Fat Mass Index | Because BMI and Body Fat Percent Suck

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fat mass index explained

Have you ever heard of fat mass index (FMI)? Probably not. Most people haven’t. But you should be aware of it because it can give you a more reliable indicator of your relative fat content.

What is fat mass index?

Fat mass index is a simple indicator of how much fat weight you have relative to your own height.

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The good thing about fat mass index is that it doesn’t penalise you for having lots of muscle (good news for muscly fit people!).

If used in conjunction with other clinical biomarkers (like blood pressure, blood sugar, lipids, waist circumference, etc), fat mass index can also help identify health risks for developing diseases like type 2 diabetes and heart disease.

How is fat mass index calculated?

Fat mass index is calculated by dividing your fat weight in kilograms by your height in metres squared.

Notice how this differs from body mass index. BMI considers your TOTAL weight on the “meat scale” relative to height.

Fat mass index, on the other hand, looks at just the fat portion of that total body weight. Therefore, your fat mass index will be a much smaller number than BMI.

Here’s a sample FMI calculation:

Your fat weight = 10 kg (22 pounds)
Your height = 1.72 m (5’6″ feet)

Step 1:  multiply your height by itself (so 1.72 x 1.72 = 2.96)
Step 2:  divide your fat weight (10 kg) by 2.96
Step 3:  FMI = 10 / 2.96 = 3.38 kg/m2

FMI vs BMI: the tyranny of numbers

Before we dig further into fat mass index, let’s set the scene by giving a quick rundown of body mass index (BMI). The formula for BMI is your weight in kilograms divided by your height in metres squared (kg/m2).

Body mass index has lots of enemies because it does not factor body composition into account. This poses a problem for some people because, if you have a lot of muscle (i.e., bodybuilders), it can classify you “overweight” or “obese,” even if you’re low in body fat.

When talking to clients, I find many of them tend to zero in on body fat percentage as the end-all-be-all holy grail of health. Unfortunately, body fat percentage can be just as misleading as body mass index because it is dependent upon your fat-free mass (i.e, muscle and bone). If you’re low in muscle, then this artificially inflates your percentage of body fat even though you have a low or normal amount of fat in kilograms (what is commonly known as “skinny fat.”).

Enter fat mass index to the rescue.

Fat mass index has the advantage over body fat percentage and body mass index because it is not influenced by fat free mass. But to be fair, it is influenced by height so depending on if you are very tall or very short, this could plausibly influence your numbers a bit.

To demonstrate how BMI misses the mark and FMI puts us back on track, let’s refer to the example above and quickly calculate a body mass index based on a total body weight of 100 kg (220 pounds) and the same height of 1.72 metres (or 2.96 metres squared).

BMI =  100 kg / 2.96 = 33.78 kg/m2

So what we can see here is that of your total BMI of 33.78, only 3.38 of that comes from body fat (based on the earlier sample calculation). So this person would be considered “obese” by BMI standards but in actual fact the person is very low in fat relative to height.

Is body mass index reliable?

For all the time I’m spending bashing body mass index, I think it’s important to point out that it’s not completely worthless and must be put into context.

  • First, when we use BMI in population studies where we measure the height and weight of, say, 100,000 people, it is true that a population with a higher BMI is at higher risk of health problems. The effect of high BMIs from bodybuilders in the sample would likely be washed out by the large sample size.
  • Second, body mass index should not be used as a stand-alone health indicator. It must be corroborated by other health biomarkers such as body composition, central obesity measures, blood sugars, blood lipids, blood pressure, etc.
  • Third, a person could carry a bit of body fat, have an elevated body mass index, yet be very athletically fit from regular exercise. In other words, this person could be metabolically fit on the inside from the conditioning effects from exercise.

Fat mass index in pictures

The image below gives a side by side comparison of two people each with low body fat, but one has low muscle and the other is high in muscle.

The “skinny fat” person on the left might have a high body fat percentage would be considered underweight as per BMI.

The person on the right is low in fat mass but high in muscle mass and would be considered “obese” as per BMI (clearly a false positive).

In both cases, their fat mass index would be low based on their actual fat weight.

fat mass index skinny fat

*For more on healthy body fat levels, have a read of my article: How much body fat should I have?

What is a normal fat mass index?

To give you a practical understanding of fat mass index, let’s put it into perspective with some normative values.

In a Swiss study of 5635 participants, percentile values for fat mass index were determined  for men and women aged 18 to 98. For the purpose of this article, I have extracted the 25th and 75th percentile data since this casts a wide enough net to see where most people tend to gravitate.

Fat mass index chart (women)

Age25th Percentile (kg/m2)75th Percentile (kg/m2)
18 – 344.66.6
35 – 544.87.3
55 – 746.510.3
75+7.511.4

Fat mass index chart (men)

Age25th Percentile (kg/m2)75th Percentile (kg/m2)
18 – 343.25.0
35 – 543.76.0
55 – 744.37.2
75+5.27.6

Source: Schutz et al. International Journal of Obesity (2002) 26: 953–960

So what do fat mass index numbers mean in practical terms?

Example 1
Let’s say you’re a 40-year old man and your fat mass index is 3.7. This would put you at the 25th percentile. When translated to plain English, this means that 25% of people have LESS fat weight than you relative to height and 75% of people have MORE fat weight than you relative to height.

Example 2
If you’re a 55-year old woman and your fat mass index is 10.3, then this means that you have more fat weight relative to height than 75% of other women, and 25% of other women have more fat weight relative to height than you do.

Measuring fat weight?

In order to calculate your fat mass index, you first need to find out how many kilograms of fat you have in your body.

I recommend getting a DEXA scan, as it is now a much more accessible and inexpensive technology that can give you a more accurate measurement of your fat mass and lean mass than methods like bioimpedance analysis (BIA) or skinfold calipers (the skin pinch test you see in gyms). Results from BIA are subject to fluctuations in body water and skinfold calipers are notoriously prone to tester error (i.e., poor technique).

That’s it. Once you have your fat weight in kilos, then it’s just a simple case of plugging your number into the fat mass index formula above.

Take home message

Fat mass index is slowly gaining momentum in the health field as a valuable biomarker. It addresses the limitations presented by BMI and body fat percent and, when used in conjunction with other health indicators, can provide a greater understanding of your individual level of health and fitness.

Recommended Reading

Schutz, Y, Kyle, UUG, Pichard, C. Fat-free mass index and fat mass index percentiles in Caucasians aged 18–98 y. International Journal of Obesity (2002) 26, 953 – 960. doi:10.1038=sj.ijo.0802037 (link)

Peltz, G., et al. The role of fat mass index in determining obesity. Am J Hum Biol. 2010 Sep-Oct; 22(5): 639–647. doi: 10.1002/ajhb.21056 (link)

More on health body fat levels from this website: How much body fat should I have?

Please be sure to leave a comment below if you have any questions or would like to make suggestions for improving this article. Thank you.

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