How do you know if i am underweight?

if u are a kid and still go to a pediatrician (kid doc), this is my advice. if you are an adult, (18, and over) and you dont go to a pediatrician, sorry, i doubt this answer will help u.

at your pediatrician's office, they will hav ur growth chart. You can measure your height and weight. After you put all of that info on your chart, u can see if you are average, below, or over the normal weight and height for ur age. hope this helps.