Body Mass Index (BMI), is a method of estimating a person’s level of body fat based only on measurements of height and weight. Doctors and researchers frequently use BMI as a measure of overweight or obesity because it is easy and convenient to measure and understand.
The standard formula for calculating BMI is: weight (kg) / height (m)^2.
What can you learn from your BMI?
The World Health Organization (WHO) provides clear categories to help us make sense of BMI. The table below shows the WHO classification for each range of BMI for adults older than 20 years of age:
BMI Range Category
< 15 Very Severely Underweight
15 - 16 Severely Underweight
16 – 18.5 Underweight
18.5 – 25 Normal
25 – 30 Overweight
30 – 35 Moderately obese
35 – 40 Severely obese
40+ Very severely obese
The WHO considers a BMI of over 25 to be overweight and a BMI of over 30 to be obese. Being overweight or obese is a major risk factor of a number of serious chronic diseases, including diabetes, heart disease, osteoarthritis, and some types of cancer like endometrial, breast and colon cancer. A common misconception is that only severe obesity leads to increases in the above risks; however, what is not widely known is that the risk of serious health problems begins to increase when a person is only slightly overweight.
The WHO recommends that individuals with high BMI take steps to reduce their risk of these diseases by reducing body fat and achieving normal body weight.
BMI’s usefulness as a measure comes from how simple and convenient it is to calculate. However, its simplicity is sometimes also its weakness. Because BMI only considers height and weight, and does not consider more sophisticated measurements that require advanced equipment and technique to measure (like body fat %), you should only consider BMI as a rough, inexact measure of body fat.
Because it is inexact, BMI is not completely appropriate for every individual. Individuals with high weight-to-height ratios (for example, muscular athletes or bodybuilders) may have a high BMI but at the same not be overweight or obese. Likewise, individuals with naturally very slender frames may have apparently normal BMI, but actually could be carrying significant excess body fat.
Where to learn more
We recommend users who are interested in learning more about this topic to visit the WHO fact page regarding overweight and obesity.