March 10, 2021 by Strategic CommunicationDo you want to lose, gain or maintain your weight? The number of calories you need to eat a day depends on factors, such as physical activity level, age and sex. Natalie Allen, registered dietitian and clinical assistant professor of biomedical sciences at Missouri State University, shares with Insider some key facts to keep in mind about calorie intake.