Body Mass Index

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Matthew Toohey

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BMI

Weight in Pounds x 703

(Height in Inches)

Source:http://www.cdc.gov/healthyweight/assessing/bmi/childrens_bmi/about_childrens_bmi.html

Weight (lbs)
Height (ins)

Body Mass Index

BMI, or Body Mass Index is a number that helps us understand how fit we are because it incorporates both height and weight. If a person is six feet tall and weighs 180 lbs, this may be an ideal weight but would be be too much weight in someone who is only five feet tall!

Here is the formula: Your BMI is equaled to your weight in pounds times 703 divided by your height in inches squared.

Or you can simply use our calculator:

So what do the numbers mean?

Well, it depends on a few factors. BMI is a static measurement for adults:

BMI STATUS

Below 18.5 Underweight

18.5 to 24.9 Normal

24.9 to 29.9 Overweight

Above 30.0 Obese

While a very muscular man may be up a few tics on the scale and still be healthy, in almost all instances these numbers don’t lie.

For children, the number fluctuates along a curve. If you used the calculator for your toddler and are now freaking out because his BMI is 14,don’t worry: he is probably just right. We don’t have a rigid scale for children. We have a percentile curve, like a growth curve, because BMI is typically lower in children and it fluctuates until it normalizes at the adult values listed above.

Here’s a great example (this is for a boy) from CDC:

You can see that BMI in a two year old ranges between 14.5 and 19.5 and actually declines to a low point between 4 and 7 years old. Then comes a steady rise until adult values are reached after age 20. The lines indicate percentiles and the dark line in the middle is the 50th percentile, or the average. You can see in the example of a 10 year old boy that a BMI of 23 is in the obese range while you or I might consider 23 a perfect target number.

Remember, that is a curve for boys. The curve for girls is very close. Here are links to the CDC BMI curves for both boys and girls:

Boys Girls

A Final Note on BMI

There is a reason for the ranges on growth charts and on these BMI charts: everyone is a little different. Not everyone is going to plot exactly at the 50% mark. People come in different body types. It is important to take this into account and not to panic about where a child plots on one of these curves. It is equally important not to lie to ourselves. “That’s the way all the boys on his side are,” or “she just has big bones,” are not appropriate reasons to plot in the obese range. Take these numbers with a pinch of salt. Talk to your pediatrician about the trend from year to year (which is more important than a single number) but don’t ignore a problem if it is staring you right in the face. If a child plots in the obese range, talk to your doctor and take steps to change this with diet and exercise.

Matthew Toohey MD. December 23, 2011.