How To Calculate Your BMI (Body Mass Index)

Posted by on Thursday, June 28, 2012

As a species, we couldn’t be more obsessed with our weight. Health stores (and even supermarkets) are stocked with shelves filled with slimming tablets; magazines and newspapers are all too ready to report on the new ‘celebrity diet’; an incredibly common (read: popular) style of ad on the internet is the sort that goes: “Lose 3 stones with this one weird old trick!”.

This obsession with our weight isn’t a new phenomenon by any means, but attitudes changes over time, and different cultures have alternative views on what is considered the ‘ideal’ weight.

While we could argue that culturally we are too concerned with our weight, that is not to say that we shouldn’t be aware of what our weight is. Our weight is a good indicator of many things and can tell us whether we get enough exercise or whether we eat too much.

But in order to find out whether we are of a healthy weight, we need some sort of reference to compare our weight to. For instance, a 6 foot tall man should not be the same weight as a woman at 5 foot 7 inches.

What is the Body Mass Index?

This is where metrics such as the Body Mass Index (BMI) come in. The BMI is a way of seeing if your weight is appropriate for your height; technically it is defined as your weight divided by your height squared. It gives you a general idea of whether you are too heavy or too light for your height – a high BMI means you are heavy for your height, a low BMI tells you that you are light for your height.

However, despite the BMI being a good way of getting a rough idea of whether you are a healthy weight, it was designed (and should still be used as such) for individuals who have an average body composition or live an inactive lifestyle. This was back in the 1840’s – a lot has changed in the science of fitness and obesity since then.

This is because the BMI does not measure the percentage of body fat, it is simply a ratio of weight and height. If you live a very active lifestyle and have a high percentage of muscle to fat, your BMI can tell you that you are overweight, despite actually being very healthy. This must be taken into account when you use a metric such as this.

Calculating Your BMI

The actual calculation is very simple: BMI = mass (kg) / height2 (m).

If you would like an online tool to calculate your BMI, try this offering from the NHS. They even offer a BMI calculator and tracker as an iPhone app, which is well worth having a look at.

BMI Chart

Click for a larger image

The Body Adiposity Index, An Alternative to the BMI

If you’d like an alternative to the BMI (or another metric to compare it against), the Body Adiposity Index (BAI) may interest you. It has designed by Richard Bergman to be a more accurate way of seeing whether an individual is overweight or obese.

What Bergman found was that the ratio of hip circumference to height was more strongly correlated to body fat than the BMI was. If you’d like to calculate your BAI, try this online calculator.

It is very important to say, as a closing note to this article, that while these tools can be useful, they are primarily useful to doctors and health professionals – if you have reason to believe you are over or underweight, please consult a professional.

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