FFMI. Almost sounds like FML, doesn't it? Well, you might be saying FML if you realize how low your number is on here.
Fat-free-mass-index is just what it sounds like: an index designating how much lean body mass (AKA fat-free-mass) you have.
Before I ramble too much, let me give you the actual calculator:
*rounded to 1 decimal
So, what does this calculator do exactly?
Well, to put it simply, it provides a solid check of how close you are to your natural potential for lean body mass. Many people wonder how big they can get without drugs, and this serves as an excellent source for that.
People argue over the actual limit for FFMI. A thoroughly designed study (they had 157 participants and even examined records from bodybuilders in the 1950s and 1940s) found 25.5 to be the rough "peak" for drug-free athletes, while the average for those athletes was about 22. To put this in perspective even further, the average index for steroid users was 25.
Now FFMI is not perfect. If you take two individuals with the same height but totally different bone structures (one has 8 inch wrists and the other has 6 inch ones), then the FFMI won't be the same.
If I had to give my 2 cents, I would say the following are natural limits:
Bad genetics - 24
Average genetics - 25
Great genetics - 26
Let's take me for example: I'm 5' 10", I weigh 176 lbs (weighed first thing in the morning after using the bathroom), and I have approximately 12% body fat (according to my handy calipers).
My FFMI, based on this, is roughly 22.4. My immediate goal is to hit an FFMI of 23, and if I'm lucky enough, to one day achieve 24. I've never had good genetics to begin with, so as much as I'd love to one day have an FFMI of 25, I just don't see it happening. So if you too have a skinny frame with tiny wrists and ankles, don't expect to hit an FFMI of 26.
Please don't let this discourage you: like I said, I personally have bad genetics and have already accepted I'm nearing the size I can achieve without steroids. Just get out there and keep busting your butt with your training, diet, and recovery!