Poor dietary habits, medical conditions or excessive exercise can cause you to be underweight. Anorexia and bulimia nervosa are eating disorders most commonly associated with being underweight. Being underweight can cause fatigue, osteoporosis and irregular menstrual cycles in women if left untreated. According to the Centers for Disease Control, in 2008, 1.6 percent of Americans over the age of 20 were considered to be underweight. You can use a simple formula, termed the body mass index, to determine if you are underweight.
Use a tape measure to record your height. Your height is a necessary measurement that factors into your body mass index, which will help you determine if you are underweight. Have a friend, spouse or family member check your height. Make sure you have removed your shoes and socks before your height is determined. Write down your height in inches.
Step on a scale to get your weight, which is a necessary component in determining your body mass index. Make sure you have removed all clothing and that the scale is at zero before you step on it. Check your weight twice and write down the average.
Square your height, or multiply your height by itself. For example, if your height is 5 feet, or 60 inches, multiply 60 times 60. Write down this number.
Divide your weight by your height squared, which was the number you calculated in Step 3. Write down this number and multiply it by 703. The result is your body mass index. You are underweight if your body mass index is less than 18.5.