Fat-Free Mass Index (FFMI): What it is, why it matters, and why is mine so low?
Could this be the metric I've been looking for?
I had never heard of the fat-free mass index (FFMI) before, and yet my results mentally assaulted me in a way that only a betrayal of a close friend could.
Before we get into why that is, or what my results were, let’s look at what it is and what it measures.
What is the Fat-Free Mass Index?
The FFMI is a measure of your lean body mass – your total weight minus your fat mass – compared to your height. The exact formula is:
Here Lean Mass is measured in kilograms, and the height is in meters.
Ever heard of the body mass index (BMI)? Well, it’s the same formula as the above, just swap out “lean mass” with “total mass.” Since total mass is just Fat Mass + Lean Mass, BMI, on its own, is insufficient to learn where you should focus next. If you look at BMI alone, you are guessing at the person’s overall health.
For example, let’s say someone has a BMI of 30. That person could either be A) obese, or B) ripped. Without knowing the rest of the person’s composition, you would miss the mark.
FFMI is neither an exact nor a better replacement of BMI. But it is much more useful in telling us something about ourselves than BMI. BMI needs the additional context that FFMI provides. Let’s take my example.
My FFMI results
I’m 6’3” and weight around 170 lbs. On December 4, 2024, my DEXA results showed a BMI of 21.4, and an FFMI of 15.8. A BMI of 21.4 is squarely in the “Normal” range. An FFMI of 15.8 is in the “did-I-really-just-watch-the-entire-season-of-Love-is-Blind” range (aka a very low).
If I continued to only look at my BMI data, I might think
“alright, nothing to see here - I’m average. Not shocking, given the fact that I haven’t been in great shape lately.”
But again, BMI isn’t enough. It can’t tell me where I should focus my attention, other than perhaps thinking “I should exercise more.” The FFMI, on the other hand, shows a glaring (helpful) hole in my fitness. According to this percentile chart from this 2018 study, and confirmed by the very helpful printed out DEXA sheet, my FFMI of 15.8 is the third percentile.
In other words, 97% of the population has more muscle for their given height than I do. Honestly that sentence is still a bit hard to type out for me. In fact, if it weren’t for Claude, I wouldn’t even know it’s a problem.
But this very important, very helpful context is missed completely by my boringly-average BMI.
I imagine most people’s FFMI results are rather boring. 40th percentile. 70th percentile. Average people, average results. But for me, FFMI really helped highlight the true depth of the gap in my fitness I always knew was there: I’m super skinny.
It’s not like I didn’t know I was skinny before I got the scan. I’ve always known. I’ve been around this exact weight since high school. But FFMI helped me shift mentally from “I’m kind of skinny” to “I’m too skinny.” Wait. That sounds negative. I get that. But know that I write “too skinny” with a more science/medical view than a judgmental, I-have-body-image-issues vibe.
To say “too skinny” differently, the risk of poor health outcomes is much higher at my extremely low FFMI than if I was in the 50th percentile. For me, having the number helps me move beyond my own insecurities (e.g. I look too skinny) to look at the problem objectively (e.g. I am too skinny for my health).