Calories tell us about the energy content of food and beverages. If you want to know how many calories should I eat to gain weight you need to understand how a calorie is related to your weight. Our bodies store extra calories as body fat when we consume more calories through food and drink than we burn. If this keeps happening, we might eventually gain weight. To maintain a healthy weight, a person needs about 2,500 kcal (10,500 kJ) per day.