What is Body Mass Index BMI (Adults vs Children) | BMI Calculator |

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What is BMI

How BMI is calculated - BMI calculator, Ranges, Limitation, meaning.

BMI stands for Body Mass Index. It's an indicator of how much fat you have in your body, regarding your height and weight. According to many Health governmental institutions, an adult person with a healthy weight should have a BMI number between 18.5 and 25.

Though it's not very precise, it has an overall accuracy of 80%, BMI still the best option when studying the weight evolution of large groups of individuals. BMI was first used in the 19th century by a Belgian mathematician called Adolphe Quetelet (1), to measure obesity and overweight among the population, and worldwide governmental organizations still use it today for the same purpose.

It might be astonishing that professionals of the medical world uses such an "outdated" indicator, but it works. Think of it as the wheel, humans invented the wheel thousands of years ago, but we continue to use it today.

Resource: CDC BMI recommendations

BMI Ranges for Adults

BMI Ranges for Adults

World Health Organization (WHO) classifies the BMI in 6 categories. You can find these categories in the table below.

BMI Ranges Meaning World Health organization Classification
Below 18,5 Underweighted
Between 18,5 and 25 Ideal weight
Between 25 and 30 Overweighted
Between 30 and 35 Obese/Class 1 obesity
Between 35 and 40 Severe Obesity/class 2 obesity
Beyond 40 Morbid obesity/class 3 obesity
Ressources WHO BMI Ranges

What your BMI says about you

Your BMI is higher than 30 - Obesity

Having a BMI higher than 30 is classified as being obese. You have to reconsider your lifestyle and diet to avoid serious health problems. Obesity can trigger various decease and decrease your longevity.
risks related to obesity are:

  1. fatty liver disease 
  2. type 2 diabetes 
  3. pregnancy problems 
  4. heart disease 
  5. cancer 
  6. kidney disease 
  7. sleep apnea 
  8. osteoarthritis 
  9. high blood pressure 
  10. strokes

Your BMI is Between 25 and 30 Overweighted

Unless you lift weights and you have a relatively high muscle mass, you're overweighted. You're not far away from having a healthy BMI, so you should make some adjustments in your diet and lifestyle to prevent your BMI from going up furthermore.

To help you reduce your BMI, you can also try to use a supplement designed to make weight loss easier, such as Trimtone. Trimtone is made from natural ingredients that can help you with the following:

  1. Boosting the metabolism to burn calories faster
  2. Control hunger
  3. Increase energy love

For further info about it, you can take a look at Trimtone review.

Your BMI is Between 18,5 and 25 - Ideal weight

When your BMI is around 18.5 and 25 your weight is considered healthy, try to maintain it the way it is.

Your BMI is below 18.5 - Underweight

If your BMI is below 18.5, it means that you are underweight. this situation can occur for the following reasons: 

Underweight causes

  1. Insufiset nutrient intake: The first apparent reason for being underweight is not to eat enough or to eat poor nutrient foods. 
  2. Genetics: If some of your family members are underweight, your low BMI may be the result of a genetic characteristic among your family as a fast metabolism. Which leads to a higher calorie-burning rate. 
  3. Exessive physical activity: If you're working out a lot or maybe your day job requires significant physical effort. It may lead to underweight. 
  4. Physical or mental illness: some undiagnosed medical problems can be the source of a low BMI. 
Resources: Underweight Adults NHS

Risks related to underweight and low BMI

  1. Weak immune System: Underweight involves immune system issues. Your body becomes more vulnerable to multiple infections and diseases due to a lack of nutrients. 
  2. Reduction of the muscular and bone mass: A study conducted on 1767 women showed that there is a direct correlation between a low BMI.And risks to develop sarcopenia and Osteoporosis. 
  3. Health problems related to malnutrition: Underweight is usually associated with malnutrition problems. Like anemia and vitamin deficiency.
  4. General fatigue and low energy.
Resources: Underweight impact on health 

What to do when you are underweight?

  1. Make some changes in your diet: Do some modifications in your diet and make it more balanced and healthier. 
  2. Consult with your doctor: it's necessary to see a doctor to discover the reason behind your low BMI

How BMI is calculated

Metric System

If you want to calculate the BMI with the metric system (kilogram/meter), The BMI formula is the body weight (kg) divided by the height squared.
BMI= Weight(kg)/Height²(m²).
Imperial System 
To convert this formula to imperial US measurement system you need to multiply it by 703 the for becomes then:
BMI= 703*Weight(pounds)/Height²(inches²).

BMI Limitations

BMI is a measure of a person's body fat. But As I said earlier, BMI has an accuracy of 80%. It will give the majority of people a reliable estimation of their body's fat percentage and overall view on their weight situation. But in some cases, it's not advised to use it. With no further due, let us go through each circumstance where BMI may not be efficient.

BMI inaccuracy: Athletes and persons with significant muscular mass

BMI doesn't take into consideration the muscular mass of the body. Therefore, when calculating the BMI of some athletes and persons who've got a significant muscular structure, the result may be misleading.

Not all athletes are affected by this matter. A marathon runner has a relatively common muscle structure, which is not the case of an NFL player, who needs to have a relevant muscle structure to perform well. 

Let us take the example of Aaron Donald, who is an outstanding Pro NFL player, his current weight and height are respectively about 73 inches and 284 lbs.

According to the BMI formula, Aaron Donald has a BMI of 37.43, Which is considered to be extremely obese. It's completely absurd because Aaron has a body fat percentage around 10% - 15 %, he has almost no fat in his body. A healthy body fat percentage for an average person is around 20%-25%. Thanks to an ACC statistic study, I could find the average BMI for athletes. You can find the results of their study in the table below.

Resources: Athletes Average BMI, Aaron Donald Measurements
Average Athletes BMI
League Sport BMI value
NBA Basketball 24,85
NFL American Football 31,1
MLB Baseball 27,01
NHL Hookey 26,47
PGA Golf 25,16
MLS Soccer 23,47
UFC Mixed Martial arts 24,21

BMI Aged Persons (Above 65 Years old)

As a person ages, its muscular and bone mass may decrease. This mass reduction start occurring around the age of 65. It might start before that age in some cases.

If this occurs, the BMI of this person can be underrated. Since the BMI is the ratio of body weight divided by body height when bone and muscle mass is reduced, the BMI value drop-down. This is why experts say that older persons should have a BMI of around 25 to 27.

The medical name for Muscular mass loss is sarcopenia. While the medical name for bone mass loss is osteopenia.

Resources: BMI US national library of medicine - sarcopenia, and osteopenia

BMI for children and teens

The formula to calculate BMI for children and teens is the same as for adults.

  1. Metric measurement system: BMI= Weight(kg)/Height²(m²).
  2. Imperial measurement system:703*Weight(pounds)/Height²(inches²).
However, the interpretation and classification of the result are not the same. Because teenagers and children are still in a growth phase.

After determining the BMI for a child, to see if your child's weight is healthy. You need to use BMI percentile charts which take into account the children specificities depending on their age and gender. These charts are based on data collected and treated by the CDC during the last 50 years in the US.

BMI Child range - using BMI percentile Charts

 The classification of children and teens BMI using percentile charts is shown in the table below:

BMI Percentile child and teens ranges
BMI child Range - percentile charts Interpretation
Below the 5th percentile Underweight
Between the 5th and 85th percentiles Healthy weight
Between the 85th and 95th percentiles Overweighted
Greater than the 95th percentile Obese

How to use BMI percentile child Charts

As an example, let's take a boy of 10 years old with a weight of 100 pounds and a height of 60 inches, His BMI is 70x100/60²= 23.34 kg/m². According to standard BMI classification, He has a healthy weight, but when we place his BMI on the charts, it shows us that this boy's BMI is above the 95th percentile. Meaning that he has a weight higher than 95% of other boys of the same age in the US.

How to use BMI percentile child Charts

The boy then is considered to suffer from obesity. 

You can download official BMI percentile charts for boys and girls aged from 2 to 20 from the CDC website by using the links below:

US CDC PDF percentile charts for Boys.
US CDC PDF percentile charts for Girls.

Ressource: BMI for children and teens

Average BMI of Countries ranking - 2020 numbers

The average BMI of the world population is 24.21. This may seem reassuring since the average worldwide BMI is below 25. But the truth is that some countries have a real problem with overweight and obesity.

In 2020, The worldwide population is around 7.7 billion. There are 1.6 billion persons affected by overweight ( 25<BMI<30) and 979 Million persons who suffer from obesity (BMI higher than 30). The overall worldwide obesity rate is about 13%. The United States is the country with the highest number of obese people in the world. Over 119 Million US citizens suffer from obesity. It might be interesting to notice that the country with the lowest average BMI value is Eritrea 20.5.

You can find in the table below the list of the top 30 average BMI per country, ranked from the highest to the lowest obesity rate.

Countries BMI ranking

Ranking Country name Average BMI 2020 Obesity Rate Population in 2020 Obese Population in 2020
Worldwide  24,21 13% 7 769 874 493 979 002 959
1 Kuwait 30,00 38% 4 270 571 1 618 546
2 United States 28,80 36% 331 002 651 119 822 960
3 Jordan 28,90 36% 10 203 134 3 622 113
4 Saudi Arabia 28,50 35% 34 813 871 12 324 110
5 Qatar 29,20 35% 2 881 053 1 011 250
6 Libya 27,70 33% 6 871 292 2 233 170
7 Turkey 27,80 32% 84 339 067 27 072 841
8 Egypt 29,20 32% 102 334 404 32 747 009
9 Lebanon 27,80 32% 6 825 445 2 184 142
10 United Arab Emirates 28,80 32% 9 890 402 3 135 257
11 Bahamas 28,40 32% 393 244 124 265
12 New Zealand 27,90 31% 4 822 233 1 485 248
13 Iraq 28,00 30% 40 222 493 12 227 638
14 Fiji 27,20 30% 896 445 270 726
15 Bahrain 28,20 30% 1 701 575 507 069
16 Canada 27,20 29% 37 742 154 11 096 193
17 Australia 27,20 29% 25 499 884 7 394 966
18 Mexico 28,10 29% 128 932 753 37 261 566
19 Malta 27,20 29% 441 543 127 606
20 South Africa 27,30 28% 59 308 690 16 784 359
21 Argentina 27,70 28% 45 195 774 12 790 404
22 Chile 27,80 28% 19 116 201 5 352 536
23 Uruguay 26,80 28% 3 473 730 969 171
24 United Kingdom 27,30 28% 67 886 011 18 872 311
25 Syria 28,10 28% 17 500 658 4 865 183
26 Dominican Republic 26,70 28% 10 847 910 2 994 023
27 Algeria 26,20 27% 43 851 044 12 015 186
28 Oman 26,90 27% 5 106 626 1 378 789
29 Tunisia 26,80 27% 11 818 619 3 179 209
30 Palestine 27,60 27% 5 101 414 1 367 179
31 Suriname 27,40 26% 586 632 154 871
32 Hungary 26,30 26% 9 660 351 2 550 333
33 Lithuania 26,60 26% 2 722 289 715 962
34 Morocco 25,60 26% 36 910 560 9 633 656

Bottom Line

BMI is an easy way to measure your body fat and get an idea of how healthy your weight is. According to WHO, a healthy BMI has a value between 18 and 25 kg/m2. However, BMI is not very accurate when it comes to measuring body fat for athletes and older persons.

For children and teenagers, the BMI is calculated the same as for adults. But you need to use BMI percentile charts to have the right interpretation. Even if the BMI was used for the first time in the 19th century. Major health authorities still use it to measure overweight and obesity in the populations.

2 comments for "What is Body Mass Index BMI (Adults vs Children) | BMI Calculator |"

  1. How do the statistics become worldwide when many African countries are not included?

  2. the calculation of the worldwide BMI takes into account all the countries.
    We only displayed the top 30 countries in this post.
    Besides, some African countries are in this it top 30: Egypt, South Africa,Algeria, Tunisia, Morocco


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