Being underweight is defined as having a body mass index (BMI) below 18.5. This is estimated to be less than the body mass needed to sustain optimal health.
Conversely, over 25 is considered overweight and over 30 is considered obese.
Use this calculator to see where you fit on the BMI scale (opens in a new tab).
However, keep in mind that there are many problems with the BMI scale, which only looks at weight and height. It does not take muscle mass into account.
Some people are naturally very skinny but still healthy. Being underweight according to this scale does not necessarily mean that you have a health problem.