Fat-free mass index. It is lean body mass in kilograms divided by height in metres squared — the same shape as BMI, but with the fat removed first. Where BMI cannot tell muscle from fat, FFMI measures only the part that is not fat.

How do you calculate FFMI?

First find fat-free mass: FFM = weight × (1 - (%BF)/100). Then divide by height in metres squared. A 180 lb man at 17.5 percent body fat has 148.5 lb of lean mass, or 67.4 kg, and at 1.778 m tall that gives 67.4 ÷ 3.161 = 21.3.

For men, 18 to 20 is average, 20 to 22 is above average, and 22 to 23 looks visibly muscular. Above 23 is superior, and roughly 25 is where the natural ceiling is usually placed. Subtract about 3 from every figure for women.

What is normalised FFMI?

An adjustment for height, FFMI + 6.1 × (1.8 - h) with height in metres. Taller people tend to score lower on the raw index for the same build, so the correction puts everyone on a common scale referenced to 1.8 m.

Is an FFMI of 25 really the natural limit?

It is a well-known reference point rather than a hard biological wall. The figure comes from a 1995 study of pre-steroid-era bodybuilders whose normalised FFMI clustered just under 25. Some drug-free athletes do exceed it, particularly those with unusual frames, so treat it as a strong statistical pattern and not a verdict.

Is FFMI better than BMI?

For anyone who trains, yes. BMI counts muscle as though it were fat, which is why it labels lean athletes overweight. FFMI needs a body fat measurement first, though, so it inherits whatever error that measurement carries.

Does FFMI work for women?

Yes, with a shifted scale. Women naturally carry less lean mass for their height, so the equivalent bands sit roughly 3 points lower — around 15 to 17 for average and 19 to 20 for visibly athletic.

FFMI Calculator: Fat-Free Mass Index and Limits

BMI’s central flaw is that it cannot tell a pound of muscle from a pound of fat, which makes it useless for anyone who trains. FFMI fixes that by stripping the fat out first and asking a better question: how much lean mass do you carry for your height? It is the number strength athletes actually care about, and it comes with an interesting side effect — a fairly well-known ceiling on what is achievable without pharmaceutical help.

FFMI Calculator formula — The FFMI formula

The FFMI formula

Two steps. First strip out the fat:

Then index it against height, exactly as BMI does:

The normalised version corrects for the fact that taller people score lower on the raw index for the same apparent build:

The 1.8 is the reference height in metres. A person exactly 1.8 m tall gets no adjustment; someone shorter gets a small deduction and someone taller a small addition.

Worked example

A man weighing 180 pounds at 70 inches tall with 17.5 percent body fat.

Step 1 — fat-free mass in pounds.

Step 2 — convert to kilograms.

Step 3 — height in metres, then squared.

Step 4 — the index.

Step 5 — normalise for height.

An FFMI of 21.4 is above average — someone who clearly lifts, without looking like it is their profession. For comparison his BMI is 25.8, which the standard chart calls “overweight”. That gap is the entire argument for using FFMI.

FFMI Calculator worked example diagram — A visual read of the worked example above — the same numbers the calculator returns for its default inputs.

Reference bands

FFMI (men)	FFMI (women)	Interpretation
under 18	under 15	Below average lean mass
18–20	15–17	Average, untrained
20–22	17–19	Above average, trains regularly
22–23	19–20	Excellent, visibly muscular
23–25	20–21	Superior, years of serious training
25+	22+	Beyond what is typical drug-free

The women’s column runs about three points lower throughout. That is a difference in physiology, not effort — average lean mass relative to height simply differs between the sexes.

About the FFMI 25 ceiling

The number comes from a 1995 study that measured bodybuilders from before anabolic steroids were available and compared them with contemporary users. The drug-free group’s normalised FFMI clustered just below 25; almost nobody exceeded it.

It is worth being careful about what that means. It is a statistical observation about a sample, not a physical law. A handful of drug-free athletes with unusual frames do measure above 25, and the figure also inherits the error of whatever body fat measurement fed into it — a body fat reading three points too low will push FFMI up by roughly 0.7.

So an FFMI of 26 is genuinely unusual and worth a second look at your body fat number. It is not proof of anything.

FFMI against BMI

	BMI	FFMI
What it measures	total weight for height	lean mass for height
Needs body fat %	no	yes
Muscular person	reads “overweight”	reads correctly
Sedentary person at normal weight	reads “normal”	reads low
Good for populations	yes	not routinely used
Good for lifters	no	yes

The two disagree most in the cases that matter. A 200-pound athlete at 10 percent body fat and a 200-pound sedentary person at 30 percent share a BMI and have nothing else in common; their FFMIs differ by about six points.

Its one real weakness

FFMI is only as good as the body fat figure you feed it, and body fat is the hard part. A tape measurement carrying ±3 percentage points of error translates to roughly ±0.7 FFMI points — enough to move you a whole band.

Use the body fat calculator with the Navy tape method for a consistent input, and measure the same way every time. As with body fat itself, FFMI is far more useful as a trend than as a single figure. Gaining half a point over a training year is a real, measurable result; the absolute number carries more uncertainty than it appears to.

Common mistakes

Using total weight instead of lean mass. That is just BMI with extra steps.
Mixing units. The formula wants kilograms and metres. Convert first, or use the fields above which do it for you.
Comparing raw and normalised figures. They differ by up to about a point at the extremes of height. Pick one.
Using the male bands for women. Subtract about three points.
Trusting an impedance scale’s body fat reading. Those swing several points with hydration alone, and every point of error moves FFMI by roughly 0.23.
Treating 25 as a pass/fail line. It is a statistical pattern from one study, not a boundary.

If your height and weight are in metric, the inches to cm converter handles the conversion the other way; the calculator above expects pounds and inches.

Sources and assumptions

This calculator assumes:

Body-fat percentage comes from your own measurement, so the result is only as accurate as that input.
The normalised FFMI adjusts to a reference height of 1.8 m.

Sources:

Kouri, E. M. et al., Fat-free mass index in users and nonusers of anabolic-androgenic steroids, Clin J Sport Med (1995), PubMed

Figures last reviewed: 5 October 2026. Educational estimates, not professional advice.

About the formula: FFMI = fat-free mass in kilograms ÷ height in metres squared. The normalised version adds 6.1 × (1.8 − height in metres) so that tall and short people can be compared on the same scale.

FFMI Calculator