What is the fat-free mass index? How is it calculated? And does a FFMI of 25 really represent a natural limit to the maximum amount of muscle mass you can gain without using drugs?
Let’s find out…
What is the Fat-Free Mass Index (FFMI)?
The fat-free mass index (FFMI) is a height-normalized index of fat-free mass. It’s calculated by dividing fat-free mass (in kilograms, or kg for short) by your height (in meters squared).
FFMI = fat-free mass in kg ÷ (height in meters) ²
The higher the number, in theory at least, the more muscular you are.
It’s basically the same formula used to calculate body mass index (BMI), the only difference being that the fat-free mass index uses lean rather than total body weight.
Strength athletes will often have a high BMI despite the fact they’re relatively lean, mainly because they’re carrying around a lot more muscle than the typical overweight or obese individual.
Given that excess body fat is linked with many of the health risks associated with being overweight or obese, it doesn’t make a lot of sense to put them in the same category as someone who’s carrying around a lot more fat.
The FFMI is often used to help determine the maximum amount of muscle mass it’s possible to gain without using drugs.
Some will also use it to tell if a popular fitness celebrity on YouTube or Instagram is secretly on the sauce, but pretending to be natural.
If you want to know what your FFMI is, just enter your height, weight and body fat percentage below, and the FFMI calculator will crunch the numbers for you.
The average untrained caucasian male has a FFMI of around 19 . Getting to a lean (8-10% body fat) FFMI of around 22 is a significant achievement.
If you can make it to a FFMI of 23-24 while staying close to 10% body fat, you’re going to look damn impressive.
|Below 18||Below Average|
How is FFMI Calculated?
Let me show you how the FFMI is calculated, using myself as an example.
These were my stats at the time:
- Height: 5ft 10 inches
- Weight: 175 lbs (79.6 kg)
- Body fat percentage: 10%(ish)
Here are the steps I’d go through to calculate my FFMI.
1. Convert pounds, feet and inches to kilograms (kg) and meters.
- 175 lbs ÷ 2.2 = 79.55 kg
- 5ft 10 inches = 1.78 meters
2. Calculate lean body mass.
- Body fat percentage = 10%
- Lean body mass = 90%
- 79.55 x 0.9 = 71.6 kg
3. Multiply height by itself to get height squared.
- 1.78 x 1.78 = 3.17 meters ²
4. Divide lean body mass by height squared.
- 71.6 ÷ 3.17 = 22.6
That gives me a FFMI of 22.6. Because of rounding, this is slightly different to the number you’ll get if you plug those numbers into the FFMI calculator.
How to Calculate Lean Body Mass
Lean body mass is the difference between total weight and fat weight.
Lean body mass = total weight – fat weight
To calculate lean body mass, you need to know how much you weigh, and what your body fat percentage is.
- To calculate fat mass, multiply your weight by your body fat percentage.
- To calculate lean body mass, subtract the result from your weight.
For example, let’s say that you weigh 200 pounds, with 20% body fat.
Fat mass = 200 pounds x 0.20 = 40 pounds
Lean body mass = 200 – 40 = 160 pounds
In this example, you have 40 pounds of fat, and a lean body mass of 160 pounds.
Although the terms fat-free mass and lean body mass are often used interchangeably, they’re not the same thing.
Fat-free mass refers to everything in your body that isn’t adipose tissue. It includes the weight of:
- Internal organs
- Connective tissue
Lean body mass includes all of those things, along with a small amount (roughly 5% or so of total body weight) of essential fat.
Lean body mass = fat-free mass + essential fat
But for our purposes, they’re close enough, and I’m going to use the terms interchangeably from now on.
Is a FFMI of 25 the Limit to What Can Be Achieved Naturally?
Some say that a FFMI of 25 is the limit to the amount of muscle mass it’s possible to gain naturally. If someone has an FFMI above 25, you know for sure they’re on gear.
In fact, while the number of people who can reach a fat-free mass index of 25 without anabolic drugs are in the minority, it’s not a hard limit.
At best, it gives you a rough idea as to what’s possible. But it doesn’t take into account genetically-gifted individuals who are exceptions to the rule.
The idea that a fat-free mass index of 25 represents some kind of natural upper limit to muscle growth comes from a study published back in the 1990s, titled Fat-Free Mass in Users and Nonusers of Anabolic-Androgenic Steroids .
For the study, a group of 134 men were recruited from gyms in Boston and LA. To this sample, the researchers also added data from 23 men who’d been using testosterone cypionate as part of a separate trial.
Of these 157 men, roughly half admitted to steroid use, while the other half said they were clean.
Here’s how the researchers sum up their findings:
“In an examination of 157 athletes, comprising 83 steroid users and 74 nonusers, we calculated normalized FFMI using height, weight, and body fat based on skinfold measurements. With this simple measurement, we found that athletes who had not used steroids all had values of less than 25, whereas a large proportion of steroid-using athletes easily exceeded this limit.”
However, the study does have a number of important limitations that limit the conclusions we can draw.
First, any assumption that a FFMI of 25 represents an upper limit to muscle growth would need to be based on data from a large number of people who were a) genetically predisposed towards gaining muscle and b) at, or every close to, their natural muscular limits.
An unspecified number of competitive bodybuilders and strength athletes, who were presumably close to their maximum muscular potential, did participate in the study.
However, most subjects were typical gym-goers recruited from local gyms. The only requirement for taking part in the study was that you were at least 16 years old, and had been lifting weights for at least two years.
In other words, it’s likely that many subjects had some way to go before they got anywhere close to their natural muscular limits.
How Height and Body Composition Affect FFMI
What’s more, the findings of the study can’t be extrapolated to people with a high level of body fat.
That’s because gaining fat tends to increase lean body mass, which means that fat individuals will end up with a higher FFMI value. On the flip side, those with less body fat will end up with a lower FFMI.
The average body fat percentage in the drug-free group was around 13%, with only six men having body fat levels greater than 20%.
Individuals with a high body fat percentage are more likely to exceed a FFMI of 25, as they usually have more lean body mass (not all of which is muscle mass) than someone who’s leaner.
I’ve come across a number of online discussions from people boasting about how high their FFMI is. But when I saw their pictures, they were a lot fatter than they thought. Lots of guys think they’re around 15% body fat, when in reality they’re probably closer to 25%.
That’s because the various methods for calculating body composition, from body fat scales to DEXA scans to underwater weighing, aren’t particularly accurate.
That’s important to keep in mind when you’re trying to calculate your fat-free mass index. The body fat percentage you enter into a FFMI calculator is a potential source of error that can skew the results.
Taller individuals also tend to have a higher fat-free mass index han shorter ones.
When the researchers looked at the tallest and shortest drug-free participants, the shorter athlete had an FFMI value of 23.5, while the tall one scored 25.4. However, the shorter athlete appeared more muscular than the taller one.
To correct for the effect of height, the researchers came up with a normalized FFMI, in which the FFMI was normalized to that of someone 5 ft 11 inches (1.8 meters) in height. Here’s what the equation looks like:
Normalized FFMI = FFMI + 6.1 x (1.8 – height in meters)
But even then, the normalized FFMI for both individuals – 24.8 for the short guy and 24.6 for the tall guy – was almost identical. Even though the shorter guy appeared more muscular than the taller one, their FFMI was the same.
Can some people attain a fat-free mass index over 25 without taking drugs? Yes.
Is it common? No.
Not everyone is blessed with favorable muscle-building genetics, and the people that get there are in the minority.
Even the researchers themselves don’t say that everyone with a FFMI over 25 is on gear.
Rather, they suggest using it as a screening method to assess for drug use. It’s more of a simple first step towards establishing whether or not someone is taking drugs, rather than the final word.
FFMI vs Muscle-to-Bone Ratio
One of the weaknesses with the FFMI is that it doesn’t take into account the size of your frame.
That matters, because the size of your skeleton has a big impact on the maximum amount of muscle mass it’s possible to build. Research shows that people with a larger frame tend to have more fat-free mass (FFM) than people with a smaller frame .
Francis Holway, an exercise and nutrition researcher from Buenos Aires, likens the human skeleton to an empty bookcase. The wider the bookcase, the more books it will hold. Likewise, the bigger the skeleton, the more muscle it can support.
Here’s a brief snippet on this subject from The Sports Gene: Inside the Science of Extraordinary Athletic Performance:
One bookcase that is four inches wider than another will weigh only slightly more. But fill both cases with books and suddenly the little bit of extra width on the broader bookcase translates to a considerable amount of weight. Such is the case with the human skeleton.
In measurements of thousands of elite athletes from soccer to weight lifting, judo, rugby, and more, Holway has found that each kilogram (2.2 pounds) of bone supports a maximum of five kilograms (11 pounds) of muscle. Five-to-one, then, is a general limit of the human muscle bookcase. The limit for women is closer to 4.1 to 1.
Holway experimented on himself, spending years in heavy weight training with a diet high in protein and supplemented by creatine. But as he closed in on five-to-one, inhaling more steaks and shakes only added fat, not muscle.”
Unlike FFMI, bone mass reflects the size of your entire frame. As a result, the muscle-to-bone ratio gives you a better idea of your maximum muscular potential. The bigger your frame, the more muscle mass it can carry.
All of which might sound interesting. But it doesn’t tell you anything useful, as you probably don’t know off the top of your head how much your skeleton weighs.
Would a DEXA scan give you the answer? Unfortunately not. While a DEXA scan measures bone mineral density, it doesn’t tell you the weight of those bones.
You could track down a suitably qualified anthropometrist (look for someone trained by the International Society for the Advancement of Kinanthropometry), who can measure bone breadth at various locations.
This approach has been used in several studies to calculate the muscle-to-bone ratio [3, 7], which you can then use to estimate your maximum muscular potential.
Maximum Muscular Bodyweight Calculator
Another popular way to estimate the amount of muscle you can expect to gain, which takes your height, wrist size, ankle size, and body fat percentage into account, is Casey Butt’s Maximum Muscular Bodyweight calculator.
It’s based on equations that Casey developed after analyzing data from some 300 title-winning drug-free (allegedly) bodybuilders and strength athletes from 1947 to 2010.
I say allegedly, because we have no real way of confirming their drug-free status.
The majority of anabolic steroid usage in the US began in the 1950s. However, testosterone was synthesized in the 1930’s, and there is speculation that some athletes were using it as early as the 1936 Olympic Games. Adverts from CIBA Pharmaceuticals for synthetic testosterone were targeting bodybuilders in the late 1940’s.
How many of the bodybuilders and strength athletes used in Casey’s equations were taking anabolic drugs?
We don’t know, and we’re probably never going to find out. But it’s another potential source of error to throw into the mix.
What’s more, this method assumes that wrist and ankle size correlates with bone size throughout the body. This is true in some cases [4, 5], but not for everyone.
That is, some people can have a relatively large frame, but smaller wrists and ankles. Others may have larger wrists and ankles, but a relatively small frame.
And even with an identical bone structure, there are numerous other factors that affect your muscular potential.
Finally, the calculator is an attempt to establish the maximum amount of muscle someone of your height and bone structure can potentially gain.
It’s an upper limit of potential based on the achievements of elite drug-free bodybuilders, rather than a prediction about the amount of muscle you can realistically expect to gain.
The numbers used in the equation come from elite bodybuilders and strength athletes, who were first in line when the lean and muscular genes were being handed out.
Most people are doing remarkably well to get anywhere close to the numbers predicted by Casey’s equation, let alone match them.
There are weaknesses and limitations to the various methods used to estimate your genetic muscular potential, and none should be treated as the final word about how much muscle it’s possible to gain.
However, it’s useful to be aware of them so you don’t end up wasting time, effort and money trying to hit a muscular body weight that’s beyond your reach without the use of drugs.
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