Wednesday, 3 April 2013

Measure It: What are BMI and BFP?

Autumn and Spring, the transitional seasons, are a great time to take stock of your health and fitness. If you have been working out all summer or winter, it can be a good time to see how much you have improved. Or, if you are about to buckle down to a new program you can check out where you should focus. To help you with this, this is the first in a series - ‘Measure It’ - looking at some of the different health and fitness factors you should keep an eye on.
Today I’m going to look at BMI and BFP, two measures of your body fat to see if you are overweight and at increased risk of heart disease.


BMI stands for your Body Mass Index.

It is based on your weight to height ratio. It is one of the most common measures used because it is very easy, though does have some problems.
To calculate it, all you need is to know your height and have access to a set of scales. The basic equation is your weight divided by the square of your height, done in kgs and meters.
BMI = Mass (kg) / (Height (m)2)
So, I’ll be brave and show you mine.
I’m currently around 76kgs and I’m 171cms tall.
Therefore, my BMI = 76/1.712 = 25.99.

If you can’t do the maths yourself, you can also just plug in the numbers to an online calculator such as the one at NSW Government Health.
But what does that mean exactly? Well, that’s where the BMI chart comes in.

CategoryBMI range – kg/m2
Very severely underweight less than 15
Severely underweight from 15.0 to 16.0
Underweight from 16.0 to 18.5
Normal (healthy weight) from 18.5 to 25
Overweight from 25 to 30
Obese Class I (Moderately obese) from 30 to 35
Obese Class II (Severely obese) from 35 to 40
Obese Class III (Very severely obese) over 40
(Table care of Wikipedia).

So, according to the chart, I’m just bordering on overweight, so probably should get it down just a bit more, and can lose quite a bit and still be healthy. In fact, according to BMI I can actually get down to 54kgs and still be ‘healthy weight’.
And this is where the problem comes in. I’m reasonably tall, and have pretty good muscle development compared to most females. A few years ago I got myself down to 61kgs, and my family were worried about me. There is actually very little chance I could healthily get myself down to 54kgs. So is the BMI chart wrong?
The BMI formula does not hold up very well if you are very tall and/or have quite a bit of muscle. As muscle is heavier than fat, and volume is cubed not squared, increases in height and muscle skew the results. So you still need to be sensible.
Having said, if you are over 30, you might want to stop making excuses and get at least between 25-30.


A more reliable formula, but harder to measure accurately, is your BFP:

Body Fat Percentage.

It looks at the percentage of your body weight that is fat (though keep in mind, this includes essential body fat, not just the excess stuff you want to get rid of). For unfair reasons (ability to grow healthy children) women need a higher percentage of fat, so essential fat is about 3-5% for men and 10-16% for women. However, that doesn’t mean that’s normal.
Generally body fat percentages are as follows:

Essential fat

So, if you can work out what your body fat percentage is now, you can work out how much you can and or should lose.

There are some high-tech ways to measure this, but it is unlikely you will be able to get your hands on these (dual energy X-ray absorptiometry anyone?). The next best thing is to get a personal trainer to do it at a gym using one of the skinfold methods, though these are not perfect.
There are scales out there that measure your body fat for you, but these are not great, reliability wise. However, if you stick to the same set of scales, it should at least tell you if you are going up or down.
But if you want to do it yourself, here are some sort of accurate ways.

1.I love this one from – ‘take a look’. Compare yourself to the images of what people with the different body fat percentages look like, and see which one you think you are. (check out for their pictures… I’m not sure if I want to disturb you all with half naked men and women :D) Only problem with this is that most of us are deluding ourselves in one way or another!

2.Body Fat Calipers: using these you can do your skin fold test. The calipers are not expensive to buy, and you can use an online calculator like: to work out the results. This is probably one of the most effective self-administered measures of body fat.

3.Others suggest using the hip to waist ratio. This is a good indication of fat composition and can tell you if you are at risk of having fat in places which have a higher association with health problems. However, it won’t tell you your fat percentage overall.

Just to compare to BMI, when I was rowing (my muscliest) I had a body fat percentage of around 22% (yeah, I was a lightweight) and weighed 69kgs. According to that, I had around 15kgs of fat, including my essential fat. So, if I dropped down to 54kgs, and managed to keep my muscle bulk, I would have been well into my essential fat stores (if it could be done.) So, body fat percentage better than BMI. However now that I’ve lost a lot of my muscle tone, I could probably get down lighter than I could then.

So there you have it.

My recommendation, work out your BMI just to see (and because it is easy) then see if you can book in to get a personal trainer to give you a proper skinfold test every 3- 6 months to see how you are going. If you are interested, buy your own calipers and give yourself the skin fold test.
Whichever way you do it, I do recommend getting an externally validated estimate of your fat levels, because you may be surprised. Don’t try to excuse it away, do something about it!

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