Lean Body Mass vs Fat-Free Mass
The two terms are used interchangeably — the only difference is a small pool of essential fat. See the distinction and calculate your lean body mass.
Lean Body Mass Calculator
Required: the lean-mass formulas use biological sex (male or female) as a variable.
Range: 30–300 kg
Enter your height in centimeters (e.g., 170 cm)
Used only to flag whether the result needs an under-19 caveat — it does not change the formula. Leave blank if you'd rather not say.
Know your body fat %? Enter it for the most accurate result (lean mass = weight × (1 − body fat%)). Leave it blank to use the height & weight formulas. Don't know it? Estimate it with the Body Fat % Calculator.
The average of the Boer, James and Hume estimates — the most balanced default. A measured body fat % above always overrides the formula estimate.
*This calculator is for informational purposes only. Please consult a healthcare professional before making any health decisions. See our medical disclaimer for more information.
Quick Answer
Lean Body Mass vs Fat-Free Mass
The two terms are used interchangeably, and the difference is small. Fat-free mass (FFM) excludes all fat. Lean body mass (LBM) includes a small amount of essential fat — roughly 2–3% of body weight — stored in the bone marrow, nervous system and organs. So lean body mass is very slightly larger than fat-free mass, and this calculator (like most) reports lean body mass: your weight minus your fat mass.
The Distinction, Precisely
Body fat comes in two kinds. Most of it is storage fat — the visible, expendable fat under the skin and around the organs that a diet is designed to reduce. A much smaller amount is essential fat, built into the structure of your bone marrow, brain, nerves and internal organs, where it does a job rather than sitting in reserve. The two definitions differ only over that essential fat. Fat-free mass is strict: it subtracts every gram of fat of both kinds, leaving pure muscle, bone, organ tissue and water. Lean body mass is slightly more generous: it keeps that small essential-fat fraction bundled in with the lean tissue. Everything else — all the muscle, all the bone, all the water — is identical between them. That is the entire technical difference, and it is why the terms are used as synonyms almost everywhere outside a research lab.
The Arithmetic of the Gap
Because the difference is that small pool of essential fat — commonly put at around 2 to 3% of body weight — the numbers barely move. Take an 80 kg person: essential fat of roughly 3% is about 2.4 kg. So their lean body mass and their fat-free mass differ by only about that much, out of a lean compartment that might be 60 kg or more. Expressed as a share of the lean figure, that is a difference of a few percent at most, and it is roughly constant from one measurement to the next. This is exactly why treating the two as the same number causes no practical trouble: if you enter a measured body fat percentage above, the tool computes weight minus fat mass and reports it as lean body mass, and you can read that as your fat-free mass too without recalculating anything.
Which One This Suite Reports
Every tool in this suite reports lean body mass — your weight minus your fat mass — which for all practical purposes doubles as your fat-free mass. Enter a measured body fat percentage above for the exact figure; without one, the calculator estimates lean mass from your height, weight and sex. To get that body fat reading in the first place, the Body Fat % Calculator uses the US Navy circumference method, and once you know your lean mass the Katch-McArdle Calculator turns it into a metabolism estimate that works from lean tissue rather than total weight. The full breakdown across formulas lives on the Lean Body Mass Calculator.
Lean body mass is computed as weight minus fat mass from a measured body fat %, or estimated with the Boer (1984), James (1976) and Hume (1966) formulas. The essential-fat difference between LBM and FFM is small and definitional. These figures are general estimates for education, not medical advice.
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