The Body Mass Index
Many people aim to manage their bodyweight. Some focus on changing their body composition for aesthetic purposes whilst others have a health incentive. Body mass index (BMI), the ratio of weight to height, is a measurement for determining a healthy bodyweight.
To calculate your BMI, simply divide your weight (in kilograms) by your height (in metres squared).
For example, if you weigh 70kg and stand 1.8m tall, then your BMI would be:
70 kg / (1.8 m x 1.8 m = 3.24 m²) = 21.60…
This is an inexpensive and easy screening method for determining weight category: ‘underweight’, ‘healthy weight’, ‘overweight’, and ‘obesity’.
For most adults, an ideal BMI is between 18.5 to 24.9.
If your BMI is:
below 18.5 – you're in the ‘underweight’ class.
between 18.5 and 24.9 – you're in the ‘healthy weight’ class.
between 25 and 29.9 – you're in the ‘overweight’ class.
between 30 and 39.9 – you're in the ‘obese’ class.
BMI does not measure body composition. For example, muscle is denser than fat, so a heavyweight boxer may be perfectly healthy despite having a BMI classed as ‘obese’.
The following are some examples of how certain variables can influence the interpretation of BMI:
On average, older adults tend to have more body fat than younger adults for an equivalent BMI
On average, women have greater amounts of total body fat than men with an equivalent BMI
Muscular individuals, or highly-trained athletes, may have a high BMI because of increased muscle mass
For this reason, health professionals may consider other factors when assessing your health, such as your waist-to-hip ratio and blood pressure, though BMI remains a basic measure of health.
