The BMI Formula

Learn about the history of the formula and how the body mass index is calculated
Adolphe Quetelet

The History of theBMI Formula

The earliest Body Mass Index formula was invented by a Belgian man named Adolphe Quetelet. He was an astronomer, an author, a statistician, a sociologist and a mathematician. He was a polymath – highly educated in both the arts and sciences and lived from 1796 to 1874. Sometime between 1830 and 1850, he developed what he called the “Quetelet index”, which is now known as the Body Mass Index.

It is reported that he was intending to find the definition of a statistically normal man. He not only looked at things such as physical normality, but he also looked at factors such as age of marriage and other behaviours.

When it came to the physical body, he was looking at proportionality. So – the taller someone becomes, the bigger their arm width should be, the bigger the circumference of their head should be and the more they should weigh. However, as part of his project, he realized that weight didn’t correlate proportionally to height, but instead to the square of height.

The simple fact is Quetelet wasn’t looking at anything in relation to obesity or even body leanness, yet his formula is his most widely known creation.

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What isBody Mass Index (BMI)?

In the early 20th century, after modern advances allowed us to better understand the negative effects of obesity on health and mortality, the insurance companies and the medical community struggled to come up with a way to measure a person’s level of body fat. Inventions such as using a caliper to measure body fat by measuring thickness of the skin provided the medical community a way to get an individual’s body fat percentage.

However, actuaries, sociologists and medical researchers found that it was difficult to gather data on body fat for an entire population of people. Since gender, height and weight were very easy to get, they struggled with formulas to determine obesity percentage among a larger group of people using the data available.

In 1972, an obesity researcher published a report on a whole bunch of different formulas that were in use at the time and was able to prove that the Quetelet formula most closely corresponded to the sample population’s body fat percentage as identified by the medical community. Since his sample population included more than 7,000 people from five countries, his work was taken as truth and the Body Mass Index, in its modern form, was born.

The formula used to calculate BMI is actually quite simple to follow. In words, it reads as such: weight (in kilograms) divided by height (in metres) squared. Or for those mathematically inclined readers, the formula would be: W/H2

Let’s look at an example: Joey is 173 centimetres tall. He weighs 81.6 kilograms. The numerator is the weight in kilograms, which is 81.6. Since the denominator requires the height to be expressed in metres, simply divide the centimetres by 100 to get the right number. In Joey’s case, take 173/100, which equals 1.73 metres tall.

To finish getting the proper denominator, we need to take the height and square it. 1.73 multiplied by 1.73 equals 2.9929, which is the final denominator. The formula has now become 81.6/2.9929. After calculating and doing a small amount of rounding, the final result becomes 27.3.

This means that Joey’s BMI number is 27.3, which indicates that Joey is categorized as being overweight. Should Joey lose 6.6 kilograms, he would then be placed into the healthy range of the BMI scale. However, if Joey were to add on 8.1 kilograms, he would be placed into the obese category.

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How is the BMI FormulaUsed in Modern Medicine?

In 1972, the Body Mass Index formula was used primarily by researchers and statisticians. However, the medical community quickly adopted the new measurement for their patients. The fact is that the body mass index formula is quick, practical and cheap. And because it’s been proven to be an accurate indicator of body fat percentage for the majority of the population, it’s also effective.

Since that time, BMI has continued to be used among researchers to gather data on obesity and track health trends among different ethnic groups, genders, age groups and countries. Insurance actuaries regularly rely on BMI to determine how healthy someone is, which affects rates that consumers pay for private health insurance or life insurance.

Also since that time health organizations of major countries began adopting the BMI formula as a standard for use by individuals. It has been implemented at the general practitioner’s office, in schools, in clinics and even been published for individuals to do their own measurements. The calculator and its corresponding categories can be used to determine those who are overweight, but also can be used to help identify someone who may be suffering from an eating disorder, such as anorexia nervosa.

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