Body Mass Index Calculation for Weight Status

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Use the BMI calculation to determine weight status. By calculating BMI one can get an indication of whether they are underweight, of normal weight, overweight or fall in the obesity range.

What is BMI - Body Mass Index?

Body Mass Index (BMI) is an indicator of relative body fat that can be used to predict a person’s risk for weight associated diseases. It is determined using a simple calculation that takes into consideration the ratio of weight to height, and defines what weight category a person falls into.

The Body Mass Index Calculation

BMI is defined as the weight in kilograms divided by the square of the height in metres (kg/m2).
As such, the body mass index calculation uses the following equation:

BMI = Weight / (Height)2

Examples of Calculating BMI

If a person weighs 60kg and is 1.75 metres tall, the person’s body mass calculation is as follows follows:

BMI = 70 / (1.75)2 = 22.9

If a person weighs 85kg and is 1.75 metres tall, the person’s body mass calculation is as follows follows:

BMI = 85 / (1.75)2 = 27.8

If a person weighs 100kg and is 1.75 metres tall, the person’s body mass calculation is as follows follows:

BMI = 100 / (1.75)2 = 32.7

How Does Calculating BMI Correlate to Normal Weight, Underweight, Overweight or Obese?

According to the WHO, a person is:

of normal weight if their BMI lies between 18.5 and 25 kg/m2.
underweight if their BMI falls below 18.5 kg/m2
overweight if their BMI lies between 25kg/m2 and 30kg/m2
obese if their BMI falls above 30kg/m2

Restrictions of the Body Mass Index Calculation

While the WHO endorses the use of BMI to define one’s weight status, there are some restrictions to using this tool. The following must be considered when using the body mass index calculation:

BMI values are age-independent, and therefore may underestimate body fat in older persons.
BMI values do not take into consideration muscle mass, and therefore may overestimate body fat in athletes and other people such as men who tend to be more muscular than women.
BMI ranges do not take into consideration body frame size, and therefore may not correspond to the different populations. This may also result in overestimates of body fat in many men.
BMI values may overestimate the mount of body fat in pregnant women.

Implications of Calculating BMI

By calculating BMI one can get an indication of whether they are underweight, overweight, obese or of normal weight. If a person falls outside the "normal" BMI range, they may want to consider seeking help from a health professional to get their body back on track, since both underweight and overweight and obesity can lead to many different health complications.