Lean body mass

Lean body mass, fat-free mass and muscle are three things

They get used interchangeably and they are not the same. Fat-free massis everything that is not fat: muscle, bone, organs, water. Lean body massincludes the small amount of essential fat inside the nervous system, bone marrow and organs, so it runs slightly higher — by around 2–3% in men and 5–12% in women.Muscle mass is a subset of both, and considerably smaller than either. None of these equations measures muscle.

Boer, James or Hume — and why they disagree

Boer is the default here and the usual clinical choice, including in heavier patients. Hume is older and behaves similarly.James is the one to watch: it has a non-linear term in weight over height squared, which means it degrades badly at high BMI and can produce lean mass figures that fall as weight rises. The page flags that when your BMI is high enough for it to matter — nobody else does, and it is the most common way these calculators mislead.

All three were fitted for clinical use — drug and contrast dosing — on predominantly Western adult samples. They are estimates of body size, not measurements of your body composition, and nothing here should be used to set a clinical dose.

The better route, when you have it

If you know your body fat percentage, lean mass is just weight multiplied by one minus that fraction. No regression, no population sample, no assumption about what someone your height typically carries. Enter a figure above and the page switches to it and says so.

The catch is that the answer is then only as good as the body fat figure. A bioimpedance scale reading moves with hydration; a tape estimate carries three to four points of error. Both still beat an equation that has never seen you.

FFMI: lean mass against your height

Fat-free mass index is lean mass divided by height in metres squared — BMI's arithmetic applied to the lean part only. It is the sensible way to compare lean mass between people of different heights, which raw kilograms cannot do. The height-normalised version adjusts to a 1.8 m reference so very tall and very short people are not systematically flattered or penalised.

What lean body mass is actually for

Three uses, mostly. It feeds Katch–McArdle, which estimates resting metabolic rate from lean mass rather than from height and weight — theBMR calculator will run it if you give it a body fat figure. It gives a sounder basis for protein targets at high body fat, which theprotein calculator uses. And it is the thing a sensible deficit is trying to preserve, which is the whole argument for keeping protein high and lifting while you lose.

Adults only — do not apply these to children. If accuracy genuinely matters, DXA or another direct method is what you need.