# Modern BMI Calculator for Men & Women – Body Mass Index Calculator

The Body Mass Index or BMI is a numeric score calculated using the mass (or weight) and height of an individual. The BMI score serves as a guideline to evaluate if an individual is underweight (score under 18.5), normal weight (18.5 to 24.9), overweight (25 to 29.9), or obese (30 or more).

Use our modern BMI calculator below to quickly and conveniently evaluate your BMI. You can either use the Metric system or the Imperial system.

## How to Calculate Your BMI?

To calculate your Body Mass Index (BMI) you just need to perform a simple division. There are 3 different ways to calculate your BMI depending on which measurement system you use.

### How to Calculate Your BMI in the Metric System?

To calculate your BMI in the metric system simply divide your weight in kilograms by the square of your height in meters. This give the following BMI formula:

BMI_{metric} = \frac{Weight {\scriptsize (in\ kg)}} {Height^2 {\scriptsize (in\ m)}}

### How to Calculate Your BMI in Imperial Measurement?

Alternatively if you are more familiar with the imperial system, there are 2 ways to calculate your BMI. If you want to use your height in feet and inches, please use the following BMI formula:

BMI_{imperial} = 703 * \frac{Weight {\scriptsize (lbs)}}{(12*Height {\scriptsize (ft)}+Height {\scriptsize (in)})^2}

If you prefer to use your height in inches only, the following BMI formula is slightly more simple:

BMI_{imperial} = 703 * \frac{Weight {\scriptsize (lbs)}}{Height^2 {\scriptsize (in)}}

## What Is a BMI Calculator?

A BMI calculator is a specialized weight loss calculator that allows to calculate one person’s BMI, an acronym for Body Mass Index. At the top of this page you can find an awesome BMI calculator to calculate your BMI in a snap.

A BMI calculator computes the mathematical formulas presented in the paragraph above. Instead of having to calculate manually the BMI it is possible to calculate your BMI instantly by using the BMI calculator above.

If you know your weight in pounds and your height in feet and inches the BMI calculator above computes the BMI using your measurements in the imperial system. Additionally if you know your weight in kilograms and your height in meters this BMI calculator also allows you to compute your BMI in the metric system.

## What Is the Body Mass Index?

The Body Mass Index is a simple numeric score calculated using only 2 variables: the mass of one person and her height. BMI is the acronym for Body Mass Index. The BMI can be calculated using the imperial measurement system if we use pounds as a measure of mass and feet and inches as measures of height. The BMI can also be calculated in the metric system if we use kilograms as a measure of mass and meters as a measure of height.

The BMI is defined by dividing the body mass by the square of the body height. The resulting BMI is expressed in units of kg/m2.

Because the BMI is a simple numeric value it is often used as a quick and easy shortcut to evaluate to which extent a person is underweight or overweight. Indeed as the resulting score falls under a common classification we can assess the degree of overweight and obesity by looking up into a BMI table or on a BMI chart.

## History of the Body Mass Index

Between 1830 and 1850 the Belgian statistician Adolphe Quetelet introduced the preliminary notions that led to the invention of the BMI. The modern BMI we use nowadays was proposed by Ancel Keys et al. in their scientific publication in the Journal of Chronic Diseases in 1972.

Originally the BMI proposed by Keys et al. was developed as a tool to evaluate the statistical propensity to obesity of a population of individuals as a whole. It was not developed to evaluate the body fat of a single individual. Instead the simplicity to calculate the BMI and to understand it by everybody resulted in the widespread adoption of the obesity indicator.

The BMI is also called the Quetelet index to honor the eponym scientist who introduced it as stated above.

## How the BMI Calculator Works

A BMI calculator calculates body mass index by dividing the weight in number of kilograms by height in meters squared.

BMI = Body Mass (in kilograms) / Body Height (in meters) squared

For example an individual weighting 81 kg and measuring 1.85 m, the BMI is calculated like that:

• BMI = 81 / (1.85 x 1.85) = 23.67 kg / m2

The body mass index is convenient for the numerical assessment of the state of the patient’s body. In most cases, it correctly displays the presence and degree of obesity.

To calculate your BMI using the BMI calculator above as follow:

1. Choose the measurement system you are familiar with:
• imperial system
• metric system
• in pounds for imperial
• in kilograms for metric
• in feet and inches or only inches for imperial
• in meters for metric
4. Press the button “Calculate Your BMI”

The BMI calculator will then immediately compute your BMI score and display it.

## BMI Classification According to the World Health Organization (WHO)

The World Health Organization (WHO) is the United Nation (UN) international agency that promotes health worldwide. The table below presents the commonly accepted BMI classification according the the WHO.

### Underweight – BMI < 18.5

According to the WHO BMI table above a BMI below 18.5 corresponds to the underweight category.

It is often considered more desirable to be underweight than overweight, but being severely underweight can lead to dangerous health problems as well. There are multiple reasons why an individual might become underweight and consequently there are many ways to remedy this condition. Indeed being underweight can results from genetics, to physical factors or even psychological factors.

Our own genetics plays a crucial role in the physiological reasons of being underweight. People with favorable genes might have a better digestive capacity. That means that their metabolism performs better at absorbing and digesting food and nutrients in a healthy way for their body.

Physical illness might as well play a role in begin underweight. The flu virus, the COVID-19 virus or variants, and even other types of viruses might cause significant weight loss. Loss of appetite is a common side effect of certain medications. Patient under a new medication and experiencing a significant loss of appetite should consult with their physician to check whether this comes from the new medication or is due to another cause.

Too much stress can be a major psychological factor at play causing weight loss due to a lack of appetite. Intense stress often comes from a stressing job. In this case an individual should consider finding a new job or even a change of carrier. But intense stress might come also from individual circumstances. For example a loved one might face a critical illness or condition which causes a lot of stress to her family. This type of intense stress might be harder to alleviate as one might not be able to control such a situation.

Depression is also a major psychological factor causing suppression of appetite. A significant part of the worldwide population experiences depression for years if not tens of years. Among them those who suffer from loss of appetite are at severe risk of being underweight due to how long experiencing depression can last. Fighting depression is hard. Some find refuge in medications but those often work for a limited time and never treat the root causes of the depression. Instead talking therapies and behavioral therapies are in measure to help a very large number of individuals suffering from depression.

It is important to be aware that a BMI under 16.5 is considered severely underweight and it is important to consult a physician or a dietician as she might help regain weight. In particular if there is a suspicion of anorexia it is urgent to seek medical assistance as this might be a life-threatening condition depending of the degree of thinness.

### Normal Weight – BMI 18.5 – 24.9

Being in the 18.5 to 24.9 BMI range is the ideal range one should aim to. Indeed that means that an individual is neither underweight nor overweight. As an obvious and fortunate consequence she is far less likely to be exposed to risk factors associated with other BMI scores out of the ideal range.

It is also important to be aware that a normal weight should be taken care of and kept in control as we might experience weight variations in our daily lives. A balanced and healthy diet like the Mediterranean diet or a more challenging diet like the ketogenic diet should be maintained. A large consumption of vegetables, fruits and proteins with a controlled amount of carbs and fat should be aimed for. Comfort food, junk food and empty food (food empty of healthy nutritional elements) should be avoided the most as they crush the healthiness of your metabolism.

It is also important to maintain a moderate and regular exercise routine. Some might walk daily 30 minutes either while bringing their kids to school or during their lunch break while walking around their office. Other might take the time to go to the gym for practicing workouts at least once a week. As long as you find what works for you to have physical activity on a regular basis you should be able to maintain your ideal BMI range.

### Overweight – BMI 25 – 29.9

It is very common to be in this BMI range as a lot of individuals face some degree of overweight. Even though some might be tempted to consult a dietician this degree of overweight is largely self manageable. Two types of actions can be taken: actions related to your alimentation and actions related to your physical activity.

As mentioned above it is important for one individual to find what works for her related to her physical activity. Some persons are able to integrate activity in their daily life while other dedicate time to sport or exercise. Both are valid and useful and will help in controlling your weight. The important fact to remember is that it’s crucial to maintain some level of physical activity.

Regarding the change of regimen. Most overweight persons have relatively healthy eating habits. They eat daily and consistently fruits and vegetables. They eat meat, fish or plant-based proteins. But where they might be room for improvement is with the carbs and fat. Many persons love bread and eat a lot of bread while they eat a meal. Bread does not cause weight gain per se but we tend to eat too much bread and these calories add up to our daily intake. The resulting calories that your body does not burn are stored into fat.

Another important aspect to consider is that the type of fats we eat play an essential role in how our body will be able to digest them. Indeed saturated fats and trans fats are the ones that make us gain weight. On the contrary unsaturated fats have not been associated with weight gain and are the type of fats that should be favored in your diet. Examples of monounsaturated fats are olive and peanut oil, avocados, almonds and pecans. Examples of polyunsaturated fats are fish, walnuts, sunflower and corn oil. These types of fats should be favored to turn your weight in the right direction.

### Obesity Class I – BMI 30 – 34.9

A BMI between 30 and 34.9 corresponds to the first class of the obesity.

It is important to consult a physician to evaluate the risks for the health of a patient suffering of obesity. Indeed an increased BMI is a major risk factor for these conditions:

• cardiovascular diseases like heart disease and heart attack
• type 2 diabetes
• musculoskeletal disorders like especially osteoarthritis or joint pain
• several forms of cancer among which are breast, liver, prostate, ovarian, and kidney

These are all potentially life-threatening conditions if not followed under medical guidance. That’s why it is imperative to be aware that an increased BMI is a major risk factor, and even more importantly to act accordingly and consult a physician.

### Obesity Class II – BMI 35 – 39.9

A BMI between 35 and 39.9 corresponds to the second class of the obesity.

As for the first class of obesity it is crucial to consult a physician as the larger the BMI the larger it is considered a risk factor regarding the life-threatening conditions mentioned in the previous section. It is equally important to devise a plan of action to correct the overweight and recover a lower BMI.

A proper plan of action might include:

• Non-medical actions consisting mainly in a change of diet, a reduction in calorie intake and an increase in physical activity.
• Medical therapy: some physicians might consider prescribing medications to reduce the likelihood of the life-threatening conditions mentioned above.

### Obesity Class III – BMI 40+

A BMI over 40 corresponds to the third class of the obesity. It is also sometimes referred as morbid obesity. A BMI over 50 is sometimes referred to as extreme obesity.

Even more than for the two first classes of obesity it is vital to consult a physician because a high BMI is considered a major risk factor in the life-threatening conditions mentioned above. It is vitally important to devise a plan of action under medical guidance.

A proper plan of action should start with non-medical actions and medical therapies as described in the previous section. Additionally bariatric surgery should also be considered for an obesity of class III. It will more often than not be recommended by doctors when one individual does not experience significant enough weight loss with the previously mentioned actions. The most common bariatric surgery called gastric bypass consists in reducing the size of the stomach to reduce the amount of food one can eat during a meal and reduce the absorption of nutrients. Bariatric surgery tend to be very effective but since it’s a major surgery it shouldn’t be considered lightly and discussed extensively with your physician.

## What Are Overweight and Obesity?

Overweight and obesity result from excess accumulation of fat in the body. They are considered a major risk factors in multiple noncommunicable diseases and life-threatening conditions.

As BMI is both simple and easy to calculate and comprehend by anybody, it is widely used to categorize individuals as overweight and obese. Our modern BMI calculator allows to calculate your BMI in a snap.

As we have seen above overweight and obesity are categorized as follow for adults:

• BMI ≥ 25 is the definition of overweight
• BMI ≥ 30 id the definition of obesity

As the BMI has been originally designed as a statistical measurement tool it is most efficient as a statistical measure of overweight and obesity for a whole population of individuals like a group of age, a state or a country. Two advantageous features that contribute to its usefulness is that:

• BMI is the same for males and females
• BMI is the same for adults of any age

At the individual level though it should be considered as an approximate indicator because the level of adiposity between individuals might vary. This is also tru for the distribution of fat in the body.

### Overweight for Children Below 5

In contrast to adults where BMI is constant with the age, when considering children their age needs to be taken into account when evaluating the degree of overweight and obesity.

Rather than classifying the BMI according to categories with established thresholds, the BMI is compared to its standard deviation against the population of children of the same sex and age. The standard deviation is a statistical tool that measures the degree of variation among a dataset. A low standard deviation indicates that a value is close to the mean or the average while a high standard deviation indicates the opposite.

For children below 5 the BMI formula is different than for adults. The children BMI is simply the ratio of their weight divided by their height. This ratio is then compared to the whole population of the children in the same class of sex and age using the WHO Child Growth Standards:

• A ratio of weight divided by height > 2 standard deviations above the median is considered overweight.
• A ratio of weight divided by height > 3 standard deviations above the median is considered obese.

### Overweight for Children Between 5 and 19

For children aged between 5 and 19 it takes 4 steps to evaluate their level of overweight:

1. Calculate the BMI using the same formula as adults (and NOT the ratio of weight divided by height used for children below 5 years old).
2. Compare their BMI to those in the same class of age and sex using the WHO Child Growth Standard dataset.
3. Evaluate how many standard deviations from the mean their BMI is.
4. This give the category of their overweight or underweight.

The 2 charts below make it easier to visualize and understand how to evaluate the overweight for children between 5 and 19 years old.

## What Causes Overweight and Obesity?

The human body needs to consume food to source energy and sustain a healthy metabolism. Any food we eat provides a quantity of energy measured in calories. Once these calories are ingested, our body metabolizes them to source energy and function properly.

The root cause of overweight and obesity is the asymmetry between the calories ingested on a daily basis and the necessary calories needed by our body to function properly. When the ingested calories excess the necessary calories, the excess calories are stored into fat in the body. When this happens over a repeated period of time like weeks, months or years this conducts to overweight and eventually to obesity for the more severe form.

Since the end of World War II and the subsequent technological advancement, the multi-millennial scarcity of food reversed gradually into an abundance of food. This new providence took decades to propagate worldwide starting in Western countries before reaching Asia and propagating into Africa.

A very large percentage of humanity nowadays can walk or drive to their nearest grocery store to choose among thousands of tasty foods. Among these there a lot of energy-dense foods to choose from. These energy-dense foods are typically high in carbohydrates or sugars and high in fats. If one ingest an excess of energy during the day whereas our body does need as much, the excess is metabolized into fat in our body.

A second worldwide phenomenon started in the nineteenth century accelerated over the course of the twentieth century and is still largely at play in the twenty-first century: the urbanization of the world. Since the progress of technology reduced the manual labor necessary to produce food for the whole population, these men and women migrated to cities to find a decent living and raise their families.

While living in cities, distances are smaller and mechanized means of transportation are easily available reducing physical activity. On top of that the economy shifted from agricultural and industrial production to services resulting in a large and increasing part of the population sitting in front of a desk 8 hours a day. These joint phenomena favor sedentariness and reduce the daily physical activity for so many of us leading to overweight and obesity.

## How Does Obesity Affect Health?

An adult with a BMI level above 30 falls into the obesity category which can drastically affect her health. Individuals with severe obesity are at risk of being affected by many diseases and conditions like type 2 diabetes, heart disease, elevated cholesterol levels, high blood pressure to cite only a few. These happen on top of obesity and might lead to reduce health significantly and decreased life expectancy among these individuals.

### Type 2 Diabetes

Diabetes is a chronic disease resulting from elevated blood sugar levels in the blood. Type 2 diabetes is the most common form of diabetes among adults. Whereas type 1 diabetes can not be prevented, type 2 diabetes is largely a consequence of poor daily living habits. It can be prevented by eating healthy, avoiding smoking, and exercising regularly.

Technically type 2 diabetes occur when the body becomes resistant to insulin. This prevents the glucose to be absorbed properly by cells. If the level of glucose present in the blood goes out of control this can be lethal. The medical consequences of diabetes are extremely severe as they include heart attack, vision loss, leg amputation, nerve damage, and kidney failure. All these are life-threatening conditions.

### Hypertension

Hypertension or high blood pressure is a medical condition in which the pressure in the arteries is too elevated for extended periods of time. Blood pressure is measured around the arm. It is characterized by the systolic and diastolic pressures which are the minimum and maximum blood pressures respectively. For most adults hypertension is diagnosed if the blood pressure stays above 130/80 or 140/90 mmHg for extended periods of time.

Hypertension is caused by either reduced cardiac output or resistance to blood flow. The latter being the most common among individuals suffering from high blood pressure. Hypertension increases the risks of heart attack, stroke, and many vascular and cardiovascular diseases. It is also a risk factor for dementia.

### Cardiovascular Disease

There are two types of cardiovascular diseases:

1. Vascular diseases that involve the blood vessels such as coronary artery disease, peripheral arterial diseases, or cerebrovascular disease like stroke.
2. Cardiovascular diseases that involve the heart such as heart failure, cardiomyopathy, cardiac dysrhythmias, or inflammatory heart diseases like endocarditis or myocarditis.

The exact causes of each cardiovascular disease vary but there are many common risk factors at play. Age is the most significant risk factor: it triples each decade we live. Men tend to be more at risk than women, although the research is still contradictory for women past menopause. Tobacco is a major risk factor accounting 10% of the direct causes of cardiovascular diseases. The lack of physical activity and a poor diet are major risks factors as well.

### Respiratory Diseases

Respiratory diseases or lung diseases affect our vital capacity to breath and inhale oxygen. There are many types of respiratory disease. Obstructive respiratory diseases are characterized by a reduction of respiration in the airway that limits the amount of air available to breathe. The most well-known obstructive respiratory disease is asthma. Restrictive lung disease result from a reduction of the lung compliance which is their capacity to expand in order to breathe properly. Respiratory tract infections can occur in many parts of the respiratory system. The common cold is the most common upper respiratory tract infection whereas pneumonia is the most common lower respiratory tract infection.

### Other Conditions

There are many other life-threatening conditions and diseases where obesity is a major risk factor:

• Cancer: In the US obesity may cause up to 90,000 cancer deaths per year.
• Kidney Disease: Type 2 diabetes and hypertension are risk factors for kidney disease and kidney failure. As explained extensively above they are two of the main consequences of obesity.
• Infertility: Fatty tissues can cause an enlargement of ovaries which can prevent the ability to get pregnant.
• Joint Damage: Obesity is a risk factor in joint issues such as back pain, arthritis, or spinal disorders.
• Alzheimer’s Disease: Some scientific studies have found that obesity might play a role in dementia and Alzheimer’s disease.
• Heartburn: Technically called Gastroesophageal Reflux Disease (GERD), acid secretions escape the stomach and damage the esophagus. Common symptoms are heartburn, indigestion, coughing at night, or throwing up food. Obesity is linked to higher risk of GERD and inflammation of the esophagus.
• Liver Disease: Obesity is the major cause for fatty liver out of alcoholic induced fatty liver. Fatty liver diseases are very common for most individuals with severe obesity. Fatty liver disease ultimately result in cirrhosis and liver failure.

## References

[1] Ancel Keys, Flaminio Fidanza, Martti J. Karvonen, Noboru Kimura, Henry L. Taylor. Indices of relative weight and obesity, Journal of Chronic Diseases, Volume 25, Issues 6–7, 1972, Pages 329-343, doi:10.1016/0021-9681(72)90027-6

[2] World Health Organization – Body Mass Index

[3] Frank B Hu, Eric Rimm, Stephanie A Smith-Warner, Diane Feskanich, Meir J Stampfer, Albert Ascherio, Laura Sampson, Walter C Willett. Reproducibility and validity of dietary patterns assessed with a food-frequency questionnaire, The American Journal of Clinical Nutrition, Volume 69, Issue 2, February 1999, Pages 243–249. doi:10.1093/ajcn/69.2.243

[4] Harvard School of Nutrition – Types of Fat

[5] World Health Organization – WHO child growth standards for children aged under 5 years

[6] World Health Organization – WHO growth reference for children aged between 5–19 years

[7] Garabed Eknoyan, A History of Obesity, or How What Was Good Became Ugly and Then Bad, Advances in Chronic Kidney Disease, Volume 13, Issue 4, 2006, Pages 421-427. doi:10.1053/j.ackd.2006.07.002.

[8] World Health Organization – Diabetes

[9] US National Library of Medicine – Lung Diseases

[10] Centers for Disease Control and Prevention – Overweight & Obesity