Is a simple enough question but the answer is not that straight forward. The generic calculation for BMI is

weight (kg) / [height (m)]2

So for example our test subject weighs 76.2kg and is 182cm tall, so their BMI would come to:

76.2 / 1.822 = 23

This is then checked against the adult chart:

BMIResult
Below 18.5Underweight
18.5 – 24.9Normal/Healthy
25.0 – 29.9Overweight
30.0 and aboveObese

In the above example our test subject is rated as normal or healthy, which is good. However this rating does not take into consideration a lot of important factors such as amount of muscle they have or their gender. Ignoring these factors can make the above measurement highly inaccurate and therefore misleading.

A good example of how the above reading can be misleading comes from the elite Navy SEALs in the American Navy. The navy regularly measured personnel BMI to ensure they were stay fit and active, however the results were indicating that the elite SEALs were obese. The issue with the readings was coming about because the SEALs had a high level of muscle which weighs more than fat, as such they were considered heavy for their size.

The American Navy then set about developing a new test to allow for muscle, to do this they determined several other factors to include on top of height and weight. For men this test included measuring the waist (at belly button) and neck (at narrowest). For women the test measured waist (at narrowest), hips (at widest) and neck (at narrowest).

The formula the navy developed was:

For men: % body fat = 86.010 x log10(abdomen - neck) - 70.041 x log10(height) + 36.76

For women: % body fat = 163.205 x log10(waist + hip - neck) - 97.684 x log10(height) - 78.387

It is worth noting that the above formula is American and therefore in inches, so when completing this with metric readings you will need to convert to inches (1cm = 0.393701inch).

If we return to our initial test subject, their waist measurement is 81cm and their neck is 37cm. Adding in these variables gives our test subject a body fat percentage of 13%. On the original table this could be seen as underweight, however the new formula also relies on a different table which places our test subject at the higher end of athletic.

CategoryWomen (% fat)Men (% fat)
Essential Fat10%-13%2%-5%
Athletic14%-20%6%-13%
Fit21%-24%14%-17%
Acceptable25%-31%18%-25%
Obese32% +26% +

The approach from the Navy instantly tries to correct the two major flaws with the standard BMI test, it allows for gender and it tries to allow for muscle mass.

As a final test we put our fearless test subject on an Aria Fitbit scale, this provided a reading of 17.6% body fat. While we can’t share the formula for the Aria calculation we can share the approach they use:

The Aria measures body fat percentage by using a body impedance method. It has four sets of ITO electrodes that shoot a small 180 micro amp current up one leg and down other to measure body impedance.

The Aria’s rating are closely aligned to that of the Navy ratings, the Aria reports 14%-31% for women is the healthy range and for men it is 6%-24%.

So which one should I use?

Unfortunately that is not something we can tell you as everyone is different. However that said, we have found personally that the Navy method is more accurate for us.

Next steps at Interesting Fitness is to ship our test subject off for a professional Dexa Scan and body fat percentage measurement, after this has been completed we will update this article with the comparison.