Fat-free mass index (FFMI) is a method of calculating the amount of muscle mass you have per unit of height. Although imperfect, research shows FFMI is a reliable indicator of whether or not someone is taking or has taken steroids to aid muscle building. 

You can do an FFMI calculation by dividing your fat-free body mass (your total body weight minus the weight of your body that’s fat, also called “lean body mass”) in kilograms by your height in meters squared, or simply entering your height, weight, and body fat percentage into the Legion FFMI Calculator.

How to Calculate FFMI

Use the following FFMI formula to calculate an FFMI score for men and women:

Fat-free mass = Weight [kg] × (1 – (Body fat [%]/ 100))

Alternatively, use the fat-free mass index (FFMI) calculator above. 

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What’s The Difference Between FFMI and Normalized FFMI?

FFMI measures the muscle mass you carry relative to your height. The problem with only using FFMI is that as height increases, so does FFMI, even if someone hasn’t taken steroids.

Normalized FFMI accounts for this by adjusting FFMI to the average height of the group being studied.

In a major study on this topic, the participants’ average height was 5’11, so this is generally the height used when computing normalized FFMI (it’s also fairly close to the average height of American men—5’10).

The key thing to remember is that normalized FFMI is more accurate when comparing the FFMIs of different people, especially those taller or shorter than average.

Is FFMI Supported By Research?

Yes.

In a renowned study by McLean Hospital, scientists calculated the FFMI of 157 young male athletes, including several elite bodybuilders and world-record holding strongmen, as well as some gents undergoing a testosterone injection study.

Of these athletes, 31 admitted to steroid usage in the past, 52 admitted to taking steroids within the previous year, and 74 claimed they had never taken steroids. The researchers also had everyone take a urine test to attempt to validate their claims.

After analyzing the data, the scientists found the steroid users had an average FFMI of 24.8 while the purported non-users averaged at 21.8.

Notably, no natural participants had an FFMI above 25, but many “enhanced” individuals did.

This study, along with others, suggests that individuals with a normalized FFMI over 25 likely have used or are using steroids.

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Does an FFMI Under 25 Guarantee No Steroid Use?

Not necessarily.

Many steroid users aren’t as defined or developed as pro bodybuilders with FFMIs above 25. In fact, many look far more “normal” than you might think.

There are various reasons for this: Some have sub-par genetics. Others take smaller doses. And still others neglect the training, diet, and rest crucial for building muscle. Some (“influencers,” usually) even lie about their height, weight, and total body fat percentage to avoid being pegged as drug users.

For example, you could maintain a very “aesthetic” physique lifting weights just once or twice per week for 30-to-45 minutes, eating fast food every day, and paying no attention to your calories and macros . . . if you also take steroids. Steroids even allow you to build muscle, stay lean, and eat quite a bit of food without going to the gym, which is why some sedentary people take them, too.

Past steroid use also creates lasting cellular changes that can enhance your ability to gain and retain muscle in the future. In other words, every pound of muscle you gain while on steroids contributes to your overall muscularity for the rest of your life.

There are plenty of people who say they’re “natural” because they’re not currently on drugs, while neglecting to mention they took steroids in the past. In many cases, people with suspiciously high normalized FFMIs (24+) used steroids to permanently prop up their FFMI, even though they don’t take them anymore.

At bottom, then, here’s what the evidence says about determining whether someone is natty or not:

  • If someone’s FFMI is over 25, it’s almost certain they’re taking or have taken steroids.
  • If someone’s FFMI is between 24 and 25, then it’s likely they’re taking or have taken steroids, though their muscle may have been attained naturally.
  • If someone’s FFMI is between 22 and 23, they maintain a fairly normal body fat percentage (they aren’t “shredded” year round), and they’ve been training consistently for 3+ years, they’re probably natural.
  • If someone’s FFMI is between 21 and 22, they’re also probably natural.
  • If someone’s FFMI is between 18 and 21, they likely don’t even lift so who cares. 🙂

+ Scientific References