Use our weight gain calculator to find out how many calories you need to gain weight and pack on some mass. You will also find out how many grams of carbohydrates, protein and fat you need to add extra weight to your physique. If you want to gain weight, you absolutely have to consume more calories than you burn. Remember, you need to eat big to get big. One pound of body weight is equal to 3,500 calories, so eating an extra 500 calories per day will cause you to gain one pound per week. Along with increasing your calories to gain weight, you should also increase your level of exercise in order to increase lean muscle mass.