Hello, Height and weight ratio is depends on Sex, age and Ethnicity. In your case we are not able to know about your age, sex and Ethnicity but still we try to throw some light on problem. If you feel that you are underweight then you need primary physician and professional dietician consultation. Primary physician after thorough body check up may ask you for some lab blood tests like Complete Blood count to rule out Anemia and it s type and Stool Routine microscopy to rule out any kind worm or parasitic intestinal infection. Long term Anemia and parasitic infection are probable causes of malnutrition and underweight. Professional Dietician will advice you proper diet chart in which your daily calories requirements and protein, fat, carbohydrates intake will be mentioned, according to your body constitution,sex, age and Ethnicity. Treating physician may ask you to start daily physical exercises under professional trainer supervision. Thanks & Take Care, Dr. Suraj D.