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What is Obesity?

Body Mass Index

Obesity is defined as an excessively high amount of body fat or adipose tissue in relation to lean body mass. Body Mass Index (BMI) is a common measure expressing the relationship (or ratio) of weight-to-height. It is a mathematical formula in which a person's body weight in kilograms is divided by the square of his or her height in meters squared (wt/(ht)2.

The BMI is more highly correlated with body fat than any other indicator of height and weight. Individuals with a BMI of 25 to 29.9 are considered overweight and are approximately 20 pounds above appropriate weight for height. Individuals with a BMI of 30 or more are considered obese and are 30 or more pounds over appropriate weight for height. For adults over 20 years old, BMI falls into one of these categories:

Adult (21 and over)

BMI

Weight Status

Below 18.5

Underweight

18.5 – 24.9

Normal

25.0 – 29.9

Overweight

30.0 and Above

Obese

Children and Adolescent (2-20)

BMI

Weight Status

Underweight

<5th percentile BMI for age

Normal weight

≥5 to <85 BMI for age/gender

At risk for overweight

≥85th to <95th BMI for age/gender

Overweight

≥95th BMI for age/gender

Obese

Not used in children/teens

Below is an example of calculating a BMI using the English system and in the metric system.

  • English Formula: BMI = (Weight in Pounds) / (Height in Inches X Height in Inches) X 703
    Example: A person who weighs 220 pounds and is 6 feet 3 inches tall has a BMI of 27.5.
  • Metric Formula: BMI = Weight in Kilograms / (Height in Meters)2
    Example: A person who weighs 99.79 kilograms and is 1.905 meters tall has a BMI of 27.5.

Use of BMI for Children

The terms obese, overweight and at risk for overweight are defined differently in pediatric populations than in adults (see chart below). Body Mass Index (BMI) is the primary measure utilized to define weight stats in both adults and children. BMI is calculated using the person’s weight and height and is a helpful indicator of weight status. In adults, BMI is a fixed measurement without regard to gender or age. In children and adolescent, BMI is age and gender specific and therefore the BMI measurement in this population changes with age. Because of these differences between adult and children’s BMIs, the BMI for the pediatric population must be plotted on the CDC growth charts enabling on to determine BMI-for-age percentiles. The Children and Adolescents chart above summarizes the categories by BMI and percentages in children.


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