Body mass index (BMI) is calculated by formula BMI = Weight in Kilograms / (height in meters x height in meters). BMI below 18.5 is underweight. 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. Other reasons are mostly medical conditions like TB, cancer, malabsorption, hyperthyroidism, and anorexia nervosa etc. Advice 1.     Adopt natural ways of gaining weight 2.     Exercise – 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 excercises 3.     Take natural high protein like egg, soya bean, pulses, meat, and milk. 4.     Choose food with nutritional calories like potato, banana and dry fruits 5.     Chew the food properly to make it easily digestible 6.     Avoid excess intake tea and coffee after meal as they contain tannin which hinders absorption of minerals like iron Thanks