It would be best if you had more calories than your body burns to gain weight. Healthy weight gain requires increased muscle and fat. Gaining excess fat, however, can be very unhealthy. That’s why turning to unhealthy foods that people often equate with weight gain (such as candy or fast food) is not a successful or safe weight gain. For weight gain, adding 1-2 pounds per month is recommended by eating no more than 500 extra calories per day. Protein is an important part of any weight gain strategy, as protein is a muscle-building block. Without protein, your body can not create muscle effectively, so the extra calories you eat will go toward fat. One possible downside to increased protein intake is decreased appetite, which may make it difficult for certain people to meet their higher calorie targets. It is recommended for weight gain to eat about 0.7-1 grams of protein per pound of body weight, but you can go much higher depending on your total daily calorie schedule. Many that are underweight, however, should contact a dietitian to develop a meal plan for them.