Body Mass Index (BMI) – The Complete Guide

The most recent obesity statistics from the World Health Organization (WHO) make grim reading. The WHO published an updated list of statistics in February 2018 and found that global obesity has tripled since 1975. There are now 1.9 billion overweight adults around the world, 650 million of whom are obese.

The terms ‘overweight’ and ‘obese’ are defined as “abnormal or excessive fat accumulation that may impair health” by the WHO. In other words, aesthetics doesn’t enter the equation; carrying too much weight is bad for your health. There are numerous tools used to calculate whether or not a person is overweight. Even with the advances in medical technology, the Body Mass Index (BMI) is one of the most frequently used measurements of a person’s physical shape and general health, some 150 years after it was first conceived.

1. What is the Body Mass Index (BMI)?

The body mass index, occasionally called the Quetelet index, is a value taken from a person’s height and weight. It is also determined via a BMI chart table and is an attempt to quantify the level of tissue mass (comprised of bone, fat, and muscle) in a person.

The result is used to determine if that individual is obese, overweight, normal weight or underweight depending on where they fall within the BMI category ranges.

1.1 How is BMI Calculated?

The body mass index formula is easy to determine. It’s a simple calculation that takes into account your weight and height.

• The imperial BMI formula = Weight (LBS) x 703 ÷ Height (Inches²)
• The metric BMI formula = Weight (KG) ÷ Height (Metres²)

If you wish to calculate your BMI using the imperial system, here’s the height and weight conversion figures:

• 1 kilogram = 2.2 pounds
• 1 foot = 12 inches
• 1 inch = 2.54 centimetres

If you are 5ft 4 inches by the imperial system, you are 162.56cm or 1.63m (rounded up) by the metric system.

Example BMI Calculations

Why the difference? Because there is a certain element of ‘rounding up’ or ‘rounding down’ involved e.g. 26.784 becomes 26.78 and so on.

But where does this place you on the BMI category chart? In the next section we cover the bmi categories and explain the classifications.

1.2 Body Mass Index Categories

Technically, the BMI scale will place you in one of six categories. The table below lists the BMI categories along with the BMI score associated with each category.

The following body mass index chart allows you to manually get your BMI score and includes a classification of the BMI category you reside within.

1.3 BMI for Children

Although the BMI calculator works in the same way for children, the measurement is used differently. While the BMI ranges remain the same for adults throughout their life, these figures change in children because kids are still growing and do so at different rates. As a result, BMI is a measure of weight for height compared to children of the same age. This calculation results in a body mass index percentile.

For instance, if your child is in the 50th percentile, it means that 50% of children of the same age have the same BMI figure or less. The 85th percentile means that 85% of kids have the same or lower BMI and this is a figure where your child is classified as being ‘at risk’ of becoming overweight. At the 95th percentile, your child is classified as ‘overweight’.

Here is what that reading looks like when based on BMI percentiles.

1.4 BMI Charts

Body Mass Index charts are useful for visualizing the ranges associated with each BMI category. You can use them to easily locate your height and weight to determine your BMI score and the associated BMI category you fit within.

BMI Chart for Adults (Men & Women)

BMI Chart for Kids & Teens (Boys)

BMI Chart for Kids & Teens (Girls)

1.5 History

The Body Mass Index is sometimes called the Quetelet Index after its creator, a Belgian mathematician, astronomer, sociologist, statistician and all-around genius named Lambert Adolphe Jacques Quetelet. He came up with the idea sometime between 1830 and 1850 as part of his development of ‘social physics’.

Even so, it was seldom used and only started to become popular in the latter part of the 20th century. The term Body Mass Index was first coined by Ancel Keys and other authors of a paper published in the Journal of Chronic Diseases in July 1972. According to the paper, BMI was “at least as good as any other relative weight index as an indicator of relative obesity.” At least Keys acknowledged that it was “not fully satisfactory.”

Prior to the 1980s, physicians used weight for height tables which differed according to gender. It was during this decade that the BMI calculator became the international standard for obesity measurement. Once governments around the world began noting the obesity problem in society, and started launching healthier lifestyle initiatives, the public at large became aware of BMI.

Initially, the threshold for being overweight was 27.8 but it was lowered to 25 internationally. In 1998, the United States National Institutes of Health followed suit and also reduced the BMI figure. This move resulted in 30 million Americans becoming classified as overweight overnight!

2. What is the Body Mass Index Used for?

Even though the Body Mass Index is over 150 years old, major health authorities such as the CDC and NIH in the United States still believe it is a ‘fairly reliable indicator of body fatness for most people.’

2.1 Public Health Statistics

Even today, BMI is used as the official measure of national obesity rates. For instance, the European Union continues to use it as a yardstick for the obesity epidemic and also suggests that people with higher BMI are at greater risk of diseases such as hypertension, cancer, and cardiovascular disease.

According to the Harvard T.H. Chan School of Public Health, your risk of developing conditions such as type-2 diabetes increases progressively as your BMI rises above 21.

2.2 Weight Range Analysis

The BMI range is also considered to be an accurate measure of your weight range. Therefore, if you have a BMI of 27, you are considered overweight regardless of what you actually weigh and your body composition or gender is not taken into account.

2.3 Screening Tool for Weight Problems

If a physician calculates a higher than normal BMI, the next step is to see if you are at risk of certain health problems. In fact, the main purpose of BMI is to determine whether you’re likely to develop a serious medical problem later in life.

In France for example, BMI is used as a screening tool for child malnutrition. The CDC champions the BMI scale as an effective means of determining whether children and teenagers are underweight, overweight, or obese.

2.4 Fashion Industry

The fashion industry’s obsession with exceedingly thin models has ensured that the BMI calculator has a permanent home there. The industry is constantly under fire for forcing women, in particular, to attain what the UK Women’s Equality Party calls “an unattainable level of thinness in women.” The party has called on all models with a BMI below 18.5 to be seen by a physician from an accredited list who will decide if that woman is healthy enough to work.

There is a law to that effect in Israel where male and female models with a BMI under 18.5 much obtain a medical certificate confirming a ‘normal’ BMI reading. For reference, Kate Moss, one of the most famous models ever, had a BMI of just 15 at the height of her fame. By any measure, including BMI, she was severely thin.

3. Global Body Mass Index Statistics vs. South Africa

BMI measurements vary around the globe and some nations have a greater ‘obesity’ problems than others according to the BMI scales. For a better understanding of global BMI scores, we have compiled statistics from six regions globally. First of all, here is the average body mass of people in different continents expressed in kilograms:

As you can see, Asians are significantly lighter than Americans for example and while they are shorter on average, it is evident that residents in North America have a higher BMI figure.