What Is BMI, What Are Its Limitations, and How Is It Calculated?

Body Mass Index, commonly abbreviated as BMI, is one of the health indicators meant to reveal whether you weigh too little, just the right amount, or too much and need to lose weight. However, you may have calculated it yourself before and noticed that the result doesn’t always fully reflect reality. For instance, in bodybuilders, it often indicates overweight or even obesity.

What should we make of it then? Can we really trust BMI and rely on it to keep track of our weight and overall health?

What Is BMI?

Body Mass Index, more commonly known as BMI, is a straightforward tool for assessing body weight. The number you get from the BMI calculation gives you a basic idea of how well your weight aligns with your height. Its simplicity and quick calculation likely explain its widespread use and popularity. It was first devised in 1830 by a Belgian statistician and mathematician, Adolphe Quetelet. At the time, scientists needed a method to quickly and efficiently gauge the weight of populations, helping to identify potential health risks. BMI not only helps track obesity rates, but also highlights underweight cases, which is particularly important in developing countries. ^[4,8]

Nowadays, based on BMI, we can say that in 2019, around 59% of adults living in European Union countries were either overweight or obese. The percentage was higher among men (63%) than women (54%). Using these results and statistics, countries can then devise strategies to tackle the issue effectively. ^[17]

BMI was originally designed for tracking populations, not for evaluating individual weight or predicting personal health issues. Over time, however, it started being used for these purposes as well. This is where its biggest flaws become evident. It largely overlooks individual differences across the population, such as fat distribution, age, or gender. However, these factors are crucial in determining whether someone’s weight poses a greater health risk.

How Do You Calculate BMI?

Calculating BMI is very straightforward. You only need to know your body weight in kilograms and your height in metres, and then use this formula:

BMI = body weight (kg) / height (m)²

To illustrate this, let’s for example consider Anna, who weighs 60 kg and is 165 cm tall. Her BMI calculation would look like this:

BMI = 60/1.65² = 22.03 kg/m

With this simple calculation, Anna can determine that her BMI result falls, as we will discuss later, within the ideal weight range for her current height. When combined with other methods, such as techniques for measuring body fat, she can gain a clearer understanding of whether her weight is optimal from the standpoint of health.

You can calculate your BMI anytime you like. For an even easier calculation, you can use our BMI calculator.

What Does My BMI Result Indicate?

The value you obtain from the BMI calculation gives you an idea of whether you have a normal weight, are underweight, overweight, or obese. Based on this, you can further estimate how your weight might impact your health. Let’s have a look at the specific ranges and what they mean.

Brief classification of BMI

You will most often come across this simpler classification in the form of a BMI table established by the World Health Organization (WHO).

	BMI
Underweight	<18.5 kg/m2
Optimal weight	18.5 – 24.9 kg/m2
Overweight	25 – 29.9 kg/m2
Obesity	≥30 kg/m2

What do these values mean in practice?

• A BMI below 18.5 kg/m² is considered very low and indicates underweight. In this case, it is likely necessary to gain weight to maintain a healthy body.
• A BMI of 18.5 – 24.9 kg/m² indicates a healthy body weight within the optimal range. While we cannot determine the exact body fat percentage, this result suggests at first glance that the risk of health problems is low.
• A BMI of 25 – 29.9 kg/m² indicates that you are overweight. If this weight is due to excess fat, your body will respond positively if you lose a few extra pounds. Achieving this can be aided by adjusting your diet, following a well-thought-out training plan, and making overall lifestyle improvements.
• A BMI over 30 kg/m² indicates obesity. This is associated with numerous health risks, so it’s important to start losing weight through a reduction diet. However, obesity is often a complex health issue, making it wise to address it with a doctor—specifically an obesity specialist—and to consult with a nutrition therapist about dietary changes.

More detailed classification of BMI

For an even better understanding, there is also a more detailed classification in the form of a BMI table, also from the WHO, which divides obesity into several degrees based on severity.

	BMI
Severe thinnes	<16.5 kg/m2
Underweight	16.5 – 18.4 kg/m2
normal weight	18.5 – 24.9 kg/m2
Overweight	25 – 29.9 kg/m2
Class 1 obesity	30 – 34.9 kg/m2
Class 2 obesity	35 – 39.9 kg/m2
Class 3 obesity (morbid obesity)	≥ 40 kg/m2

Unfortunately, BMI alone does not always provide a clear picture of whether your weight is healthy or not. To gain a complete understanding, you ideally need to know an individual’s fat and muscle mass percentage. For instance, a slim person with a lot of muscle, such as a bodybuilder, might fall into the overweight or obese category based on BMI. Conversely, it’s also possible for someone to be classified as underweight or of normal weight while actually having minimal muscle mass and excessive fat percentage. This situation is referred to as “skinny fat,” which poses health risks like elevated cholesterol levels, insulin resistance, and other metabolic issues. ^[7]

Such problems mainly arise when you use BMI to assess individuals. However, when screening larger populations—where BMI is more suitable—these shortcomings are less pronounced. After all, most people are not bodybuilders, but have a typical body composition.

If you do calculate your BMI to get the best possible idea of your status, also find out what your body fat percentage is. Devices like the InBody can provide this service, along with other methods that we will describe in the article.

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Limitations of BMI

1. It does not take body composition into account

Do you remember the bodybuilder who may be classified as obese according to BMI? This happens because the calculation does not consider the percentage of muscle and fat in the body. In general, it tends to calculate based on a “typical” person with an average amount of muscle. However, when it comes to an above-average muscular athlete, the high weight due to muscle mass is interpreted by BMI tables as overweight or obesity.

We’re talking about a significant limitation of BMI, particularly because obesity has a specific definition. It refers to the excessive accumulation of body fat that poses a health risk. On the other hand, an excess of muscle mass is not a concern. ^[9]

As you may know, muscles are denser than fat, which is why athletes often receive misleading results from the BMI calculation. It’s quite easy to classify a muscular athlete with 8% body fat as obese. Furthermore, BMI can be discouraging when you are losing weight while engaging in strength training. Your physique may change dramatically, and you may appear slim, but the scale and BMI might suggest otherwise.

Therefore, it’s ideal to measure both body fat and muscle mass before drawing any conclusions. A higher BMI primarily indicates weight that is above the “normal” range, but it does not automatically mean an excess of fat and overweight. ^[10]

If you’re curious about what else, besides muscle mass, can contribute to a higher weight, be sure to check out our article: Why the Scale Shows You a Higher Number and It’s Not Fat

2. It ignores fat distribution

It’s not only important how much body fat you have, but also where it is stored. We primarily distinguish between two types of stored fat—subcutaneous and visceral. Subcutaneous fat is the one we can see at first glance. It tends to accumulate on the thighs, buttocks, and under the skin of the abdomen. This primarily poses an aesthetic issue.

In contrast, visceral fat may not be visible at first glance because it is stored around the organs in the abdominal cavity, yet it is far more risky. It is associated with issues such as cardiovascular diseases, type 2 diabetes, and even some types of cancer. ^[1,14]

However, the BMI calculation cannot assess which type of fat you have in excess. Therefore, a person with a higher BMI, who has most of their stored fat in the lower part of their body, may not have a problem. Conversely, someone with a BMI above the normal range, whose fat primarily accumulates around the abdomen, should be more cautious.

3. It does not take gender into account

The aforementioned body fat distribution is directly related to gender. Fat is typically stored differently in men and women. You’ve likely heard of the pear and apple body types. The pear shape is typical for women, as they often have most of the fat stored in the hips and thighs. In contrast, fat in the male body tends to accumulate around the abdomen, giving it an apple-like shape. Men are also more prone to excessive storage of visceral fat.

In practice, this means that a man and a woman with the same BMI value can have significantly different health risks. A woman with most of her fat stored in the lower body can be perfectly healthy even if she is overweight. Conversely, a man with excess fat mainly around his abdomen might need to reconsider his lifestyle and lose weight in order to avoid issues such as high blood pressure, cholesterol, or blood sugar levels.

4. It does not take age into account

Age is another factor that determines what amount of stored fat is healthy for you. However, the BMI calculation does not take into account how old you are, and thus it cannot indicate what the ideal weight should be according to age. As a result, the assessment can sometimes be misleading. After all, the fit body of a 25-year-old woman naturally looks different from that of a healthy, active woman in her 60s. While younger people have an easier time maintaining a slim figure and an optimal amount of muscle mass, it’s physiologically more natural for older individuals to have slightly less muscle and more stored fat.

Moreover, while you can often hear that higher BMI and being overweight lead to greater health risks, the threshold for what is considered a high BMI differs slightly for seniors. In older age, the lower limit of an optimal BMI isn’t 18.5 but rather 23 kg/m². A lower BMI in older adults is actually associated with higher mortality rates. So, when calculating BMI, it’s always important to take the person’s age into account. ^[14]

5. It does not take ethnic differences into account

Another issue with BMI calculation is that it doesn’t take into account the differences in body composition across various ethnic groups. This is because the BMI formula was originally developed based on data from white populations. In practice, this makes it less applicable to other ethnicities. For example, Asians tend to have a higher body fat percentage at lower BMI values compared to white individuals. On the other hand, people of African descent often have higher bone density and more muscle mass, which also affects how their BMI should be interpreted.

This means that the same BMI value can indicate different body compositions across various ethnic groups, leading to different health risks. Ideally, when evaluating BMI, specific characteristics of each ethnic group should be considered to provide a more accurate assessment of an individual’s health. ^[3,14]

How to Determine the Amount of Body Fat?

BMI doesn’t provide much insight into the amount of fat in the body. Therefore, you can only estimate based on what you see in the mirror. However, there are several methods that can measure or estimate this.

• Waist circumference (WC), waist-to-hip ratio (WHR), and waist-to-height ratio (WHtR) can help you estimate the amount of body fat. The estimated values are obtained from tables that list values corresponding to the given parameters. For example, if a woman’s waist circumference exceeds 88 cm and a man’s exceeds 102 cm, it indicates a high health risk. ^[2]
• Using a fat caliper, you can measure the thickness of skinfolds, which allows you to estimate your body fat percentage.
• Bioelectrical impedance scales measure body fat by assessing the electrical conductivity of various body tissues. Based on this information, you can determine your proportions of muscle mass, fat, and more. This principle is utilized by well-known devices like InBody and Tanita.
• Methods such as densitometry or DEXA (dual-energy X-ray absorptiometry) provide the most accurate results, but they are primarily used for scientific purposes, and the average person will rarely encounter them in practice. ^[5]

What Are the Risks Associated With High BMI?

Let’s consider a person with a high BMI who is not exceptionally muscular. In this case, the results are not distorted by muscle mass; the individual genuinely falls into the overweight or obese category. Here, BMI serves its purpose, raising a red flag to warn you that you have too much body fat, which can lead to various health issues. What are the complications associated with overweight and obesity?

• high blood pressure
• high cholesterol levels
• cardiovascular issues, including heart attacks
• type 2 diabetes
• dna
• sleep apnea
• metabolic syndrome
• mental health issues
• some types of cancer (e.g., colorectal, prostate, breast, or uterine cancer)
• infertility
• sleep apnea
• pregnancy complications
• orthopaedic issues
• non-alcoholic fatty liver disease ^[11,12]

Do you want to know to what extent genes contribute to obesity? If so, then you should check out the article: How Do Genes Influence Your Weight and Tendency Towards Obesity?

What Are the Risks Associated With Low BMI?

Every day, you can hear about how obesity poses a danger to your health. However, the opposite problem, underweight, is not talked about as much. Yet, it can also pose significant risks.

• Nutritional deficiencies due to inadequate diet are quite common. These can include a lack of vitamins and minerals, such as iron deficiency anaemia or hypocalcaemia.
• There is a risk of osteoporosis (bone thinning) and related more frequent fractures.
• It can lead to a weakened immune system and hormonal changes.
• It causes fatigue, overall lower energy levels, and reduced physical and athletic performance.
• Underweight is often associated with eating disorders such as anorexia nervosa, orthorexia nervosa, or bulimia nervosa.
• It can also lead to hair loss and skin problems.
• In both women and men, it can cause fertility issues. ^[16]

The “female athlete triad” often occurs in female athletes and is typically linked to underweight, low caloric intake, and high energy expenditure. This condition is characterized by low energy availability, menstrual dysfunction, and reduced bone density.

You can learn more about the female athlete triad in the article: How to Combat the Absence of Menstruation and Other Symptoms of the Female Athlete Triad?

Should You Rely on BMI?

The Body Mass Index (BMI) has long been a staple in assessing body weight, and certainly holds its value in practice. However, it’s crucial to recognize its limitations to avoid being misled by potentially distorted results. If you’re using BMI to gauge whether your weight is within a healthy range, consider complementing it with body composition measurements. By understanding your fat and muscle percentages alongside your BMI, you’ll gain a more comprehensive view of your health, making the BMI a more insightful tool in your wellness journey.

How to evaluate BMI in different groups of people?

Body Mass Index is applicable for most people, but it cannot be reliably applied to certain groups.

• For athletes, BMI often gives incorrect results due to a higher muscle mass, which can push the index into the overweight or obesity range. That’s why it’s important to also consider body fat percentage and muscle mass when assessing their overall health.
• For seniors, the optimal BMI is higher than for younger adults. It’s considered healthier for them to maintain a BMI of at least 23 kg/m².
• For children and adolescents, BMI is evaluated differently than for adults. In this case, percentile charts, which take into account age and gender, are used. These charts are commonly utilized by paediatricians. ^[13]
• For pregnant women, BMI is not used to assess weight. The weight gain during pregnancy mainly consists of the baby, placenta, amniotic fluid, increased blood volume, and breast tissue. Body fat typically makes up only a small portion of the total weight gain. Instead, there are recommended weight gain guidelines for pregnant women, which are based on their pre-pregnancy weight.

What are the alternatives to calculating BMI?

Calculating BMI isn’t the only method for estimating optimal body weight. There are several other indices that can help determine it as well.

• BMI Prime is an adjusted calculation of the Body Mass Index. It is determined by dividing the BMI value by 25, which is the upper limit of a healthy BMI. The resulting number indicates the risk in relation to this threshold. For instance, a BMI Prime of 1.4 means that the individual weighs 40% more than they should for optimal BMI.
• Rohrer’s index (corpulence index) is primarily used for children and adolescents, as it better accounts for growth changes. To calculate it, you need the weight in grams and the height in centimetres. For example, a result below 110 kg/m² indicates underweight. ^[15]
• Broca index can only be applied to individuals with a height between 155 and 165 cm, which limits its usage significantly. ^[6]

What Are the Main Takeaways?

BMI is a useful tool for monitoring weight, but it has its drawbacks. It doesn’t account for the differences among individual people, such as body composition, fat distribution, ethnicity, or age. The same BMI value can represent different health risks for different individuals. Therefore, when tracking weight, you should not rely solely on the BMI value; you should also consider other factors, such as the proportion of muscle and stored fat, as well as its distribution in the body. Ideally, you should use BMI as a guiding tool and combine it with other methods for a more comprehensive assessment.

Do you have a better understanding of how BMI works now? If you enjoyed the article, feel free to share it with your friends and family!

Sources:

[1] BAJPAI, A. Waist-to-Height Ratio—The New Body Mass Index? – https://link.springer.com/article/10.1007/s12098-022-04257-2

[2] FLEGAL, K. et al. Comparisons of percentage body fat, body mass index, waist circumference, and waist-stature ratio in adults. – https://pubmed.ncbi.nlm.nih.gov/19116329/

[3] JEONG, S.-M. et al. Different correlation of body mass index with body fatness and obesity-related biomarker according to age, sex and race-ethnicity. – https://www.nature.com/articles/s41598-023-30527-w

[4] KRAKAUER, N.Y. - KRAKAUER, J.C. Untangling Waist Circumference and Hip Circumference from Body Mass Index with a Body Shape Index, Hip Index, and Anthropometric Risk Indicator. – https://pubmed.ncbi.nlm.nih.gov/29649376/

[5] KURIYAN, R. Body composition techniques. – https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6366261/

[6] LAURENT, I. et al. The use of Broca index to assess cut- off points for overweight in adults: A short review. – https://pubmed.ncbi.nlm.nih.gov/32495251/

[7] MOHAMMADIAN KHONSARI, N. et al. Normal Weight Obesity and Cardiometabolic Risk Factors: A Systematic Review and Meta-Analysis. – https://www.frontiersin.org/journals/endocrinology/articles/10.3389/fendo.2022.857930/full

[8] PRAY, R. - RISKIN, S. The History and Faults of the Body Mass Index and Where to Look Next: A Literature Review. – https://www.ncbi.nlm.nih.gov/pmc/articles/PMC10693914/

[9] WORLD HEALTH ORGANIZATION Overweight and obesity. – https://www.who.int/gho/ncd/risk_factors/overweight_text/en/

[10] WU, Y. et al. Advantages and Limitations of the Body Mass Index (BMI) to Assess Adult Obesity. – https://www.ncbi.nlm.nih.gov/pmc/articles/PMC11204233/

[11] BMI Classification Percentile And Cut Off Points - StatPearls - NCBI Bookshelf. – https://www.ncbi.nlm.nih.gov/books/NBK541070/

[12] Health Risks of Overweight & Obesity - NIDDK. – https://www.niddk.nih.gov/health-information/weight-management/adult-overweight-obesity/health-risks

[13] Child and Teen BMI Categories | BMI | CDC. – https://www.cdc.gov/bmi/child-teen-calculator/bmi-categories.html

[14] Physiology, Body Mass Index - StatPearls - NCBI Bookshelf. – https://www.ncbi.nlm.nih.gov/books/NBK535456/

[15] Ponderal Index Calculator. – https://www.omnicalculator.com/health/ponderal-index

[16] Underweight: A Heavy Concern. – https://www.todaysdietitian.com/newarchives/tdjan2008pg56.shtml

[17] WHO European Regional Obesity Report 2022. – https://www.who.int/europe/publications/i/item/9789289057738