Body Mass Index

Written by Brian Krans | Published on August 15, 2012
Medically Reviewed by George Krucik, MD

Overview

The body mass index (BMI)—also known as the Quetelet index—is a formula used to calculate a person’s body fat content based on his or her weight and height. The World Health Organization (WHO) has used the tool to calculate obesity rates since the 1980s.

One major shortcoming of the test is that it doesn’t take into account frame size or muscularity. Therefore, even a professional athlete may fall into the “obese” range on the index scale because of the weight of muscle on his or her frame.

Why Body Mass Index Is Used

You can calculate your body mass index to determine your body fat content. If your body fat level is too high, you are at risk for diabetes, high blood pressure, stroke, and heart disease.

How to Prepare for Body Mass Index

You will need to know your weight and height. For the most accurate results, use height and weight measurements taken at a doctor’s office or clinic.

The Math: How Body Mass Index Is Calculated

Your body mass index is calculated by dividing your body weight by the square of your height. If your measurements are in inches and pounds, you must multiply the formula by 703.

The Centers for Disease Control and Prevention has a body mass index calculator on its website at www.cdc.gov.

Example from CDC:

Weight = 150 lbs, Height = 5’5" (65")

Calculation: [150 ÷ (65)2] x 703 = 24.96

So, an adult who is 5 feet 5 inches tall and weighs 150 pounds has a BMI of 24.96, right on the border between “normal weight” and “overweight.”

The body mass index scale follows these guidelines:

  • Underweight: <18.5
  • Normal weight: 18.5 to 24.9
  • Overweight: 25 to 29.9
  • Obese: >30

Using the Results

If your BMI is greater than 25, ask your doctor about how to lose weight, improve your lifestyle habits, and stay in optimal health.

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