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Body mass index facts for kids

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Body mass index chart
A chart showing Body Mass Index. You can find your BMI range by lining up your weight and height. This helps show if an adult's weight is healthy. The chart is based on data from the World Health Organization.

The Body Mass Index (or BMI) is a way to check if a person's weight is healthy for their height. Health groups like the World Health Organization (WHO) and governments use it. It is sometimes also called the Quetelet Index.

How BMI Started

The BMI was created by Adolphe Quetelet. He was a scientist who worked on it between 1830 and 1850.

Later, in 1972, another scientist named Ancel Keys used the name Body Mass Index for the first time. He thought governments should use BMI to see if adults were too heavy or too light.

How to Figure Out Your BMI

To find someone's BMI, you need two things:

Then, you use a simple math formula. You divide your weight by your height, multiplied by itself (that's called the square of your height). If you use pounds and inches, you also need to multiply the answer by 703.

Here are the formulas:

\mathrm{BMI}  = \frac{\text{weight}(\text{kg})}{\left(\text{height}(\text{m})\right)^2}
 
= \frac{\text{weight}(\text{lb})}{\left(\text{height}(\text{in})\right)^2}\times 703

For example:

  • A man who weighs 78 kg and is 1.83 m tall has a BMI of:
\mbox{BMI} = \frac{78}{1.83 ^2} = 23.3
  • A man who weighs 172 lb and is 72 in tall has a BMI of:
\mbox{BMI} = \frac{172}{72 ^2}\times 703 = 23.3

What Your BMI Means

Health groups, like the World Health Organization (WHO), use BMI to help decide if adults are too heavy or too light. The WHO uses these ranges for adults:

Category BMI Range
Seriously Underweight less than 16.5
Underweight between 16.5 and 18.49
Normal between 18.5 and 24.99
Overweight between 25 and 29.99
Obese 30 or more

Some experts have different ideas about these numbers. For example, some think the Overweight range for people with Asian body types should be different. Also, some believe the lowest Normal BMI should be 20 for men and 18 for women. The definition of "overweight" for BMI changed in 1998.

Why BMI is Used

BMI is helpful for governments to understand the health of their people. For example, in the United States in 2007, many adults had a BMI above 25. This information helps the government plan health programs.

However, BMI is less useful for checking if one single person is too heavy or too light. BMI does not show how much of your weight is fat, how much is muscle, or how much is bone. For example, many athletes have a high BMI because they have a lot of heavy muscle, not because they are fat. Other people might have a normal BMI but still have too much fat. Another way to measure body size is the Body Volume Index.

Even so, BMI is popular because it is easy to use and understand. It gives helpful information for most people.

Other Ways to Measure Body Size

The Ponderal Index is similar to BMI. But instead of squaring the height, you multiply it by itself three times (this is called cubed). The formula is:

\mbox{PI} = \frac{weight(kg)}{height(m) ^3}

Some experts also suggest that the best formula might use a slightly different number for the height's power, like 2.6 instead of 2. This would look like:

\mbox{BMI} = \frac{weight(kg)}{height(m) ^{2.6}}

Images for kids

  • Correlation between BMI and Percent Body Fat for Men in NCHS' NHANES 1994 Data

    This graph shows how BMI relates to body fat for men. It also shows that BMI might not always tell the whole story about body fat.

See also

Kids robot.svg In Spanish: Índice de masa corporal para niños

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Body mass index Facts for Kids. Kiddle Encyclopedia.