How much should an 18-year-old girl weigh if she is

There is no one answer to this question, as the healthy weight for an 18-year-old girl can vary depending on a number of factors, including height, build, and activity level. However, a good rule of thumb is that an 18-year-old girl should weigh somewhere between 100 and 140 pounds. If you are concerned about your weight, it is best to talk to your doctor.