BMI is Useless for Most People, Here’s Why FFMI is Better
How to use fat-free mass index as a health & fitness metric
By: Jeremy Fox CNC, CPT
Updated: March 18, 2025
Has your doctor ever said your body mass index (BMI) is too high?
It happened to me during a routine health screening.
At 5’9″ and 205 pounds, I was shocked to hear that I was considered “Obese” with a BMI of 30.
The most frustrating part was that I worked out five days a week and was in decent shape. How could this health metric be so wrong?
The issue with BMI measurements is that they don’t consider body composition.
That’s where fat-free mass index (FFMI) could provide better insights.
FFMI Defined
While BMI centers on total body weight, FFMI focuses on lean body mass.
In other words, it quantifies how much of your weight is muscle vs. fat.
FFMI goes beyond a rudimentary height-to-weight ratio and provides more granular information about your physical fitness.
Calculating your FFMI requires knowing your height, weight, and body fat percentage.
Here’s how the calculations differ:
- BMI = weight / height²
- FFMI = lean mass / height², where lean mass = weight — (weight x body fat percentage / 100)
Understanding FFMI provides a more accurate assessment of your overall health and fitness compared to other people of your size and gender.
What Is a “Good” FFMI?
When calculating your FFMI, you get a number similar in scale to BMI. Unlike BMI, however, a high FFMI can be a good thing.
To understand what’s considered a “good” FFMI, it’s helpful to look at the typical range of values for men and women.
For men, the FFMI result is usually between 15 and 25. Here are the discriminant categories within that range.
FFMI Categories for Men
- <18 skinny
- 18–20 average
- 20–21 athletic
- 21–23 very well built
- >23 extremely muscular
Women generally have less muscle mass than men, so the FFMI scale for females ranges from 12 to 22. Here are the corresponding categories.
FFMI Categories for Women
- <15 skinny
- 15–16.5 average
- 16.5–18 athletic
- 18–20 very well built
- >20 extremely muscular
I also reviewed a study of over 5,000 men and women to better understand the relationship between FFMI and BMI.
I plotted that data in a normal distribution or ‘bell curve’ so you can see where most people fall on the FFMI scale and where you stand in comparison.
Bell curves showing the normal distribution of FFMI for men and women.
That means anyone above or below those ranges has more or less lean mass than most people.
If you fall outside the norm (like I did), BMI is not a good indicator of overall health.
BMI vs. FFMI Examples
Let’s look at some examples to demonstrate the differences between BMI and FFMI.
(hand raised) 🙋♂️
As I mentioned, I used to be rather heavy for my height. Although I lifted weights regularly, my body fat was a little on the high side.
Here’s how my BMI and FFMI compared.
Overweight Male BMI vs FFMI
- Height: 5’9″
- Weight: 205 lbs
- Body Fat: 24%
- BMI: 30.3 (obese)
- FFMI: 23.1 (extremely muscular)
Based on these numbers, my BMI was considered “obese,” while my FFMI was “extremely muscular.”
In my case, measuring FFMI revealed that my high BMI was mainly due to my having more muscle mass than most people — not that I was “fat” or “unhealthy.”
Conversely, a low BMI isn’t necessarily an indicator of good health. So, let’s look at an example of a “healthy” female.
This time, let’s say Beth is 5’4″ tall and weighs 120 pounds, which sounds good. However, her body fat is that of a typical American woman, at 31%.
Normal Female BMI vs FFMI
- Height: 5’4″
- Weight: 120 lbs
- Body Fat: 31%
- BMI: 20.6 (normal)
- FFMI: 14.2 (skinny)
Based on these numbers, her BMI is considered normal at 20.6. However, her FFMI of 14.2 reveals that she has a relatively low lean mass compared to her body fat.
Limitation of FFMI
While fat-free mass index provides more personalized health insights than body mass index, it’s not without limitations.
For example, I could have taken my above-average FFMI as a signal that my diet, exercise, and lifestyle were optimized. But that wasn’t the case.
The health screening also revealed that I had stage 2 hypertension with a blood pressure of 160/90. The most likely culprit was a high-calorie/high-sodium diet with minimal cardiovascular exercise.
I took the hint and adopted a more nutritious diet while losing body fat. A few months later, my blood pressure returned to a healthy level.
Image of author before and after weight loss transformation showing the changes in BMI and FFMI relative to body weight and body composition.
How to Use FFMI + Body Fat
Relying on FFMI alone doesn’t provide a complete picture. It’s possible to have a “good” FFMI and still be in poor health.
A sumo wrestler may have a high FFMI score due to their training and muscularity. But they also carry an excessive amount of fat, which usually isn’t healthy.
Therefore, we must also consider body fat percentage and vital signs when making lifestyle decisions.
Using FFMI and body fat percentage together, you can determine your ideal weight. Then, adjust your diet and workout routine to hit a specific target.
The graph below illustrates how body fat and lean mass intersect. Generally, it’s best to set goals that move you in a direction where the bars get taller (i.e. less body fat and/or more lean mass).
Chart created by author using Excel.
This approach also works if your BMI is normal but you aren’t passing other health metrics.
To illustrate, let’s go back to our previous example with Beth. With her low FFMI and high body fat percentage, she would benefit from adding lean mass through resistance training and increasing her protein intake.
Being more active and managing her energy balance would help Beth lose fat while improving her overall health.
Take Control of Your FFMI
Understanding your FFMI, body composition, and other vital health metrics empowers you to make better changes in your daily life.
If you’re ready to take control of your body and health, the first step is finding your body fat percentage.
You don’t need a doctor or fancy equipment, either. I have a simple calculator that helps you find your body fat percentage in the comfort of your own home.
>> Try my body fat percentage calculator
Once you know your body fat level, the next step is finding your FFMI. I have another free calculator that shows your FFMI and helps you decide what changes to make in your diet and workout program.
