Everything you weigh that is not fat. Three published formulas, and they disagree by more than you would expect.
Lean body mass
Boer formula — the three formulas span 57.8 to 62.7 kg
James (1976)62.7 kg
Hume (1966)57.8 kg
From your body fat %65.6 kg
Each lean body mass formula and what it returns
Formula
Equation
Lean mass
Boer (1984)
0.407w + 0.267h − 19.2
61.4 kg
James (1976)
1.1w − 128(w/h)²
62.7 kg
Hume (1966)
0.32810w + 0.33929h − 29.5336
57.8 kg
From measured body fat
w × (1 − bf%)
65.6 kg
The three regressions were fitted on different populations decades apart. The gap between them is the estimate's own error bar.
This calculator needs JavaScript to recalculate. The answer shown above was worked out when the page was built and is correct for the date in the address bar.
Last checked
Worked example: man, 80 kg, 180 cm, body fat measured at 18%
Method
Equation
Lean body mass
Hume (1966)
0.32810w + 0.33929h − 29.5336
57.8 kg
Boer (1984)
0.407w + 0.267h − 19.2
61.4 kg
James (1976)
1.1w − 128(w/h)²
62.7 kg
From measured body fat
w × (1 − bf%)
65.6 kg
The three height-and-weight formulas span 4.9 kg — about 8%. And the measured-body-fat figure sits 3 kg above all of them. That gap is the entire point of this page.
Why the formulas disagree so much
Boer, James and Hume all take only height, weight and sex. That means they cannot distinguish an 80 kg lifter at 12% body fat from an 80 kg untrained person at 30% — they return the same lean mass for both, which is obviously wrong for at least one of them. They are estimating what an average person of your height and weight would carry, and they were fitted on populations from 1966, 1976 and 1984 respectively, which were leaner on average than the population today. That is why they tend to read low for trained lifters.
If you have a genuine body-fat measurement, use it. Lean mass is then simply weight × (1 − body fat), which is not an estimate at all beyond the accuracy of the body-fat measurement itself. The three formulas exist for the case where you have no measurement, and in that case Boer is the usual default.
What lean body mass is used for
Three things on this site, all downstream. Katch-McArdle calculates basal metabolic rate from lean mass rather than total weight, which makes it the better calorie formula for people who are notably lean or notably heavy. FFMI is lean mass normalised for height. And protein targets are more defensible per kilogram of lean mass than per kilogram of total bodyweight, particularly at higher body-fat percentages.
Methodology
Boer (1984), James (1976) and Hume (1966), all in metric, with separate coefficients for men and women. If a body-fat percentage is supplied, lean mass is also reported directly as weight × (1 − body fat), which is the figure we recommend using when the measurement is real.
Limitations, stated plainly
The three height-and-weight formulas cannot see body composition, so they are least accurate exactly where it matters most: on trained lifters, on very lean people and on people carrying a lot of fat. The 8% spread in the example above is typical, not unusual. The direct calculation is only as good as the body-fat measurement behind it, and consumer bio-impedance scales are frequently out by 5 percentage points or more — which on an 80 kg person is 4 kg of lean mass.
This is a fitness estimate, not medical advice, and not a substitute for a professional who can actually see you.
Common questions
Which lean body mass formula should I use?
If you have a measured body-fat percentage, none of them — use weight × (1 − body fat), which is more direct. If you do not, Boer is the usual default and sits between the other two. Hume runs lowest and James highest, and the spread between them on a typical adult is around 8%.
Why is the number from my body-fat percentage higher than the formulas?
Because the formulas were fitted on general populations from the 1960s to the 1980s and assume an average body composition for your height and weight. If you carry more muscle than that average, they will read low. If your measured body fat is genuinely 18% at 80 kg, your lean mass really is about 65.6 kg and the formulas are under-estimating you. The reverse happens if you carry more fat than average.
Is lean body mass the same as muscle mass?
No, and this trips people up. Lean body mass is everything that is not fat — muscle, bone, organs, connective tissue and the water in all of them. Skeletal muscle is roughly half of it. So a 62 kg lean mass does not mean 62 kg of muscle, and a 2 kg gain in lean mass is not 2 kg of muscle either; some of it is water and glycogen.
How accurate are bathroom scales that measure body fat?
Poor for a single reading, better for tracking a trend. Bio-impedance sends a small current through your body and infers composition from the resistance, which is heavily affected by hydration, when you last ate, and how warm your feet are. Errors of five percentage points either way are common. They are more useful for direction of travel over a month than for the number on any given morning.
Should I calculate protein from lean mass or total weight?
Lean mass is the more defensible basis, particularly at higher body-fat percentages, because fat tissue has essentially no protein requirement. At moderate body fat the two approaches produce similar numbers and it does not much matter. See the protein intake calculator, which lets you pick either.