The Body Mass Index is a measure of body weight relative to height. Often referred to as “BMI,” it is a bit of a guess, albeit an educated one, as to the amount of fat someone carries. So, clearly it doesn’t measure fat directly, which is why it is an educated guess. But it is really easy find (you just need to know your height and weight), which is why is is a common tool to help determine one’s health status.
What BMI can tell you
When someone has a high BMI, it can indicate that they are too fat. But if it’s too low, it implies they are too skinny. While BMI is only an estimate, it is generally associated with diseases such as heart disease, high blood pressure and diabetes. The higher the number, the higher the disease risk.1,2, On the other hand, a low BMI leads to other health issues such as bone loss, lowered immune function and anemia.3,4,5
The BMI is useful for most but not all persons
Since the BMI does not actually measure fat, it is of course limited. And so, as I suggested, it is a simple way to make a good guess. Thus, BMI is a reliable starting-point for your average person, but it does not always mean someone has too much, or not enough fat. For instance, athletes, especially weightlifters, tend to weigh much more than ‘average’ people their height. Thus, their BMI may say “obese,” when in fact they are quite healthy. On the flip side, a long-distance runner may appear to have too little body fat according to the BMI but also be in excellent health.
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Finding a Healthy Body Mass Index for adults
To find your BMI, you only need to know your height and weight. The easiest way to figure it out is just to plug your height and weight into an online tool, like this BMI calculator. But if you’re a math nerd like me, you can figure it out yourself. The equation looks like this:
This gives you a number that can be interpreted with the following body mass index table:
| BMI | Weight Status |
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Normal body weight |
| 25.0 – 29.9 | Overweight |
| 30.0 and above | Obese |
Keep in mind, this table is for adults that are at least 20 years old. Now remember, this is only a starting point. It does not necessarily mean you are what it says you are. For instance, I am a frequent weightlifter and I also run regularly. My weight is 200lbs (90.74kg) and I am 5’10’’ (1.8m). Thus, my BMI is 28.7, putting me close to obese. This would suggest that I am prime for a heart attack, but I am relatively thin and healthy in every other measurable way. I just have a higher muscle proportion than ‘average.’ Now if I did NOT lift weights but was still a runner, I would probably weigh about 160lbs. If that were the case, my BMI would be 23, right in the middle of “normal.”
Finding a healthy BMI for kids
When it comes to kids, the BMI scale is different given the many more variables involved with little people that are constantly growing and undergoing physical and hormonal changes. The same formula is used, comparing height and weight, but it is applied based on averages, not values. In other words, an adult BMI is figured based on their height and weight and that’s it. The number they get implies their health status. With a child or adolescent, the number doesn’t matter as much as how it compares to other children of their age and gender group. As children grow, what a ‘healthy’ amount of fat is changes, as it does between boys and girls. So, as you might imagine, there are BMI charts for every age and both genders, but the following table always applies:
| Percentile | Weight Status |
| Below 5th | Underweight |
| 5th to 85th | Normal body weight |
| 85th to 95th | Overweight |
| 95th and above | Obese |
Click here for a BMI calculator for girls and boys
The BMI and your health
The National Institutes of Health (NIH) tells us that more than 2 out of 3 adults in the U.S. are overweight according to the BMI. In addition, 1in 3 are obese. Of kids ages 2 to 19, it’s almost 1 in 5. Those are staggering numbers! When you are overweight or obese, your all-cause disease risk skyrockets. In other words, this means your risk for ANY disease but especially ones that can and will kill you such as heart attacks, strokes, diabetes and even cancer. So, if you are overweight or obese according to the BMI, is probably means you need to lose weight and get into better shape. There is plenty you can do to change your diet and activity-level to significantly reduce your risk of unpleasant health outcomes.
