Body Mass Index

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To find out how much you weigh, you simply step on a scale. But your weight alone can't tell you whether you're underweight, healthy, or overweight. If you're 6'4" and you weigh 200 pounds, you're probably at a healthy weight; but if you're 5'9" and weigh 200 pounds, you're probably overweight.

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Because both height and weight are important to help determine if someone is overweight, scientists came up with a mathematical formula called Body Mass Index (BMI). This simple measurement helps doctors determine whether their patients are at a healthy weight or need to lose or gain a few pounds.

In this article, you'll learn how the BMI is calculated and what the numbers mean to your health.


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BMI Calculator

Body Mass Index is a calculation that takes into consideration both a person's body weight and height to determine whether they are underweight, overweight, or at a healthy weight. It can be calculated in inches and pounds (in the United States), or meters and kilograms (in countries that use the metric system).

In inches, the formula looks like this:

weight in pounds
(height in inches) x (height in inches)
x 703

A person who weighs 180 pounds and is 5 feet 8 inches tall has a BMI of 27.4.

180 lbs
(68 inches) x (68 inches)
x 703 = 27.4

In meters, the formula looks like this:

weight in kilograms
(height in meters) x (height in meters)

A person who weighs 99.79 kilograms and is 1.905 meters (190.50 centimeters) tall has a BMI of 27.5.

99.79 Kg
(1.905 m) x (1.905 m)
= 27.5

What's your BMI? Type your height and weight into the boxes and press "Find My BMI."

Your height: feet inches
Your weight: pounds

Studies have indicated that being overweight or obese can significantly increase a person's risk for developing several diseases, including:

Similarly, being underweight can also lead to increased health risks due to malnutrition.

In a broad sense, the BMI helps health officials get a general idea of how weight and obesity affect the health of the population. On an individual basis, it enables doctors to identify weight problems in their patients before serious health conditions arise. Patients who are overweight, or at risk for being overweight, can start on a diet and exercise program to help them bring their weight down to a healthy range.

It's important to note that BMI is just one factor involved in determining risk for disease. Diet choices, exercise, and smoking all combine to determine whether or not an individual is healthy.

In the next section we'll see how BMI is measured for children.

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BMI for Children

Young children naturally start out with high body fat, but tend to get leaner as they get older. Girls and boys also have different body compositions. To take into account the differences between boys and girls and children of different ages, scientists have created a special BMI for children, called BMI-for-age.

Doctors use a set of growth charts to track the development of children and young adults between the ages of two and 20. The BMI-for-age figures in a child's height, weight, and age to determine how much body fat he has. It compares the results to those of other children of the same age and gender, and can help predict whether children will be at risk for being overweight when they get older. You can view a complete set of the charts at CDC's growth chart page.

Each chart contains a set of curved lines indicating the child's percentile. For example, if a 15-year-old boy is in the 75th percentile for BMI, 75 percent of boys of the same age have a lower BMI. He is at a normal weight. Although his BMI changes as he grows, he can stay at around the same percentile and remain at a normal weight.

The normal BMI range becomes higher for girls as they mature, because teenage girls normally have more body fat than teenage boys. A boy and girl of the same age may have the same BMI, but the girl could be of normal weight and the boy could be at risk for being overweight.

Doctors say it's important with children to track BMI over time rather than looking at one individual number, because children can go through growth spurts.

In the next section, we'll learn about some of the controversy associated with using BMI.


Is BMI an Accurate Measure of Obesity?

It's important to note that although BMI is accurate most of the time, it may overestimate or underestimate body fat. For example, BMI doesn't distinguish between body fat and muscle mass, which weighs more than fat. Many NFL players have been labeled "obese" because of their high BMI, when they actually have a low percentage of body fat.

The BMI is not always accurate in elderly adults, who have often lost muscle and bone mass. Although their BMI might be within a normal range, they could still be overweight. BMI may also relate differently to various ethnic groups. For example, Asians may be at risk for health problems at a lower BMI than Caucasians.

Because of the possibility for error, BMI should be just one of many gauges used to assess a person's weight status and health. The National Institutes of Health (NIH) recommends that doctors assess whether their patients are overweight based on three factors:

  1. BMI
  2. Waist circumference - a measurement of abdominal fat
  3. Risk factors for diseases associated with obesity, such as high blood pressure, high LDL ("bad") cholesterol, low HDL ("good") cholesterol, high blood sugar, and smoking

Many health experts say that body fat percentage is a better indicator of weight status than BMI. But body fat isn't always as easy, or as inexpensive, to measure. Tests such as skin-fold measurements (in which a technician pinches a fold of skin on the patient's body to measure the subcutaneous fat layer just beneath the skin), dual energy X-ray absorptiometry (DEXA, which measures bone density), or bioelectrical impedance (which measures the opposition to a flow of electric current through the body -- impedance is low in lean tissue and high in fat tissue) are more precise, but they must be done by a trained medical professional.

Next, we'll learn about the history of BMI.


The History of BMI

Using a formula to calculate obesity is not a new concept. In the nineteenth century, a Belgian statistician named Adolphe Quetelet came up with the Quetelet Index of Obesity, which measured obesity by dividing a person's weight (in kilograms) by the square of his or her height (in inches).

Formula: w/h2

Before 1980, doctors generally used weight-for-height tables -- one for men and one for women -- that included ranges of body weights for each inch of height. These tables were limited because they were based on weight alone, rather than body composition. BMI became an international standard for obesity measurement in the 1980s. The public learned about BMI the late 1990s, when the government launched an initiative to encourage healthy eating and exercise.

In 1998, the National Institutes of Health lowered the overweight threshold for BMI 27.8 to 25 to match international guidelines. The move added 30 million Americans who were previously in the "healthy weight" category to the "overweight" category. Today, the NIH advises doctors and their patients to include BMI in a complete assessment of a person's body size and overall health.

For more information on BMI and related topics, check out the links on the next page.


Lots More Information

Related HowStuffWorks Articles

More Great Links


  • "Aim for a healthy weight." National Heart, Lung and Blood Institute.
  • "Bioelectrical Impedance Analysis." Impedimed Limited, March 10, 2005.
  • CDC: BMI
  • CDC: BMI-for-age
  • CDC: Growth Charts
  • BMI Charts
  • Kuczmarski, R. J. and K. M. Flegal. "Criteria for definition of overweight in transition: background and recommendations for the United States." American Journal of Clinical Nutrition, Vol. 72, No. 5, 1074-1081, November 2000
  • Mackey, Carole S. "Body Mass Index." Nutrition and Well-Being A to Z. Vol. 1. New York: Macmillan Reference USA, 2004. 71-74.
  • Platkin, Charles Stuart. "BMI is a better way to measure." North Virginia Daily, 2005.