BMI Calculator (Body Mass Index)

He developed the BMI scale to quickly estimate the degree of overweight and obesity in a given population, in order to help governments decide where to allocate health and financial resources.

Interestingly, Quetelet claimed that BMI was not useful for studying specific individuals, but to provide a snapshot of the general health of a population. However, it is widely used to measure the health of individuals. And this, as we shall see, can be problematic.

The BMI scale is based on a mathematical formula that determines whether a person is at a “healthy” weight by dividing their weight in kilograms by their height in metres squared:

BMI = weight (kg) / height (m2).

Alternatively, BMI can be calculated by dividing weight in pounds by height in inches squared and multiplying by 703:

BMI = (weight (lbs) / height (in2)) x 703

You can also use an online BMI calculator, such as the one we provide.

Once your BMI is calculated, it is compared to the BMI scale to determine if you fall within the “normal” weight range. Here is the table:

BMI	Meaning	Risk for Health
Less than 18.5	Low weight	High
18.5-24.9	Normal weight	Low
25.0–29.9	Overweight	Low to Moderate
30.0–34.9	Obese class I (Moderately obese)	High
35.0–39.9	Obese Class II (Very Obese)	Very High
40 or more	Obese class III (Extremely obese)	Extremely High

Based on this calculation, a health professional can suggest health and lifestyle changes if you do not fall into the “normal” weight category.

Some countries have adopted this BMI scale to better represent the size and height of their populations. For example, Asian men and women have been shown to have a higher risk of heart disease at a lower BMI compared to non-Asians.

While this may give the health professional a snapshot of a person’s health based on their weight, it does not take into account other factors such as age, gender, race, genetics, fat mass, muscle mass and bone density.

This is why we insist that you should be careful with BMI.

Is Body Mass Index a good predictor of health?

Body Mass Index (BMI) is a standard health assessment tool in most healthcare settings.

Although it has been used for decades as a measure of health based on body size, it has been widely criticised for its oversimplification of what it means to be healthy.

In fact, many claim that BMI is outdated and inaccurate and should not be used in medical and fitness facilities.

Despite the above, if used with care and with some clear assumptions, BMI is a good metric to know the state of health of a normal person and the health risks he or she faces.

Although on a theoretical level and in specific cases we find that BMI is not a good tool, the fact is that most studies show that a person’s risk of chronic diseases and premature death does increase with a BMI below 18.5 (“underweight”) or 30.0 or higher (“obese”).

For example, a 2017 retrospective study of 103,218 deaths found that people with a BMI of 30.0 or more (“obese”) had a 1.5 to 2.7 times higher risk of death after a 30-year follow-up.

Another study of 16,868 people showed that those in the “obese” BMI category had a 20% higher risk of death from all causes and from heart disease compared to those in the “normal” BMI category.

The researchers also found that those in the “underweight” category and the “severely obese” or “extremely obese” categories died an average of 6.7 years and 3.7 years earlier, respectively, than those in the “normal” BMI category.

Other studies have shown that a BMI above 30.0 starts to significantly increase the risk of chronic health problems such as type 2 diabetes, heart disease, respiratory difficulties, kidney disease, non-alcoholic fatty liver disease and mobility problems.

In addition, a 5-10% reduction in a person’s BMI has been associated with decreased rates of metabolic syndrome, heart disease and type 2 diabetes.

Because most research shows an increased risk of chronic disease among people with obesity, many health professionals may use BMI as a general snapshot of a person’s risk.

This is because, although there are specific cases where BMI is not useful, in most cases it is useful. But then… Who should not use BMI as a tool to predict health risk? We will look at that a little later. For now, let’s look at all the disadvantages of BMI.

Disadvantages of BMI

Despite research associating low BMI (below 18.5) and high BMI (30 or more) with increased health risks, there are numerous flaws in its use. Let’s take a look at the most common ones:

It does not take into account other health factors

BMI only answers “yes” or “no” to the question of whether a person is a “normal” weight, without taking into account age, gender, genetics, lifestyle, medical history or other factors.

Relying on BMI alone can overlook other important measures of health, such as cholesterol, blood sugar, heart rate, blood pressure and inflammation levels, and overestimate or underestimate a person’s true health.

Furthermore, although the body composition of men and women varies – men have more muscle mass and less fat mass than women – BMI uses the same calculation for both groups.

It should also be noted that as a person ages, body fat mass naturally increases and muscle mass naturally decreases. Numerous studies have shown that a higher BMI of 23.0 to 29.9 in older adults can protect against premature death and disease.

Finally, simply using BMI to determine a person’s health ignores other aspects of health, such as mental well-being and complicated sociological factors such as income, access to affordable and nutritious food, dietary skills and knowledge, and living environment.

Assume that all weight is equal

Although 1 pound or kilo of muscle weighs the same as 1 pound or kilo of fat, muscle is denser and takes up less space. Therefore, a very thin person with a large muscle mass may be heavier on the scale.

For example, a 200-pound (97 kg) person who is 175 cm tall has a BMI of 29.5, which classifies him or her as “overweight”.

However, two people of the same height and weight can look completely different. One may be a bodybuilder with a large muscle mass, while the other may have a larger fat mass.

If only BMI is taken into account, it is easy to misclassify a person as “overweight” or “obese” despite their low fat mass. Therefore, it is important to consider a person’s muscle, fat and bone mass in addition to their weight.

To give you an idea, all bodybuilders are listed as obese in the BMI calculation. But they’re not exactly what we have in mind when we think of someone obese, are they?

Does not take into account fat distribution

Although a higher BMI is associated with poorer health outcomes, where fat is located is also a consideration.

People with fat stored around the stomach area have a higher risk of chronic disease than those with fat stored around the hips, buttocks and thighs.

For example, in a review of 72 studies involving data from more than 2.5 million people, researchers found that people with fat in the stomach area had a significantly higher risk of all-cause mortality, while those with fat distribution in other parts of the body had a lower risk.

In fact, the authors pointed out that BMI does not take into account where fat is stored in the body, which can misclassify a person as unhealthy or at risk of disease when, in fact, they are perfectly healthy.

Can lead to weight bias

A medical professional is expected to use their best judgement, which means that they will take the BMI result and consider their patient as a unique individual.

However, some health professionals use BMI alone to measure a person’s health before offering medical recommendations, which can lead to weight bias and poor quality health care.

People with a higher BMI more often report that their doctors focus solely on their BMI, even if the appointment is for an unrelated problem. Serious medical problems often go unnoticed or are incorrectly considered to be weight-related problems.

In fact, studies have shown that the higher a person’s BMI, the less likely they are to attend regular medical check-ups because of fear of judgement, mistrust of the health professional or a previous negative experience. This can lead to delayed diagnosis, treatment and care, with consequent health risks.

May not be relevant for all populations

Despite the widespread use of BMI among all adults, it may not accurately reflect the health of certain racial and ethnic populations.

For example, numerous studies have shown that people of Asian descent have a higher risk of chronic disease at lower BMI cut-off points compared to whites.

In fact, the World Health Organization developed Asian-Pacific BMI guidelines, which provide alternative cut-off points for BMI.

Our calculator will not work for you if you are of Asian or black descent (we are working on a calculator that takes these into account).

Numerous studies have shown that these alternative cut-off points better identify health risk among Asian populations. Still, more research is needed to compare these cut-off points with Asian Americans across generations.

In addition, blacks may be misclassified as overweight despite having less fat mass and more muscle mass. This may suggest that chronic disease risk occurs at a higher BMI cut-off point than in people of other races, especially among black women.

In fact, a 2011 study found that black women were considered metabolically healthy at cut-off points 3.0 kg/m2 higher than non-black people, further calling into question the usefulness of BMI for all racial and ethnic groups.

Finally, relying solely on BMI ignores the cultural importance of body size for different groups. In some cultures, greater fat mass is considered healthier and more desirable. Health professionals must take into account what “health” means for each individual and for the community in which they live (as psychological effects can also have an impact on health).

Given that important health decisions, such as surgical interventions and weight loss, are based on BMI and weight, it is important that all health professionals go beyond BMI to ensure that they are making patient-centred recommendations.

Who should not worry about the Body Mass Index?

After all the above, we can conclude that the Body Mass Index is a good tool, as long as we take into account the particularities of each case and the limitations of the tool.

In this sense, BMI is not used for bodybuilders, long-distance athletes, pregnant women, the elderly or young children. This is because BMI does not take into account whether weight is carried in the form of muscle or fat.

People with a higher muscle mass, such as athletes, may have a high BMI but no increased health risk. Those with lower muscle mass, such as children who have not completed their growth or the elderly who may be losing some muscle mass, may have a lower BMI without being a problem.

During pregnancy and breastfeeding, a woman’s body composition changes, so it is not appropriate to use BMI.

Therefore, use our BMI calculator with care and with these aspects in mind.

Also, if you are black or Asian, you should use BMI with care (if you are black, a higher BMI need not be a problem, whereas if you are Asian, the risks start earlier).

Alternatives to BMI

Despite the many shortcomings of BMI, it is still used as a primary screening tool because it is convenient, cost-effective and accessible in all healthcare settings.

Moreover, as mentioned above, BMI IS useful if its limitations are known.

However, there are alternatives to BMI that may be better indicators of a person’s health, although each has its own advantages and disadvantages. Therefore, it is best, in the end, to use several tools and get an overall picture of a person’s situation.

Let’s see what these alternatives are:

Waist circumference

A larger waist circumference – one greater than 35 inches (85 cm) for women or 40 inches (101.6 cm) for men – indicates a greater amount of body fat in the abdominal area, which is associated with a higher risk of chronic disease.

Benefits

It is easy to measure, as only a tape measure is needed.

Disadvantages

Does not take into account different body types and builds.

Waist-to-hip ratio

A high ratio (above 0.80 in women or above 0.95 in men) indicates a greater accumulation of fat in the stomach area and is associated with an increased risk of heart and chronic diseases.

A low ratio (less than or equal to 0.80 in women or less than or equal to 0.95 in men) suggests greater fat storage in the hip area, which is associated with better health.

Benefits

It is easy to measure, requiring only a tape measure and a calculator.

Disadvantages

Does not take into account different body types and builds.

Body fat percentage

Body fat percentage is the relative amount of body fat a person has.

Advantages

Distinguishes between fat mass and fat-free mass and is a more accurate representation of health risk than BMI.

Disadvantages

Convenient assessment tools (such as skinfold measurements, portable bioelectrical impedance analysis and household scales) carry a high risk of error.

More accurate tools (such as dual-energy X-ray absorptiometry, underwater weighing and BodPod) are expensive and inaccessible to many.

Laboratory testing

Laboratory tests are various blood and vital sign measurements that can indicate the risk of chronic diseases (e.g. blood pressure, heart rate, cholesterol, blood glucose levels, inflammation).

Benefits

These tests provide a more detailed review of a person’s metabolic health and do not rely solely on body fat as a measure of health.

Disadvantages

Most of the time, a single laboratory value is not sufficient to diagnose or indicate risk.

Regardless of the assessment tool used, it is important that health professionals do not rely on a single test. For example, a health professional may measure a person’s BMI and waist circumference and, if a concern arises, may perform a blood test.

In conclusion

Body mass index (BMI) is a highly controversial health assessment tool designed to estimate a person’s body fatness and risk of ill health.

Research generally shows an increased risk of chronic diseases as BMI increases above the “normal” range. In addition, a low BMI (below 18.5) is also associated with poor health.

That said, BMI does not take into account other aspects of health, such as age, gender, fat mass, muscle mass, race, genetics and medical history. Moreover, using it as the sole indicator of health has been shown to increase weight bias and health inequalities.

While BMI can be useful as a starting point, it should not be the only measure of your health. Keep it in mind!