Body mass index (BMI) below 18.5 is underweight. BMI is calculated by formula BMI = Weight in Kilograms / (height in meters x height in meters). If you want to gain weight then gradual weight gain advisable. There are various causes of underweight like inadequate food intake, prolonged meal time gaps, and increased physical activity weight increasing food intake. Natural ways of gaining weight is advisable. Advice 1. Start workout in gym at least for half an hour for 4 – 5 days in a week regularly. Start with a balanced mix of cardio, weight training and flexibility exercises 2. Take natural high protein like egg, soya bean, pulses, meat, and milk. Choose food with nutritional calories like potato, banana and dry fruits 3. Drink 3 – 4 liters of water daily 4. Chew the food properly to make it easily digestible 5. Avoid excess intake tea and coffee after meal as they contain tannin which hinders absorption of minerals like iron 6. Eat at least three major meals Thanks