Those that are interested in tracking their weight have most likely heard the term Body Mass Index (BMI). I’ve mentioned BMI here in the past, but have never really talked about exactly what it is, or how it’s calculated.
Your BMI is a measure of your body weight in relation to your height. While BMI doesn’t directly measure your percent body fat, it’s often used as a metric for determining whether or not you are underweight, overweight, or obese.
How to calculate BMI
There are actually two different (but equivalent) formulas for calculating body mass index. Ultimately, BMI is expressed as kg/m^2, so if your measurements were done in imperial units (pounds and inches) you’ll have to use an alternate formula.
Metric BMI calculation
In metric units, the BMI calculation is done as follows:
BMI = (mass in kg) / (height in meters)^2
So for me… I currently weigh roughly 232 pounds (105.2 kg) and I’m 6′5″ tall (1.96 m). This works out to a BMI of 27.4.
Imperial/English calculation
In imperial units, the calculation is done as follows:
BMI = (pounds x 703) / (height in inches)^2
As above, my height is 77 inches and I weigh 232 pounds, which works out to a BMI of 27.5. The slight difference in results is due to rounding.
How to interpret BMI values
So… Now that you know your Body Mass Index, what does it mean? According to the National Institutes of Health (NIH), BMI values can be broken up into the following categories
- Underweight = Below 18.5
- Normal weight = 18.5 – 24.9
- Overweight = 25.0 – 29.9
- Obese = 30 or greater
These ranges should only be applied to adults.
Interpreting BMI in children
In children, Body Mass Index calculations are done as above, but the values are generally compared to those of all children of the same sex and age rather than to arbitrary thresholds. In this way, you can get a sense for the relative ranking within the population as a whole.
According to the CDC, a BMI percentile in the range of 5%-85% correspends to healthy weight. Below 5% is considered underweight, 85%-95% is considered overweight, and greater than 95% is obese.
Criticisms of Body Mass Index
I never been a big fan of BMI, and I’m not alone. Because BMI is based only on weight and height, it doesn’t take into account body composition, and can thus be inaccurate when it comes to determining whether or not someone is under/overweight.
For example, BMI estimates for athletes or those with a muscular build might be inflated despite the individual being in good physical condition. Likewise, BMI values for older people or those who have low muscle mass might be artificially low.
As such, BMI alone is often a poor indicator of morbidity and/or mortality even though many in health-related fields rely on it for such things.
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