What’s the Big Deal About Body Mass Index (BMI)?

June 26, 2020

BMI is an acronym that gets thrown around a lot in the fitness world, and if you don’t know it by now, it’s time to learn. 

BMI stands for Body Mass Index, and it’s calculated by dividing your weight in kilograms, by your height in meters squared. 

It seems like a weird calculation, but it’s extremely useful for determining the right weight for your height, as well as if you may be over or underweight. 

According to the Centers for Disease Control and Prevention (CDC), “BMI doesn’t measure body fat directly,” but it correlates it to where you can determine what fat level you need to be at for a healthy weight. There are multiple categories for weight levels for BMI and they are flexible depending on a person’s build or physique. A BMI higher than 30 is obese, while a BMI of between 25-30 is considered overweight. Underweight BMI is less than 18.5 and a normal weight BMI is between 18.5 and 25 kg/m². 

Body Mass Index (BMI): What You Need to Know- YOUTOHEALTHY

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Body Mass Index (BMI) in Adults

BMI was originally used for population statistics, as it seemed easier to determine results for larger groups as opposed to individuals when it came to healthy weight. 

You may be wondering why BMI matters then, if it’s mainly for groups. Well, besides determining a healthy weight, BMI provides a simple numeric measure of a person’s thickness or thinness, allowing health professionals to discuss weight problems more objectively with their patients. 

Therefore, BMI is also used for individuals. But you’ll find it coupled with more helpful metrics such as waist circumference to give a fuller picture of your health. 

This can be extremely helpful for people struggling with eating disorders, such as binge eating or anorexia. Discussing issues objectively allows for the doctors to be sensitive and goal-oriented, pushing their patients on the right track.

Using the BMI calculations for adults, and then comparing it to statistical data on child BMI, you can calculate the BMI for a child. For children, if you’re BMI is over 95% of the data shown, you’re considered obsese, and if you’re BMI is under 5% of the population, you’re considered underweight. 

So how do you stack up compared to other people? 

Well in the U.S, BMI has shifted a bit.

“In 1998, the U.S. National Institutes of Health and the Centers for Disease Control and Prevention brought U.S. definitions in line with World Health Organization guidelines, lowering the normal/overweight cut-off from BMI 27.8 to BMI 25. This had the effect of redefining approximately 29 million Americans, previously healthy, to overweight” (CNN, 1998).  

In 2015-2016, the U.S. National Health and Nutrition Examination Survey illustrated that “ 71.6% of men and women had BMIs over 25” (CDC 2015), being considered overweight. 

While becoming older gives way to more health problems, studies have shown that for older women, they are more likely to become obese. Unfortunately, overweight and obese individuals are more at risk for diseases such as hypertension, type 2 diabetes, and osteoarthritis, than the rest of the population.

However, there are limitations to BMI. According to mathematician Nick Trefethen:

“BMI divides the weight by too large a number for short people and too small a number for tall people. So short people are misled into thinking that they are thinner than they are, and tall people are misled into thinking they are fatter.”

This is also seen in the data analyzed from BMI testing. According to Medical News Today (2015):

“The BMI overestimates roughly 10% for a large (or tall) frame and underestimates roughly 10% for a smaller frame (short stature). In other words, persons with small frames would be carrying more fat than optimal, but their BMI indicates that they are normal. Conversely, large framed (or tall) individuals may be quite healthy, with a fairly low body fat percentage, but be classified as overweight by BMI.” 


BMI gives you an estimate of what your weight and height ratio is, and therefore what your correct weight should be. This estimate may be faulty, as some people could be overweight or underweight due to a medication, and not reflect their true weight.

The other issue is that BMI does not distinguish between muscle and fat, so those who are muscular may be put in a category that does not properly reflect their fitness. BMI is especially inaccurate for people who are athletic, as their higher amount of muscle can put them in the overweight category, even though their body fat percentage is lower than average.

BMI is important to have as a base for knowing how fit you are and how to move forward to your ideal weight, but know that your fitness is more than a BMI number. It’s a series of choices, it is a lifestyle, and if anyone is capable of keeping up with your fitness, it’s you.

More about Kenna Castleberry

Kenna Castleberry is a science communicator finishing her M.Sc. at Imperial College, London. Her hobbies include running, archery, hiking, yoga, reading, and podcast listening.

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