BMI: How to calculate body mass index? Calculator + formula

BMI: How to calculate body mass index? Calculator + formula
Photo source: Getty images

Obesity or being overweight is a concern for many people. Almost everyone wants to know where they stand with their weight. This is where the body mass index comes in. Calculating it is not complicated at all. Whether you need to know it for health reasons or purely out of curiosity, we have a way to find out.

The calculation of BMI (short for Body Mass Index) helps doctors, for example, to determine the nutritional status of an individual. It is only a guideline, but it is very helpful.

What is BMI?

Loosely translated, BMI stands for Body Mass Index. It is one of the most widely used methods of measuring obesity. It is not a completely accurate indicator, but its advantage is its speed of calculation.


In terms of history, its importance has not been as great as in recent years.

It started in 1836, when a Belgian insurance company wanted to check the health status of its clients. It asked the Belgian astronomer, statistician, mathematician and sociologist Adolphe Quetelet to design a system that could do this. This is why the index is sometimes called the Quetelet index.

Dr. Quetelet assessed the male population of Belgium. Based on the measurements and results, he determined the body mass index BMI.

However, it did not catch on and fell into near oblivion. It was only in the early 1970s that it began to re-emerge as the best and quickest way to measure obesity.

It became popular even though it was known to be less accurate.


BMI should not be the only method for diagnosing obesity. It is only appropriate to use it as a control or comparative figure.

Its advantage stems from the simplicity of its calculation, measuring without tools or medical supervision. All one needs to know is the height and weight of the person being measured. However, this is where its disadvantage begins. It does not take into account any other factors that are important in determining overweight or obesity. It is therefore labelled as less accurate.

BMI is mostly applicable for statistical surveys. Its calculation should therefore not be taken as an absolute indicator. Rather, it should be seen as an approximate guide to be used as one of many means.

How to calculate BMI?

It is calculated by dividing a person's weight by the square of their height.

BMI = m/h²

  • m = body weight in kilograms
  • h = body height in metres

The BMI (body mass index) calculator will help you with the calculation.

The table shows the measurement values with the BMI results

Classification Values
Malnutrition < 18,5
severe malnutrition < 16
moderate malnutrition 16-16,99
mild malnutrition 17-18,49
Normal weight 18,5-24,99
Overweight ≥ 25
Slightly overweight 25-29,99
Obesity ≥ 30
Grade 1 obesity 30-34,99
obesity grade 2 35-39,99
Grade 3 obesity ≥40

The data in the table are based on the WHO 2007 World Health Organisation. Valid for the European population.

We have mentioned that BMI is only a statistical indicator. In some people, the clinical condition may be inconsistent with the meaning of the measured value. A person with a large amount of muscle mass, for example a bodybuilder, may have a BMI of 30 and not be obese.

His high BMI is due to the large amount of muscle mass.

A man wearing light-coloured trousers and black socks is standing on a digital scale. He is measuring his weight. The dial reads 84.7 kilograms. The scale is white, glass, on a light-coloured floating floor.
To calculate your BMI, all you need to know is your own weight and height. Source: Thinkstock

Conversely, older people with a low muscle mass ratio may be at an ideal weight according to their BMI results. However, from a health point of view, they will be obese.

Read also the article on obesity and overweight.

BMI limits also vary for different populations. Different values apply to Asians, others to Europeans and so on.

Measuring BMI in children

In children, it is based on a precise body measurement. By the age of two, the head circumference is added to the weight and length of the body.

BMI is calculated in the same way as for adults, but is assessed using percentile charts. Gender is also taken into account in this assessment, as there is a difference in physical development between boys and girls.

Table of BMI and height assessment by percentile for children

Percentile zone Height rating BMI rating
<3 very small Malnourished
3-10 small low weight
10-25 smaller slim
25-75 medium proportional
75-90 taller robust
90-97 tall excessive weight
97 < very high obese

The index may be distorted, especially in adolescent boys. A gain of muscle mass at the expense of adipose tissue may be noted, especially during rapid development. Therefore, further confirmatory investigations are necessary in children at risk of obesity.

Other methods of measurement

There are other methods of measurement that can be used to determine the level of health risk. One simple measurement is the calculation of the WHR index.

WHR index

The abbreviation WHR is from the English term waist to hip ratio. It stands for waist to hip ratio.

This calculation can be used to determine the risk of cardiovascular disease. This index is more accurate. It looks at the waist circumference with the hip circumference.

WHR = waist circumference/hip circumference

Values are given in centimetres.

For example, if the waist circumference is 70 cm and the hip circumference is 85 cm, divide 70:85. The result is WHR = 0.8

The result of the WHR calculation gives us an indication of how fat is distributed.

In the table we give the WHR values

WHR value of a woman WHR value men Method of fat distribution
< 0,75 < 0,85 peripheral
0,75-0,8 0,85-0,9 balanced
0,8-0,85 0,9-0,95 Central
> 0,85 > 0,95 risk

The peripheral mode of fat distribution means that fat is deposited on the peripheral, external parts. This means the hips and buttocks. In the female sex, this mode is normal and genetically determined. The mode itself cannot be influenced by weight loss. It does not cause health problems.

The balanced way represents a normal figure. In this case, the risk of cardiovascular or metabolic diseases is low.

The central mode of fat distribution indicates that fat is being stored in the abdomen. There is no need to worry, but it is a sign that the figure is beginning to change from slim to cylindrical. Ideally, you should adjust your lifestyle.

A risky way of fat distribution already means a high risk of cardiovascular disease. It can be said that the level of risk depends on the size of the abdomen. In this case, it is already obesity even if the BMI index would indicate that everything is fine.

If the hips are wider than the abdomen, everything is still normal. However, the waist diameter must be normal, men under 90 cm and women under 78 cm.

Measurements should be taken in precise places.

If you want to adjust your lifestyle, read how to do it.

A Body Shape Index

This is one of the latest measurements to determine the level of risk to health. The English abbreviation is ABSI and could be loosely translated as Body Shape Index.

Primarily, it predicts the magnitude of your risk of cardiovascular or heart disease. In general, it says that your health is at risk if you are too thin or have a big belly.

ABSI = waist circumference / (BMI²⁄³ x height½)

This index takes into account the amount of intra-abdominal fat. It can also roughly estimate body composition in terms of muscle to fat ratio.

Table of cardiovascular disease risk according to the ABSI

Age Male Female
30 0,079 0,077
40 0,081 0,078
50 0,083 0,079
60 0,084 0,081
70 0,085 0,083
80 0,088 0,085

If the ABSI score is higher than in this table, the risk increases. Conversely, if it is lower, the risk decreases.

ABSI health risk table

Risk ABSI value
Very low less than -0.868
low between -0,868 and -0,272
average between -0,272 and + 0,229
high between +0,229 and +0,798
very high more than +0,798

How much energy do we take in in our diet:

Calorie tables: what are the nutritional/nutrient values of meals, foods?

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