How to Calculate BMI

Being muscular or athletic can lead to your BMI misrepresenting your health
Being muscular or athletic can lead to your BMI misrepresenting your health

The calculation of BMI (Body Mass Index) was invented by Adolphe Quetelet, a Belgian mathematician born in 1796.

His proposal was that people's weight could be deemed as relative to an ideal weight for their height.

BMI is still referred to as the Quetelet index, for which the formula is:

  • BMI (kg/m2) = mass (kg) / height (m)2

BMI is conducted the same way for both adults and children, however, the interpretation of the results is different.

For adults

BMI is interpreted using standard weight categories for adults over 20, which are the same for all ages of both men and women.

Once you have measured your BMI number, you can assess which weight status category you fall in with this table:

BMI to weight status
BMI Weight Status
Below 18.5 Underweight
18.5-24.9 Healthy weight
25-29.9 Above ideal range
30+ Obese

With height ranges added, the corresponding BMI ranges and weight status categories can be derived from a sample height with this table. For example, someone who is 5' 9" and who is 1.752m:

  • BMI (kg/m2) = mass (kg) / height (m)2 - which in this case is:
  • [BMI Range Value] = mass (kg) / 1.752m2
BMI to weight for a sample height
Height Weight Range BMI Weight Status
5' 9" (1.752m) Less than 56.2 kg Below 18.5 Underweight
56.3kg - 76.2kg 18.5 to 24.9 Healthy weight
76.3kg - 91.6kg 25. to 29.9 Above ideal range
91.7kg or more 30 or more Obese

For children

The interpretation of BMI for children and teens is both age and sex specific, with BMI obtained using percentiles to show how a child's BMI compares to other children of the same age and gender.

Age and gender are used to interpret BMI in children and teens because the amount of body fat changes with age and differs between boys and girls.

The following table displays BMI-for-age weight status categories and their corresponding percentiles.

BMI to percentile range
Percentile range Weight Status
Less than the 5th percentile Underweight
5th percentile to less than the 85th percentile Healthy weight
85th to less than the 95th percentile Above ideal range
Equal to or greater than the 95th percentile Obese

BMI cut off for Asians

The relationship between BMI and body fatness differs in Asian populations, who have been found to have a higher body fat per cent at a lower BMI compared to Caucasians.

This high body fat per cent can be explained due to factors such as body build, slenderness and muscularity, which is why universal BMI cut-off points are not appropriate for Asians.

To calculate BMI for Asians, you must use this BMI cut-off table.

BMI to weight status: Asians
BMI Weight Status
Below 18.5 Underweight
18.5-23.9 Healthy weight
24-26.9 Above ideal range
27+ Obese
Explore BMI