For as long as people have been trying to get healthier, they’ve looked for a way to gauge what healthy is. The popular Body Mass Index (BMI) is one of the quickest and most straightforward ways we can assess our fitness level.

The BMI was created in 1832 by Adolphe Quetelet, a Belgian statistician. Quetelet was interested in relating probability calculus to physical human traits. Specifically, he used the index to research the growth of an average man.

Only after World War II, when several studies linked obesity and mortality, did people start connecting Quetelet’s formula to obesity. By the 1980s, the index turned into the standard metric to measure obesity in humans. It entered the popular consciousness by the late 1990s.

How to Calculate Your BMI

Anyone whose curious where they fall on the index can use one of the many BMI calculators available on the internet. If you’d rather calculate it the old-fashioned way, there’s a simple formula you can use. Just take your weight in kilograms divided by your height in meters squared.

The formula is also available in imperial measures. In this case, you take weight in pounds divided by your height in inches squared. Then, multiply that by 703.

BMI Categories

Once you find your number, you can see which category you fall under. The World Health Organization (WHO) lists six BMI classification categories:

BMI Category
Less than 18.5 Underweight
18.5–24.9 Normal weight
25.0–29.9 Pre-obesity
30.0–34.9 Obesity class I
35.0–39.9 Obesity class II
Above 40 Obesity class III

Pros of BMI

One of the primary reasons that the index has become so popular is its simplicity. All you need to do is plug in two easily attained measurements, take that number, and see which category you fall under. Couldn’t be easier, right?

This straightforwardness has led to the index’s adoption by most of the world. Now, it’s useful as a comparative tool to study rates of obesity and malnourishment rates from country to country. One study in 2006 even called it the “best available tool for monitoring progress in the campaign against obesity.

Cons of BMI

Despite its advantages, many have criticized the ratio when it’s used to gauge obesity on a personal level. There are plenty of reasons why BMI is not a reliable measurement of fitness. Often its biggest strength — its simplicity — seems to be its greatest weakness.

For example, the formula is so simple that it doesn’t account for variations in body type, including differences in muscle, bone, and fat. One reason for this is because Quetelet never intended his creation to be used for determining obesity. Instead, it was developed to compare growth.

Additionally, the formula only squares height, which can skew individuals’ results. Specifically, it can make short people seem thinner and tall people seem fatter.

Besides BMI, there are also more accurate measurements of health that exist, such as the waist-to-hip ratio.

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So, is BMI accurate? After looking at the formula and the arguments, it’s pretty clear that it has its limitations, but that doesn’t mean it’s completely worthless. It can still give you an idea of where you are on the weight spectrum, just don’t use it as your only measurement.

Further Reading

Harvard T.H. Chan School of Public Health — Measuring Obesity — Bye-bye BMI? Tape may measure obesity better