One of the easiest ways to determine if you are underweight or overweight is to calculate your body mass index (BMI). There are many tables available online (search term: BMI table). Enter your height and weight into the tool to see your BMI. A BMI between 19 and 24 is considered normal; less than 19 is considered underweight. A BMI between 25 and 29 is considered overweight, and greater than 30 places you in the category of obese.