Lean body mass and FFMI calculator

How is it calculated?

Lean mass comes first: weight × (1 − body fat). For the sample body, 82 kg at 22.2% fat gives 63.8 kg of lean mass and 18.2 kg of fat. Because the accuracy of this step depends entirely on the accuracy of the body fat figure, the form asks where that figure came from and the page talks differently in the two cases. The multiplication itself is innocent.

If you do not know your body fat, it is estimated with the Deurenberg equation from BMI, age and sex. Because that equation takes BMI as an input, it reads muscle as fat in muscular bodies: it reports body fat HIGH and therefore FFMI LOW. Entering a measured value usually raises the result — for the sample body, a normalised FFMI of 20.3 from the estimate becomes 22.4 at a measured 14%.

FFMI is lean mass divided by the square of your height: 63.8 ÷ 1.78² = 20.1. Up to here the arithmetic mirrors BMI, except the numerator is lean mass rather than total weight. That is the whole difference.

Then comes normalisation, and this is the number that matters: FFMI + 6.1 × (1.80 − height). Raw FFMI tends to run high in shorter people and low in taller ones; the correction proposed by Kouri and colleagues in 1995 moves everyone to a height of 1.80 m. For the sample body, 20.1 → 20.3. This is the comparable version, and it is what the page prints large.

The upper end of the natural range comes from that same 1995 study: among athletes who did not use steroids, normalised FFMI did not exceed 25. That is an observation about a distribution, not a prohibition. The "remaining to the natural ceiling" line gives the lean mass gap to that upper end in kilograms — 15.0 kg for the sample body, though that figure too rests on an estimated body fat percentage.

The cut table holds lean mass CONSTANT and gives the weight at each target body fat: weight = lean mass ÷ (1 − target). For the sample body, reaching 15% means 75.0 kg and reaching 10% means 70.9 kg. It assumes every kilo lost comes from fat with no muscle loss — in practice, the deeper the deficit, the more optimistic that assumption becomes.

How to read your result

Look at the source of the body fat figure first, then at the number. An FFMI computed from an estimate carries the estimate's error, and that error is not random but DIRECTIONAL: it runs systematically low in muscular bodies. If you have not had it measured, read the number here as a floor.

Use the normalised value, not the raw one. Both sit on the page, but the normalised one is the only comparable version, so if another site gave you a very different number, that site is most likely printing the raw value without ever saying which of the two it is. Two numbers, one name.

Read BMI and FFMI together. The sample body reads BMI 25.9, which falls in the "overweight" band, yet its FFMI is 20.3 — above average for muscle. The two numbers do not contradict each other: one measures the size of the weight, the other what is inside it. This is exactly why BMI misjudges muscular people.

Read the cut table as arithmetic, not as a promise. It rests on lean mass staying constant, and that assumption only holds approximately when the deficit is moderate, protein is adequate and resistance training continues. The deeper the deficit, the more of the lost weight comes from muscle — and the weight in the table then arrives with a softer body than you expected. The number holds; the mirror does not.

Read the gap to the natural ceiling as a scale, not a schedule. Lean mass rises by a few kilos in the first year of training and the rate drops noticeably in later years. The 15 kg gap for the sample body is not a target but a distance measured in years — and it usually shrinks once an estimated body fat figure is replaced with a measured one.