Are you underweight if you are 5'10 and weigh 126 pounds?

Yes; according to most standards, you are underweight.

In the US, we use the body-mass index (BMI) to help determine whether an individual is underweight, normal, overweight, or obese. The BMI ranges for these categories is as follows:

* Underweight: <18.5

* Normal: 18.5-25

* Overweight: 25-30

* Obese: >30

BMI is calculated by dividing a person's weight (in kilograms) by their height (in meters) squared. Using this calculation, a person who is 5'10" tall and weighs 126 lb would have a BMI of 18.1, which is technically underweight.