How to calculate BMI (formula)? Classification, table, child

A simple and widely adopted way to calculate the ideal weight is through BMI, the body mass index.

The container is quite simple and, in a short time, it is possible to know if the current weight is in accordance with the general guidelines.

See more about body mass index!

What is BMI?

BMI is the acronym for Body Mass Index. It is an international measure that serves to define whether a person is at his ideal weight, below or above him.

Being at a different weight than the ideal one for your height can bring some risks. Being underweight may be caused by malnutrition, leading to stress and fatigue in milder cases, to heart failure in more severe cases.

Overweight is also a risk factor. Being a little overweight can hinder circulation a little while obesity can bring greater risks of diabetes , stroke and heart attack .

Formula: how to calculate BMI?

The formula used to calculate BMI is weight in kg divided by height in meters squared. It is represented by the following equation:

The result shows the person’s BMI, which is compared with a table that indicates the degree of obesity.

  • BMI below 16 points to severe thinness and can cause heart failure, severe anemia and weakened immune system.
  • BMI between 16 and 17 points to moderate thinness, which can lead to infertility, hair loss and lack of menstruation.
  • BMI between 17 and 18.5 is light thinness, which can result in stress, anxiety and fatigue.
  • BMI between 18.5 and 25 is considered healthy, presenting a lower risk for diseases.
  • BMI between 25 and 30 indicates overweight, which can lead to fatigue, varicose veins and poor circulation.
  • BMI between 30 and 35 points to grade I obesity, which can result in diabetes, infarction, angina and atherosclerosis.
  • BMI between 35 and 40 indicates grade II obesity (severe), which can cause shortness of breath and sleep apnea.

BMI above 40 points to grade III obesity (morbid), which can lead to reflux, heart attacks, stroke, walking difficulties and bedsores.

For example, if a person is 1.70m tall and weighs 56 kg, the account looks like this:

That person’s BMI is 19.37. According to the table, it is a healthy weight.

The measure has variations for the elderly and children, since at these ages, the definitions of healthy weight are different.

BMI table

To use the BMI table, simply locate the height and weight below:

Classification of the result of the Body Mass Index

Each BMI range has a different classification ranging from severe thinness to morbid obesity and each can have different consequences for the body. Are they:

Severe thinness

Characterized by a BMI below 16, severe thinness is a condition that can be caused by severe malnutrition. Consult a doctor, as this weight is a health threat.

Moderate thinness

Hormonal conditions, such as hyperthyroidism , can affect a person’s weight, in addition to parasites or simply a low calorie diet. You can see a doctor to make sure that nothing is wrong.

Light thinness

It may have some consequences, but in general it is not worrying. A BMI above 17 is not far from healthy.


This classification guarantees a lower risk for several diseases ranging from anemia to infarction. To be in this classification is to be at the ideal weight for your body, but it is good to remember to check your waist circumference for excess fat.

For women, the waist must be up to 80 cm. For men, up to 94 cm. The waist should be measured just below the ribs.


Overweight can cause some circulation problems in the body, in addition to fatigue, but like light thinness, if your BMI is just above 25, it is not a concern. A diet with a little less calories or a little more exercise in the routine can solve the situation.

Grade I obesity

Above BMI 30 the person is considered obese, which in itself is already a disease. Thanks to weight, the risk for various conditions increases considerably. A doctor can indicate an ideal means for regulating weight.

Grade II obesity

Being with grade II obesity means having high risks of diabetes, hypertension, as well as cancer and heart attack. Look for a doctor to deal with the situation.

Grade III obesity

Called morbid obesity, this condition poses serious health risks. Get medical help.

Elderly BMI: how is the calculation?

Elderly people naturally have less muscle and more fat than young people and adults, so their BMI is different. The count made is the same, but the table changes a little. This table is applicable for people over 60 years of age.

  • BMI below 22 indicates that the elderly person is underweight.
  • BMI between 22 and 27 is considered healthy for the elderly.
  • BMI between 27 and 30 indicates overweight.

Obesity in the elderly is still classified by BMI above 30, but healthy weight is in a different range.

Children’s BMI (infant)

Calculating children’s BMI is a challenge, and its results are controversial, as each body develops at very different speeds and in different ways. In place of numbers, percentiles are used based on children of the same age.

Percentiles are measures that divide a statistical sample (in this case, children of a certain age) into 100 parts in ascending order.

This means that the lightest child of a specific age is in the first percentile, or percentile 1, and the heaviest child is in the 100th percentile. To be in the 35th percentile, for example, is to be heavier than 35% of the people in this age group. sample, but below the other 65%.

The count made is the same, weight on height squared. If a 13-year-old, six-month-old child is 1.44m tall and 40 kg, his BMI is 19.29.

According to the WHO table, this fits her in the 50th percentile. Her weight is over 50% of children of the same age, and her BMI is considered adequate.

Another child of the same age and height, but weighing 32 kg, has a BMI of 15.43. In an adult, this would be considered severe thinness, but for that age, it is in the 3rd percentile, which is the limit of what is considered appropriate. Despite being a little bit, she is still at a healthy weight.

With 44.5 kg, this child would have a BMI of 21.46. The percentile is 85, that is, compared to children of the same age, she has a BMI greater than 85% of them. Although the number indicates a healthy body mass index in adults, for children of that age, she is overweight.

If the child has the same numbers, but 14 years and six months, the 44.5 kg is healthy, but the 32 kg are underweight.

Each month of age enters the equation for children and adolescents, so the accounts and the table are much more complicated.

Why is he important?

BMI is important because it is an international measure used to define serious diseases such as obesity. In addition, it is easy to apply, which facilitates diagnostics, in addition to allowing a doctor to eliminate the possibility of diseases, or go after a specific one based on the result of the account.

Thyroid diseases, for example, affect weight, and the simple measurement of BMI can be a clue to them.

What is the ideal BMI (male, female)?

There are disagreements about which BMI is ideal. Although the number 19 is within the healthy range, it can still be considered somewhat low.

For adults, the BMI between 20 and 22 indicates the ideal amount of body fat, and there are associations of these BMI numbers with longer lifetimes and fewer serious illnesses.

There is literature that indicates that this is the BMI range that people find most physically attractive. It is important to note that BMI 22 to 25 is also associated with good health.

Some research indicates that the BMI that indicates longest survival is between 23 and 25.

All of these numbers fall within the BMI range considered healthy for adults.

What is my BMI?

Is the BMI accurate?

The accuracy of the BMI is not perfect. There are situations in which it ends up being inaccurate, for example with athletes or fake skinny people, who are people with little weight, but a high percentage of body fat.

As muscles weigh more than fat, an athlete may have a BMI above 25, even with low body fat levels. A person with a BMI 19, but high levels of fat, on the other hand, appears to be healthy, but is not.

Certain factors are not considered by the BMI, such as the percentage of fat in the body and the circumference of the abdomen, for example, in addition to the person’s life habits.

The standard BMI does not take into account the following factors:

  • Gender;
  • Age;
  • Sports practiced;
  • Fat mass;
  • Lean mass.


We have already talked about this a little, but this is another example where the BMI is not accurate. Considering that children can go through stretch marks and their height and weight measurements vary a lot according to age and even within a specific age group, BMI cannot be applied precisely to children.


There are minor ethnic variations in the calculation of BMI, and they are not necessarily standardized. For example, a WHO group concluded that Asians with a BMI of 25 can be considered overweight, while this is the limit for the healthy in the calculation.

When not to use BMI

Some situations interfere with the accuracy of the BMI, so in these cases, it is not reliable to define whether a person’s weight is healthy. These are the situations:


Athletes have less body fat and more muscles. Muscle weighs more than fat, so the calculation becomes inaccurate, and someone with a BMI of 26 may have a healthy weight, although, according to the table, that person is considered to be overweight.

There are calculations that take into account body fat.


By having a baby growing in her uterus, the pregnant woman has extra weight on her body, and the BMI would be inaccurate, as there would be no way to calculate the weight of the woman without the baby, and pregnancy causes weight changes that leave the calculation inaccurate even if the child’s weight was removed from the equation.


There are specific formulas for children, as well as for athletes. Due to their differences with each age, stretches and the action of puberty, the use of BMI for children is extremely inaccurate.

Amputees and disabled people

Due to the lack of a part of the body, such as arms and legs, or bodily changes resulting from physical disabilities, the weight and height of the amputated or disabled person may not be representative of their health or their ideal weight, so the BMI cannot be applied to these people.

New formulas for calculating body mass

There are several variations in the BMI calculation so that there is a greater coverage. We list some of them:


After four years of research, in 2012, USP nutritionist and researcher Mirele Savegnano Mialich Grecc developed the following formula:

With this formula, the number that separates overweight from grade I obesity is not 30, but 28.

The problem with this equation is that not everyone knows a number equivalent to their fat percentage. To find this number, it is necessary to pass an exam called bioimpedance , which needs a special device, or a physical exam, often performed in gyms.


In 2013, Professor of Numerical Analysis at the University of Oxford, Nick Trefethen, proposed a new formula, supposedly more accurate. The formula changes exponent 2 (height squared, represented by height ²) to 2.5 ( height ²).

To approximate the mean of the calculation to the average of the standard BMI, a multiplier of 1.3 is added to the weight in the formula. The new version would look like this:

Applying this formula to our hypothetical person from the beginning of the text, we have the following:

The new result is only slightly different, but takes into account other details, considering geometric facts of the human body.

Interestingly, eleven years earlier, a Brazilian had already arrived at this same formula, but discarded it for an even more precise one. Hindemburg Melão Jr is a polymath – someone whose knowledge cannot be restricted to a single area – who came up with a formula that considers, in addition to geometric facts, the effects of gravity and the resistance of materials. Its formula is as follows:

The result in our hypothetical friend is:

As in the case of the British scientist’s formula, the difference is not huge. The goal with the new formulas is not to say that the previous ones were wrong, just that they are inaccurate.

My BMI is not healthy, what to do?

How to gain or lose weight is not a mystery. Any food has an amount of calories, some more, some less. If you spend more calories than you consume, you lose weight. If you consume more than you spend, you win.

Some people have a faster metabolism than others. Therefore, the diet for each person must be different. If you are unsure of what you should do to gain or lose weight, consult a nutritionist.

Here, we will give some tips in case your BMI is below or above the indicated. They can help you achieve an ideal weight.


Set goals, but be realistic, no matter if you want to lose or gain weight. Trying to lose or gain 10 pounds in a month is not the way. Stopping eating or stuffing yourself every day is unhealthy and can bring negative health results, in addition to hindering success and motivation.

Plan your diet

Planning is an essential part of losing or gaining weight. Keep it all noted. Knowing how much you are eating and having control of it helps to regulate your habits, which are the biggest enemy of the person who wants to change your weight.

Remember that a nutritionist can be of great help in planning the diet and exercise for each body.

I am overweight

If you are overweight, it means you are consuming more calories than your body expends. It is necessary to decrease the amount of calories or increase the burning of them so that the weight goes down.

Gradually decrease the caloric amount. Eating light, non-greasy foods helps, as they satiate with a lower caloric amount. Decrease pasta and carbohydrates and use more vegetables with meals.

Don’t go without eating

You will only go hungry and when you eat again, the rebound effect catches you. Stopping eating makes your body, accustomed to having food often, feel at risk of not having food again, so it starts to take advantage of everything it can ingest, storing energy in the form of fat. That means weight gain.

Eating small, spaced meals for a short time helps boost metabolism, which burns calories faster. Meals every 3 hours is recommended.

Do exercises

Just as important as reducing the amount of calories you eat is increasing your expenditure. Depending on how overweight you are, only one of these two attitudes is enough, but the set makes it easier to achieve the goals.

Aerobic activities, such as running, are the ones that most reduce the number of scales, but anaerobic activities, such as weight training , also expend a lot of calories. The main difference is that weight training creates more muscles, which are heavier than fat.

The result will not be so reflected in the scale, since the weight does not decrease so much. However, the exchange of fat for muscle mass is healthy and, as we mentioned before, BMI does not take this into account.

Exercise uses up calories, and that’s what matters here. Achieving a healthy weight goes through exercise anyway.

I’m underweight

A lot of people have a hard time gaining weight. This is due to the same factors as overweight, but reversed. Instead of leftover calories, there is a lack of calories compared to daily expenditure. Solving this problem uses the same logic as weight loss.

In the diet for fattening, there are no food restrictions, but it is good not to overdo the fries and sugar. Despite the high caloric content, these foods are bad for your health when consumed in excessive amounts.

Proteins , carbohydrates, fibers and any type of nutrient are welcome. Foods such as chicken, fish and eggs can help, and foods rich in carbohydrates such as rice, potatoes and pasta facilitate the accumulation of calories.

Keeping meals stable is important, as the secret to changing weight is habit.

Do exercises

Exercising is always good, whether you want to gain weight, lose it or keep it off. Gaining muscle mass instead of fat helps ensure healthy weight gain. Weight training can help with the scale numbers, but any exercise is recommended.

Diseases that affect weight

Some illnesses can be responsible for weight problems. If you notice any sudden changes, it is good to see a doctor. Examples of these diseases are:

Hyperthyroidism and hypothyroidism

These two diseases cause changes in the thyroid, a gland that secretes hormones. Hyperthyroidism causes an increase in the hormonal production of the gland and hypothyroidism does the opposite.

Both can cause weight changes, making it difficult to maintain the ideal weight. Treatment is done with hormones by an endocrinologist.


Stress is a major villain in the fight for the ideal weight. It can cause loss of appetite or compulsory appetite, causing changes in weight.

Absorption problems

If your body has some difficulty absorbing calories, you lose weight. This is what happens in celiac disease, Crohn’s disease and some types of intestinal infections.

Chronic infections

Parasites, HIV and tuberculosis can cause sudden weight loss.


Diabetes is characterized by a lack of insulin in the body. This lack prevents glucose from being used by the cells, which reduces the use of calories, causing weight loss.

Psychological disorders

Depression , anxiety and eating disorders cause changes in diet, which affect the patient’s weight.

BMI is useful to get an idea of ​​how healthy your weight is, however, it is not accurate and leaves many factors out. If you have problems with your weight, it is recommended to visit a nutritionist to find out if any treatment or change of habit is necessary.

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