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Measuring Body Fat

Published by marco on

Updated by marco on

The BMI, or Body-Mass–Index, has been in the news a lot lately. Whether because of runway models, whose BMIs are dangerously low, or because of kids in first-world countries, whose BMIs are dangerously high. The BMI myth by Peta Bee (The Guardian) takes a look at the utility of this measurement in determining health. As usual, now that the public (as well as insurance companies and government agencies) has glommed on to this statistic as the final say in health, scientists have taken a look at it and found it wanting.

Calculating BMI

The BMI has gained popularity because it can be calculated from easily obtainable statistics. Using weight in kilograms and height in meters, calculate:

BMI = kg / (m × m)

Americans will have to do a bit more work, either converting from pounds and inches to the metric system (yuck!) or using the formula below:

BMI = (lbs / (in × in)) × 703

The magic number, 703, normalizes the value back to the same scale as that calculated with metric inputs.[1]

Now that you know how to calculate it, realize that it’s more or less useless for athletes or any other reasonably fit and active people. The problem, in a nutshell, is that it doesn’t take body composition into account. Muscle weighs more than fat; therefore, an athlete weighs more than a non-athlete of similar build. In fact, many top athletes have BMI ratings of “obese” precisely because of this. According to other research, in order to determine actual health (rather than just obtaining a numeric evaluation of conformance with society’s beauty standards), “[t]he important thing to consider is how body fat is distributed around the body, as the real problems occur when fat accumulates in the central abdominal region”.

Two things comes out of this research:

  1. Measuring the percentage of fat in a person is very useful in determining fitness and health
  2. Measuring the distribution of fat in a person is very useful in determining risks of disease

Measuring Percentage Body Fat

According to How to Calculate Body Fat, there are a few ways to measure the percentage fat in a body.

Skinfold Calipers
The size of the fat above the hips (the “love handles”) is measured; this is accurate only when done by an experienced and trained person (though you can teach yourself, of course).
Hydrostatic Weighing
The most accurate method, but the most difficult to do, as it requires a hydrostatic weighing tank in which you are completely submerged and the amount of fat in your body is measured using Archimedes Principle.
Home Body Fat Scales

This is probably the easiest method:

“A low-level electrical current is passed through your body and the “impedance”, or opposition to the flow of current, is measured. The result is used in conjunction with your weight and other factors to determine your body fat percentage.”

However, the accuracy of this method is severely affected by “amount of water in your body, your skin temperature and recent physical activity”. They recommend not exercising or eating for at least 4 hours before the test in order to improve accuracy. If you have one of these at home, this is pretty easy to achieve, in that you simply step on the scale right after getting out of bed.[2] This method’s precision is not at all related to its accuracy: measuring under the same conditions day after day results in extremely consistent readings. Therefore, this method is very useful for determining change, as when dieting.

Measuring Fat Distribution

The most effective of the quick methods for determining overall health is waist-circumference measurement, as “it is a direct measure of the part of the body that tends to accumulate fat”. In order to normalize this measurement for all body types, including children, doctors recommend using a waist to height ratio[3]. Just the waist measurement alone, however, is enough to determine whether you’re at high-risk for heart disease:

“Having a waistband of more than 88cm (35in) in women and 102cm (40in) in men indicates the highest risk of cardiovascular and metabolic disease. There is an increased risk of the diseases for women with measurements of more than 80cm (32in) and men whose measurement is over 94cm (37in).”

Do It Yourself

Though the BMI is the de-rigour standard, it is extremely inaccurate for determining actual health or fitness. Body fat scales are relatively cheap and quite precise, if not 100% accurate, and can be used for determining changes over a period of time. Waist measurement is even lower tech and is also a fairly accurate indicator of susceptibility to the number one killer—in America anyway—heart disease. These two used in conjunction offer a relatively cheap and effective way of tracking health at home.


[1] Formulas obtained by examining the JavaScript source code on Calculate your Body-Mass Index (National Heart Lung and Blood Institute)
[2] Though most will do something else first, just take that extra bit of weight off before stepping on the scale.
[3] Again, presumably normalized using the metric system … have fun, Americans