How Accurate is 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 BMI is a useful measurement for most people over 18, and one of the best methods of assessing if people are overweight or obese.

Your BMI may not always be an entirely accurate representation of your health, with these instances including:

  • Being pregnant - or having recently given birth, and/or nursing
  • Being of muscular or athletic build
  • Being under 18

All three of these examples feature limitations that BMI does not account for, with different body shapes not resulting from excess body fat, which is liable to provide inaccurate BMI results.

BMI does not distinguish between body fat and muscle mass, which is why muscular athletes may have a high BMI because of increased muscle rather than additional body fat.

You should seek advice from your doctor if you belong to one of these groups in order to receive a more precise BMI reading.

Is BMI reliable as an indicator of body fat?

While otherwise strong, the correlation between BMI and body fat is dependent on sex, race and age.

At the same BMI level, men tend to have less body fat than women, while older people generally have more body fat than younger adults, on average.

In regard to body fat, there are two other measurements that are recommended for assessing the risk of disease. BMI is only one factor that relates to this risk.

Guidelines from the National Heart, Lung and Blood Institute recommended looking at waist circumference – as abdominal fat can lead to obesity-related diseases – and other risk factors associated with obesity such as physical inactivity and high blood pressure.