What is the Body Mass Index (BMI)?
BMI is a simple and quick way to assess whether a person has a healthy weight. It is calculated from a person's height and weight and offers a standardised way to evaluate body composition. Although BMI is a useful tool, it does not take muscle mass, fat distribution or other health factors into account, so it does not always give a fully accurate picture of health.
Formula for calculating BMI
The formula for calculating BMI is:
Where:
- Weight: the person's weight in kilograms (kg).
- Height: the person's height in metres (m).
Example of a BMI calculation:
Suppose someone weighs 70 kilograms and is 1.75 metres tall; the BMI calculation is then as follows:
In this case the person's BMI would be approximately 22.86.
BMI categories:
- Underweight: BMI < 18.5
- Normal weight: BMI 18.5 to 24.9
- Overweight: BMI 25 to 29.9
- Obesity class I: BMI 30 to 34.9
- Obesity class II: BMI 35 to 39.9
- Obesity class III (severe obesity): BMI ≥ 40
Uses and limitations of BMI
BMI is a useful tool for assessing the weight of large populations and identifying possible health risks. However, BMI does not take into account fat distribution (such as abdominal fat), muscle mass, bone density or other important factors. As a result, in athletes with a lot of muscle mass it can show a higher BMI that does not necessarily indicate an unhealthy weight.
In plastic surgery, BMI is used by insurers for the reimbursement of a procedure. For example, a breast reduction is not reimbursed if there is overweight and a BMI of 30 or higher. There is a relationship between a high BMI and complications after an operation. For this reason too, patients may be asked to lose weight first before a non-urgent procedure.
BMI is most useful as an initial screening tool, but for a full assessment of someone's health it is important to also consider other measurements and factors, such as waist circumference, body fat percentage and other health indicators.
