Health
Lean Body Mass Calculator
The three formulas look like they agree. This page shows how far apart they are, and the body-fat range that gap leaves open.
The link carries your figures, so it reopens on exactly these numbers.
The three formulas against the fat range they leave, weight by weight
The estimates track a few kilograms apart at every weight, and the body fat they imply keeps a several-point gap. The three numbers are tidier than the thing they measure.
| Weight | Lean body mass (average of three) | Implied body fat, at the highest lean mass | Implied body fat, at the lowest lean mass |
|---|---|---|---|
| 60 kg | 52.1 | 11.2 | 14.6 |
| 70 kg | 56.5 | 17.7 | 22.1 |
| 80 kgThe default above. Average 60.6 kg lean; implied fat 21.6 to 27.8%. | 60.6 | 21.6 | 27.8 |
| 95 kg | 66.4 | 27.5 | 34 |
| 110 kgHere James has started to bend — its squared term drags the estimate low. | 71.5 | 33.1 | 38.5 |
The formula
Worked example
A man 180 cm tall weighing 80 kg
- Boer: 0.407 × 80 + 0.267 × 180 − 19.2 = 61.4 kg
- James 62.7 kg, Hume 57.8 kg — a spread of 4.9 kg
- the body fat this implies runs from 21.6% to 27.8%
- a 6-point range hiding behind three tidy-looking numbers
Where this goes wrong
Reading three formulas that disagree as one answer
At 180 cm and 80 kg the three estimates run from 57.8 to 62.7 kg — a 4.9 kg spread — and the body fat they imply stretches from 21.6% to 27.8%, a full six points. A single lean-mass figure hides all of that. Worse, James is not a straight line: it subtracts a term in weight-over-height squared, which behaves at an ordinary build but bends the wrong way at a high BMI, where adding weight can push the estimated lean mass down. These formulas were fitted to normal-range bodies and estimate an average composition; for a very muscular or very heavy person they drift, and none of the three can see the difference between muscle and fat. Use the range, and where the number will drive a real decision — a drug dose, a body-composition goal — measure, do not estimate.
Questions
- Which lean body mass formula is most accurate?
- There is no clear winner for an individual. Boer is a linear fit that stays well-behaved and is the one most often reached for; Hume tends to read lowest; James adds a squared term that helps at some builds and misbehaves at high ones. The honest reading is the range they give, not any single one — and for a decision that matters, a measured method beats all three.
- What is the difference between lean body mass and fat-free mass?
- Lean body mass includes the small amount of essential fat inside organs; fat-free mass excludes all fat. The two differ by roughly 2 to 3 percent in men and 5 to 12 percent in women. These formulas estimate lean body mass, and the difference is usually smaller than the disagreement between the formulas themselves.
- Why does the James formula give a strange answer for heavy people?
- Because it subtracts a term proportional to weight divided by height, squared. At a normal build that term is a small correction, but as weight rises it grows faster than the linear part, so past a high BMI the estimate can actually fall as weight increases. That is a real limit of the formula, not a bug in the calculator, and it is why the page shows all three rather than James alone.
- Do these formulas work for children?
- No. Boer, James and Hume were fitted for adults. Children are estimated with a different equation (Peters) because body composition changes throughout growth, and this page uses the adult formulas only.