## Understanding BMI, fat index, etc

Good morning ''Doc'',
I have joined the gym about 3 weeks ago and yesterday I checked my BMI on the machine at the gym and just need clarification on what it means, please.

It gives:
Weight –  74.3
Height –  173
And BP
Age: 37 (male)

Then, what I need to understand,please, is:
Body fat estimation:
Fat index –  21.8%
Fatt mass –  16.1kg
Free fat mass –  58.5kg

It also states:
Normal fat index is –  17 –  23%
Normal fat Mass is 11.9 –  17.4kg.

What should I be keeping an eye on? My weight has increased over the last week, so I am assuming Im building muscle as well. Where should I then lose my fat / weight?

Many thanks,
Francois

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###### Our expert says:
FitnessDoc

Hi Francois

OK, so BMI is just a measure of mass to height (because you can't just use mass, because then tall people would be disadvantaged, as they should be heavier).

So BMI equals mass (in kg) divided by height squared (in meters). So yours is 74.3 divided by 1.73 squared, or 24.8. Which is fine, it's in the normal range.

The rest of the stats are all OK too. Those ranges are always a bit dicey, because you never know who you want to compare yourself to! You probably don't want to be perfectly average! So from the sounds of it, your goals will be to bring the body fat percentage down, to increase the lean mass. The combination of which, of course, means that your mass might not change, and that's important to understand.

So if you have the ability to measure this, then it would help you see if you're achieving that. The actual masses (like fat free mass, fat mass etc) are all just calculated from the percentage. So I'd be keeping my eye on the fat percentage, and the lean muscle mass. They tell you all you need to know.

Final point - remember that technology is never perfect, and that machine has "error". It's around 2%. So what that means is that if your fat percentage is currently 21.8%, and the next time it's say 23%, that's not actually different, if you follow the reason? There is day-to-day variation of around 2%, so be careful about being too rigid. I'd measure once a month, and just be patient about the values.

Good luck

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###### Posted by: fitnessdoc | 2010/09/13

Hi Francois

OK, so BMI is just a measure of mass to height (because you can't just use mass, because then tall people would be disadvantaged, as they should be heavier).

So BMI equals mass (in kg) divided by height squared (in meters). So yours is 74.3 divided by 1.73 squared, or 24.8. Which is fine, it's in the normal range.

The rest of the stats are all OK too. Those ranges are always a bit dicey, because you never know who you want to compare yourself to! You probably don't want to be perfectly average! So from the sounds of it, your goals will be to bring the body fat percentage down, to increase the lean mass. The combination of which, of course, means that your mass might not change, and that's important to understand.

So if you have the ability to measure this, then it would help you see if you're achieving that. The actual masses (like fat free mass, fat mass etc) are all just calculated from the percentage. So I'd be keeping my eye on the fat percentage, and the lean muscle mass. They tell you all you need to know.

Final point - remember that technology is never perfect, and that machine has "error". It's around 2%. So what that means is that if your fat percentage is currently 21.8%, and the next time it's say 23%, that's not actually different, if you follow the reason? There is day-to-day variation of around 2%, so be careful about being too rigid. I'd measure once a month, and just be patient about the values.

Good luck

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