What is BMI and what does a BMI calculator do?

Last updated date: 28-Apr-2022


13 mins read

BMI is an acronym for Body Mass Index, and it is used as a tool for screening patients as it is believed to indicate if the weight of an individual is within normal limits according to their height. So, as mentioned, to calculate the BMI, the parameters needed are the weight and the height put in the following formula: BMI = kg/m2, for the metric system. In other words, BMI’s main purpose is to diagnose different stages of obesity, but the Body Mass Index alone cannot replace the more specific medical tests for this condition, which should be taken whenever the first assessment of the quickly done BMI indicated towards the diagnosis of obesity.

Either the metric or the imperial systems of the BMI calculator can be used to obtain the measure of one’s body size and composition, but the BMI calculator formula must be changed accordingly:

  • BMI calculator in metric: BMI = kg/m2

      With the metric system, the heigh can be in meters or centimeters.If the individual height is measured in centimeters, it must be converted to meters, dividing it by 100, because the kg-cm BMI calculator doesn’t exist.

  • BMI calculator in imperial: BMI = lbs. x 703/in2

When using a BMI calculator in lbs. is necessary to multiply the entire formula by 703 to transform from pounds and inches to kilograms and meters.

Ultimately, the BMI calculator indicates a number by dividing one’s body weight expressed either in kilograms or in pounds with the square of their height in meters or inches, this number is, then, fit into one of five categories supposedly revealing if the individual in question is underweight, normal weight or overweight. So, in other words, the BMI calculator indicates the characterization of one’s weight.


What is body fat?

Fatty tissue, or adipose tissue in medical terms, is a specialized type of connective tissue, vital for the human body. The fatty tissue is localized both under the skin and around the organs in the abdominal cavity. The adipose tissue plays numerous roles to ensure the proper functioning of the entire human body, like shaping the form of our bodies, protecting the abdominal organs, secreting hormones that contribute to the energetic metabolism, and last, but not least, storing immense quantities of energy.

Body fat is essential for the normal functioning of numerous vital organs and having too little of it can lead to dangerous medical conditions, mainly because some of the vital organs are no longer protected by adipose tissue. On the other hand, too much body fat can lead to other serious medical issues like hypertension or high blood pressure, type II diabetes, fatty liver, and some heart diseases.

Constant body fat needs to be sustained for a healthy lifestyle, proper meals and regular exercising can help achieve that. A balance between each element of the body composition must be maintained because too much fat is as bad as too much muscle mass, and too little fat tissue or muscle mass is not advised as well, as it can do as much harm.

Back to the Body Mass Index, it can be useful for healthcare professionals to tell if the patients have a normal amount of body fat, just by taking simple measurements. It’s pretty fast and easily done and it offers somewhat a good first impression of one’s body constitution and composition, but alone it’s not enough to properly say if the patient has a healthy body weight or not. Other medical tests must be taken and a blood sample must be analyzed, to surely and successfully place a diagnosis.

For a better understanding of the BMI calculator chart, the following table will show each of the five weight categories one can fall into, by nutritional status and their numeric intervals


Nutritional status




Normal weight




Obesity class I


Obesity class II


Obesity class III

The table above applies only to adult individuals, starting with the age of 20 years old. For kids and teenagers, ages from 2 to 19 years old, the BMI calculator percentile is used, even though, the formula is the same as is for adults, each value of the BMI is being compared with others from the same age and sex categories by percentile ranking. BMI for children and teenagers needs therefore to be age and sex-specific because throughout puberty the body suffers a series of changes, weight fluctuation being one of them, so it needs to be compared to a majority having the same specifications, to assess if the BMI is normal or not. Moreover, teenage girls tend to have a larger amount of body fat than boys, this also has to be considered when evaluating a patient, so mistakes are avoided.


BMI calculator by age and sex

As mentioned before, body fat is normal in different amounts for men and women. Women have more fatty tissue due to their bodies` adjustment to giving birth and it mostly disposed on the hips and tights, whereas men’s body fat is merely placed around the abdomen region. When it comes to age, it must be mentioned that an older human body has more fat, than a younger human body, both having the same BMI value. Despite these facts, in adults, when the BMI is determined, neither age nor sex is taken into account when the formula is applied, thus the conclusion that BMI in its original form, is not enough to properly evaluate a patient’s overall health and wellbeing, was drawn later on. It was revealed that BMI doesn’t only fluctuate with age and sex just for children and teenagers and is as important for adults. So, the BMI formula and BMI calculators were adapted accordingly to be specific for sex and age.

BMI calculator for women is the same as BMI calculator for men, formula-wise, but other factors are being taken into consideration when differentiating the two, like the fact that women need a little bit more body fat for their bodies to function properly than men need. Before placing a diagnosis of a normal or abnormal nutritional status, a lot more tests are performed, like a thorough medical history exam and a complete physical exam, blood tests, and medical imaging can always help to accurately place a diagnosis.


Can BMI be used for weight loss?

An accurate BMI calculator for a specific adult, when it comes to losing weight, is not as accurate, because the value of one’s BMI can remain the same while the body composition of the same person changes, replacing body fat with muscle mass, and a BMI measurement, no matter how realistic the BMI calculator is, it can not distinguish between fatty tissue and muscle tissue. Thus, when it comes to weight loss, the BMI is not as helpful and it can actually be misleading.


Smart BMI calculator

The Smart Body Mass Index or SBMI is the newest version of the BMI, it was developed rather recently, about five years ago, and it holds improvements that helped doctors assess patients a lot more accurately than before with the classic form of BMI. There are three aspects that make SBMI superior to BMI. These new and improved aspects of this new index for body measurements are:

  • Taking age and sex into account when calculating BMI, besides weight and height;
  • Making SBMI a comparative figure, measured on a scale from zero points to 70 points, no physical units are involved;
  • Obtaining the significance of one’s body weight for their overall health, this was not achievable using BMI, as mentioned above.

The following table shows the values of SBMI.

Smart Body Mass Index

Nutritional assessment

Weight-related level of risk

0/70 – 9/70

Extreme anorexia

Very high risk

10/70 – 19/70


High risk

20/70 – 29/70

Moderate to underweight

Moderate risk

30/70 – 39/70

Normal weight

Low risk

40/70 – 49/70

Moderate overweight

Moderate risk

50/70 – 59/70

Overweight to obesity

High risk

60/70 – 70/70

Morbid obesity

Very high risk


Reversed BMI calculator

This version of BMI’s calculator could somewhat be considered as a tool for monitoring weight loss, but as the classic, BMI it’s not too accurate for this process. It operates with the same formula BMI = kg/m2, but you can calculate what weight you should have for your specific height and desired BMI. Although the reversed BMI is useful for estimating the ideal weight one should have, by statistics, it is not the standard way of evaluating the optimal weight each and every specific person should have in order to be considered healthy, medically speaking.


BMI calculator’s usefulness in different professions

  • In pediatrics medicine:

In pediatrics, the BMI calculator percentile is still used, but not very often, as its specificity to diagnose obesity is not very strong, however, it can still give the most basic image of the patient body’s composition before more extensive tests are performed.

  • In military services:

Both the US Army and Navy have developed a specific BMI calculator, each, to use as a criterion to help evaluate and eliminate candidates for military services, as each future soldier must meet the standard imposed by the US Army and Navy.

The standards for men’s and women’s body fat as required by the Navy will be listed in a table below:





Body fat percentage


Body fat percentage

18 to 21


18 to 21


22 to 29


22 to 29


30 to 39


30 to 39


Over 40


Over 40


The Army BMI calculator is not as accurate as the BMI calculator for the Navy, which is, perhaps, the most accurate type of BMI calculator. It is called the “Navy Body Fat Calculator” and it is used to estimate the amount of body fat of each Navy candidate. The body measurements needed to calculate the body fat, according to the US Navy, are:

  • Age;
  • Sex;
  • Height – for a better assessment, it’s recommended to be done barefoot;
  • Circumference of the neck – it should be measured just underneath the larynx, underneath Adam’s apple for men;
  • Circumference of the waist – it should be measured horizontally for women and men as follows:
  1. For men around the navel;
  2. For women and the narrowest part of the abdomen;
  • Circumference of the hips – it should be measured at the widest part of the hip.

Having these many parameters needed to assess the body fat, the result is more realistic than the result of the classic BMI formula and the standard is well adapted with each age group for both men and women.

  • In athletic professions and sports medicine:

When it comes to athletes, BMI can be quite misleading at first, and with no other assessments, the BMI can misguide the athlete or the physician towards the complicated diagnosis of obesity and wrongfully so. This happens even when the individual in question is visibly in good physical shape. The misleading comes from the BMI calculator’s inability to differentiate between adipose tissue and muscular tissue. Athletes are exposed, almost daily, to soliciting and repetitive physical activities, thus they gain a lot of muscle mass, along with that newly obtain muscle tissue, the bones suffer numerous processes to adapt to it, in order to properly sustain the body, as a result, the density of the bones increases, making them weigh a lot more. This process can sometimes add up to 10 kg (22 pounds). Sometimes athletes score a poorer BMI value than non-athletic individuals from the same-height category and are placed in the obesity class I category, regardless of the fact that clearly, shape-wise, the athlete is in a much better form. Again, this happens because the BMI calculator doesn’t have high specificity for fat alone, it rather compares the entire body weight to its height, so it is not quite precise. 

To avoid this confusion, an athlete's BMI calculator, the index of Fat-Free Mass or FFM was introduced, with this, the body fat of athletes could be properly estimated. The FFM, also known as lead body mass, refers to every one of the body’s components except fat tissue. The lean body mass includes bone mass, muscle mass, organs, and the quantity of water retained by the body. Even though all these components are accounted for, the fat-free mass mainly points out the muscle mass.

The following table will present the minimum amount of fat tissue required to remain healthy and the ranges of the proper amount of the body fat, as estimated by the American College of Sports Medicine:

% of body fat 

Minimal amount

Healthy amount

Healthy FFM










Can the BMI calculator be used in pregnancy?

Along with the progression of the pregnancy, a certain amount of weight gain is expected, in order to carry out a healthy pregnancy and deliver a healthy newborn baby. To evaluate this weight gain, determined by the growth of the fetus, is hard using only the classic BMI formula. To do that the BMI needs to be calculated prior to the pregnancy, thus meaning the mother’s nutritional status pre-pregnancy also holds great importance. As the pregnancy progresses the BMI is evaluated with each month of the pregnancy and then the value of weight gained during pregnancy is determined by subtracting from the values measured each month, the value measured pre-pregnancy. As this process can be hard to track, a BMI calculator for pregnancy was introduced. This pregnancy weight gain calculator uses as measurements:

  • Pre-pregnancy weight;
  • Current weight;
  • Height;
  • Week of pregnancy.

The normal amount of weight gained during pregnancy differs from woman to woman and is very much influenced by the nutritional status of the mother before the pregnancy, if the mother is underweight, more weight is needed to be gained for a healthy pregnancy, both for the mother and the fetus, on the other hand, if the mother is obese, any class of obesity, less weight is expected to gain, sometimes if weight loss is advised. The normal amount of weight expected to be gained by a normal weight mother ranges from 10 to 15 kg (26-35 pounds).


A quick history of the BMI calculator

The BMI was introduced for the first time in the early 19th century, around the 1830s, by a Belgian statistician and mathematician, called Lambert Quetelet. The BMI was initially called by the name of Quetelet’s Index. The reason he came up with this index was to help him in his research for the measurements of “the average man”. The purpose of his research was for aiding the government in allocating resources, and also for quantifying the degree of obesity of the mass population. It was meant only as a statistical tool, to give quick information about the nutritional status of the general population, its inventor specifically mentioned that it cannot be used for assessing the specific levels of fatty tissue in a singular individual nor for replacing medical tests, to diagnose obesity. Also, something else that needs mentioning is that Lambert Quetelet had no affiliation with the field of medicine.

Around the 1950s and 1960s, physicians were regularly using Quetelet’s Index and other flawed tables, for assessing their patients’ fat levels and overall health. Not only the tables for height and weight assessment were deeply inconsistent, but they were also put together by only measuring the wealthy and white population, not to mention that most of the individuals measured were men, also age was not taken into account at all. This raised a lot of problems later on because the Index was obviously not made for diagnosing obesity nor for ensuring one’s health, but it was still being used. By the 1970s, a team of certified physicians continued Quetelet’s work and tried to be more inclusive when choosing candidates for their clinical studies, still, no changes were made in the actual formula, only the name changed into Body Mass Index.



The Body Mass Index was very used in the past to diagnose different types of obesity and other weight disorders and is still used today in some fields of medicine and military services, but it cannot accurately estimate the amount of body fat of every specific individual, only based on their weight and height. So, along with the progression of medicine, the BMI was transformed and adapted to indicate as accurately as possible the amount of body fat.

Is important to know that fat tissue is absolutely necessary for a healthy body and for the normal functioning of internal organs, not only because it shapes the body, but because it ensures the protection of abdominal organs and stores a great amount of the body’s energy reservoir. Thus, a 20 to 32 body fat percentage is expected in a healthy fit woman and a 10 to 22 body fat percentage is normal for a healthy and in good shape man.

BMI calculator for kids and BMI calculator for teens are different from the BMI calculator for adults, aged at least over 20 years old, as the first two are correlated with age and sex and express in percentiles. Even though originally the BMI did not include age and sex, it should always be done with these two parameters taken into account, and so the new BMI calculator was introduced, the SBMI. The SBMI calculator is a BMI calculator by age, sex, height, weight.

Even though it is not a specific test, if your BMI reaches over 35, it usually means that something is not entirely well functioning, and it could signal an underlying issue, thus a medical doctor should always be consulted, when you are having concerns about your weight and doubts about having or not having the diagnosis of obesity.


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