Science · 8 min read · Video lesson

What Is FFMI? Every Level Explained

By Professor Mass · Written version of the video

BMI can't tell a bodybuilder from someone overweight. FFMI can. Here's how it works, what every level looks like, and where the "natural limit" really comes from.

The formula

Take your weight, remove your body fat, and divide by your height in meters squared. Most calculators also normalize the result to a height of 1.80 m, so tall and short lifters can be compared:

FFMI = lean mass (kg) ÷ height (m)² + 6.1 × (1.8 − height in m)

Example: two men are 5'10" and 175 lb. One is 25% fat, the other 10%. Same BMI. Their FFMIs are about 19 and 23. On paper they're twins. In a T-shirt nobody would confuse them.

Be honest about body fat. Guessing 10% when you're really 20% throws the whole number off. If you're not sure, use the body fat calculator first.

Every level, from 17 to 35

FFMI	Who's there	What it looks like
17	Skinny, untrained	Shirt hangs from the shoulders. The fastest level to grow from.
19	Average man	The "factory setting". Where most people start and stay.
22	Gym regular	Years of training. Lean, athletic, clearly lifts.
24	Elite natural	Great genetics plus 10+ years. Rarer than you think.
25	The "natural limit"	Where nearly every natural lifter stops.
27	Natty or not?	Possible naturally, but very rare.
30	Enhanced	Classic physique pro stage.
35+	Mass monster	Open bodybuilding. On stage, near 40.

FFMI 17: the starting line

Skinnier than about 9 in 10 men. If your thumb and middle finger overlap easily around your wrist, you were probably built to start here. The good news: this is where muscle comes fastest. 17 isn't a sentence, it's a starting line.

FFMI 22: the top of normal

In a study of 74 men who had lifted seriously for years without steroids, the average was about 22. If you've trained consistently for a few years, this is probably you. It's also the lean, athletic look most people actually want.

FFMI 24: where genetics take over

In one large experiment, about 580 people trained one arm for 12 weeks with the same routine. Some arms barely changed. Others grew 59%. That spread is what separates a 22 from a 24.

Why 25 is called the natural limit

The number comes from a 1995 study by Kouri and colleagues. They compared 74 drug-free lifters with 83 who admitted using steroids. Not one natural lifter went above 25. Almost half of the users did. The same researchers looked at Mr. America winners from 1939 to 1959, before steroids spread in the sport. Their average: 25.4.

But 74 people is a small sample. It shows where most naturals end, not where every human ends. Think of 25 as the floor where almost every elevator stops.

Natty or not at 27?

John Grimek, who dominated in the 1940s before steroids, is estimated near 27. A scan of 111 college football players found about 1 in 5 above 25, with the highest at 27.7. So 27 naturally exists, it's just rare, like being 7 feet tall. Big bones, height and water all push the number up. At this level, the honest answer is "maybe".

Above 30, the debate is basically over. In a classic 10-week trial, men who lifted on placebo gained about 4 lb of lean mass, while men who lifted on 600 mg of testosterone a week gained about 13 lb. That escalator has a price: shut-down natural testosterone, thicker blood and a growing heart.

Calculate yours, then use it

Your FFMI is most useful as a progress tracker against your own past, not as a score against the internet. Most natural lifters spend their whole training life between 22 and 24, and look great there.

Keep learning

• Muscle insertions explained
• Bone structure explained
• Video: Every Body Fat % Explained

Sources: Kouri EM et al., Clin J Sport Med, 1995 (FFMI of natural vs steroid users). Bhasin S et al., N Engl J Med, 1996 (testosterone and strength training). Hubal MJ et al., Med Sci Sports Exerc, 2005 (variability in response to training).