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How accurate is the BMI scale?

Posted in: Weight Loss 11 Jan, 2012

If you have ever had some form of a concern over your weight, you will likely have stumbled across the phrase “body mass index”, commonly abbreviated to BMI. It is a tool used by doctors and other health professionals to determine whether an individual is at a healthy weight or not, with categories ranging from severely underweight to obese class III. The popularity of usage of the BMI scale is in no small part due to its simplicity; it requires only an individual’s height and weight. However, this very advantage is also a significant drawback when it comes to accuracy, and the BMI scale is not without controversy.

In simple terms, the BMI test offers an estimate on whether a person’s weight is healthy or not by comparing their weight to their height. It does not take into account body fat percentages, a shortcoming that is acknowledged as being problematic. Other factors which could contribute to what is considered a “healthy” weight and are not accounted for by the BMI scale are ethnicity, gender and age. A common example of this disadvantage is in the case of athletes, who often have a higher muscularity rather than a higher percentage of body fat, but are misrepresented by the BMI scale as being obese.

With this in mind, why is the BMI scale used so extensively?

The BMI scale has the dual benefit of being both very simple to use and low-cost, which is the main reason why it is used and recommended by health professionals. These factors mean individuals are able to calculate their own BMI themselves without needing to visit a doctor. In addition, it is worth noting that the BMI is intended to be used as a way of identifying potentially at-risk individuals rather than as a diagnostic tool in and of itself. No doctor will make any decisions regarding your healthcare based solely on your BMI. Instead, if your BMI indicates that you are obese, further tests will be carried out to confirm this, and whether there are any other associated health risks.

In this sense, it is easy to see why the BMI scale is so favoured by health professionals as a tool to help assist in monitoring an individual’s general health.

What are the weight categories as defined by the BMI scale?

The BMI operates on a numerate scale from 16 or less to over 40. An individual’s BMI will fall within a particular range, to indicate which category they fall in to.

Severely underweight: less than 16.0

Underweight: from 16.0 to 18.5

Normal weight: from 18.5 to 25

Overweight: from 25 to 30

Obese Class I: from 30 to 35

Obese Class II: from 35 to 40

Obese Class II: Over 40 In simple terms, anyone with a BMI over 30 is considered to be obese. How can I calculate my BMI? All you need to know in order to calculate your BMI is your height and your weight. There are many websites online which have BMI calculators to find your BMI for you using this information, such as on the NHS website.


  • MarkThursday, Jan 12, 2012

    Technically I am classed as overweight, but I am really muscular and not fat in the slightest. I think it's important that people use BMI with caution and take other factors into consideration as well

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