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Measuring BMI

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Calculating BMI (Body Mass Index) can be done several ways.


The easiest method is to use a formula developed by the NAtional Institutes of Health (NIH). But it's also probably the least accurate.


I just started workout out four months ago. My trainer uses calipers to take measurements and calculate my BMI. We did it at about the six-week mark and again last night at the seventeen-week mark.


Both methods have shown an improvement for me, but the numbers are wildly different. For me, the caliper method is about 12 points better than the BMI formula method.


So, in order not to obsess about this (too late! :)), I'm thinking of getting a more reliable reading done. Probably hydrostatic (water submersion method).


Has anyone done hydrostatic measurement for BMI? Is there anything unusual or special that I need to plan for with that method?

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BMI is a very crude indicator that relates your body weight to your height. While it's better than nothing it doesn't say anything about body composition. Using BMI, a six-footer who weighs 180 lbs at 7% body fat, in other words, really buff, has the same BMI as a six-footer weighing 180 lbs at 27% body fat, which is more than a little chubby.


If the trainer was using calipers, he was measuring body composition i.e. fat/and or muscular you are. Much better indication of fitness than BMI.


Water immersion is supposed to be one of the most accurate methods. The downside is that it involves holding your breath underwater for periods of several seconds, long enough so that some people aren't up to it.


DEXA scan is also supposed to be very accurate. It involves a minimal radiation exposure, costs about the same as water immersion and doesn't involve any discomfort.


It wore out not long ago, but for years I had one of those scales that used bioelectric impedance to estimate body composition. They have been criticized as not being very accurate. But the measurement from my scale was inevitably .5% lower than a caliper measurement done by my trainer on the same day.

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I think y0u're confusing BMI with BFP, or body fat percentage. BMI is simply a calculation based on an adult's height and weight. It is used to classify people as underweight, normal weight, overweight, or obese (class 1, 2, or 3), but is not accurate in very muscular individuals (especially bodybuilders). Waist circumference is probably more accurate than BMI in classifying obesity, especially in muscular individuals (usually men). BFP is even more accurate, and the water immersion method is the most accurate, in determining a person's level of obesity.

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