What is a BMI? What is Your BMI?

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What is a BMI?

BMI stands for Body Mass Index (also called the Quetelet Index, for its developer) which is an estimate of whether the amount of body fat a person has is healthy, taking height and weight into account.

What Does the BMI Number Mean?

The number we call the BMI is a person's weight in kilograms divided by the square of their height in meters. The number is a so-called "index number" meaning that it doesn't mean anything in and of itself.

If your BMI is 25, it doesn't mean that your body has 25 of anything in particular. The number can only really be understood as having a meaning assigned to it by the people who have studied the BMI. There are are commonly-assigned ranges that tell whether a person is underweight, normal weight, overweight, or obese.

In particular, the BMI number isn't the same as body fat percentage, with which it is often confused. The purpose of the BMI is essentially a screening tool which simply tells whether a person has the "right" amount of body fat and whether it's too much or not enough. It is easy to use, which is probably the main reason it became so popular, as body fat percentage is more complicated to measure.

How Accurate is the BMI?

The CDC reports that BMI is "moderately correlated with more direct measures of body fat". In particular, there are certain categories of people for whom the index may be somewhat misleading:

  1. Athletes and others with a lot of muscle development can have a BMI that is misleadingly high - in this case, the "extra" weight is muscle, which is not a problem (in fact, it's good, from a metabolic standpoint).
  2. Very tall and very short people - Because of the way the math works out, the index tends to overestimate how overweight a tall person is. It can also underestimate the weight category for a short, small-framed person.
  1. Frame size - The index doesn't account for people who have a smaller- or larger-than-usual body frame.
  2. Age - As people age, they tend to lose muscle. This means that if they stay the same weight (and height) their BMI doesn't change, but they may have more body fat than they previously did, sometimes too much. Another way that age affects BMI is that people tend to lose height, so even if they stay the same weight, their BMI will be higher. (This has happened to me!  Bummer!)
  3. A surprising number of people with a normal BMI have what has been called (perhaps improperly) Normal Weight Obesity.
  4. Race - At the same BMI, on average, Blacks will have less body fat than Caucasians, who will have less body fat than Asians.

BMI and Children

The BMI numbers from the charts below are interpreted differently for children.  For children, if they are at or above the 95th percentile of children their age they are considered obese.  To find out the percentile of someone age 2-20:

Body Mass Chart for Boys

Body Mass Chart for Girls

For babies and toddlers from birth to 3 years old:

Body Mass Chart for Infant and Toddler Boys

Body Mass Chart for Infant and Toddler Girls

BMI Categories for Adults

There are variations of how to interpret BMI numbers in different parts of the world.  The Centers for Disease Control (United States) uses these numbers:

BMIWeight Status
Below 18.5Underweight
18.5 - 24.9Normal/Healthy
25 - 29.9Overweight
30 and aboveObese

The World Health Organization further breaks down the categories of underweight and obese.

How to Find Out Your BMI

To use this chart, choose your height in inches from the left column, and then go over to your weight in pounds in that row.  The number at the top of that column is your BMI. If you weigh more than the numbers on this chart, go to the second table.

BMI for Most Adults
BMI1920212223242526272829303132333435
Height  WEIGHT         
589196100105110115119124129134138143148153158162167
599499104109114119124128133138143148153158163168173
6097102107112118123128133138143148153158163168174179
61100106111116122127132137143148153158164169174180185
62104109115120126131136142147153158164169175180186191
63107113118124130135141146152158163169175180186191197
64110116122128134140145151157163169174180186192197204
65114120126132138144150156162168174180186192198204210
66118124130136142148155161167173179186192198204210216
67121127134140146153159166172178185191198204211217223
68125131138144151158164171177184190197203210216223230
69128135142149155162169176182189196203209216223230236
70132139146153169167174181188195202209216222229236243
71136143150157165172179186193200208215222229236243250
72140147154162169177184191199206213221228235242250258
73144151159166174182189197204212219227235242250257265
74148155163171179186194202210218225233241249256264272
75152160168176184192200208216224232240248256264272279
76156164172180189197205213221230238246254263271279287
Adult BMI Table #2
BMI3637383940414243444546474849505152
Height  WEIGHT         
58172177181186191196201205210215220224229234239244248
59178183188193198203208212217222227232237242247252257
60184189194199204209215220225230235240245250255261266
61190195201206211217222227232238243248254259264269275
62196202207213218224229235240246251256262267273278284
63203208214220225231237242248254259265270278282287293
64209215221227232238244250256262267273279285291296302
65216222228234240246252258264270276282288294300306312
66223229235241247253260266272278284291297303309315322
67230236242249255261268274280287293299306312319325331
68236243249256262269276282289295302308315322328335341
69243250257263270277284291297304311318324331338345351
70250257264271278285292299306313320327334341348355362
71257265272279286293301308315322329338343351358365372
72265272279287294302309316324331338346353361368375383
73272280288295302310318325333340348355363371378386393
74280287295303311319326334342350358365373381389396404
75287295303311319327335343351359367375383391399407415
76295304213320328336344353361369377385394402410418426

 

Sources:

Centers for Disease Control and Prevention Web site page: About Adult BMI accessed January 28, 2016

National Institutes of Health Web site pages: Body Mass Index Table 1 and Table 2, accessed January 29, 2016

Sorkin JD, Muller DC, and Andres R. Longitudinal Change in Height of Men and Women: Implications for Interpretation of the Body Mass Index: The Baltimore Longitudinal Study of Aging. American Journal of Epidemiology. 150:9 969-977

World Health Organization Web site page BMI Classification, accessed January 29, 2016

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