How to Know If You Are Underweight

Body mass index, or BMI, uses your height and weight to determine your percentage of body fat. Under this system, people are grouped into one of four categories based upon their BMI: Underweight, Normal, Overweight or Obese. According to the National Heart Lung and Blood Institute, a person is considered underweight if he or she has a BMI of less than 18.5 percent. There is a quick and painless solution if you wish to determine whether or not you fall into this category.

Things You'll Need

• Scale
• Measuring tape
• Pen
• Paper

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Instructions

• 1

  Weigh yourself on the scale to determine your exact weight in pounds or kilograms. Round to the nearest whole number if necessary and write this number on the paper.

• 2

  Use the measuring tape to determine your exact height in feet and inches or centimeters. Enlist the help of a second person if needed. Round to the nearest whole digit if necessary and write this number on the paper as well.

• 3

  Access the Body Mass Index Calculator (see Resources). Select the "Standard" tab if you measured your height and weight in feet and pounds. Select the "Metric" tab if you measured in centimeters and kilograms.

• 4

  Enter your height in the "Your Height" field on the calculator. Enter your weight in the "Your Weight" field. Select "Compute BMI" to continue.

• 5

  Look at the "Your BMI" field at the bottom of the calculator. Notice the number in the field; it represents your body fat percentage.

• 6

  Look to the right of the calculator, at the section titled "BMI Categories." Locate the range of numbers your BMI falls in to determine if you are underweight.