If you know your body fat, skip the formulas
A measured body fat percentage gives the most accurate lean mass. Someone weighing 80 kg at 20% body fat has 80 × 0.80 = 64 kg of lean mass and 16 kg of fat. The Boer, James and Hume formulas below are estimates for when you only have height, weight and sex.
The three estimation formulas
Each uses weight W (kg) and height H (cm). They were derived from different populations, so they disagree by a kilogram or two for the same person.
| Formula | Men | Women |
|---|---|---|
| Boer | 0.407·W + 0.267·H − 19.2 | 0.252·W + 0.473·H − 48.3 |
| James | 1.1·W − 128·(W/H)² | 1.07·W − 148·(W/H)² |
| Hume | 0.3281·W + 0.3393·H − 29.53 | 0.2957·W + 0.4181·H − 43.29 |
Boer is the clinical default. James factors in the weight-to-height ratio, which helps at the extremes of build.
How far apart the three answers land
For an 80 kg, 180 cm man the formulas give slightly different lean masses. The spread is normal, so treat any single estimate as a ballpark, not a precise figure.
Same man, three formulas: about a 5 kg spread from top to bottom.
Common questions
Is lean body mass the same as muscle?
No. Lean body mass is everything that is not fat: muscle, bone, organs, blood and water. Skeletal muscle is only about 30 to 40% of it in a typical adult.
Which formula should I use?
Boer is the usual default and the most widely validated. If you have a measured body fat percentage from a DXA scan or bioimpedance, use that directly instead, since it beats any height and weight estimate.
Why does lean body mass matter for drug dosing?
Many drugs distribute through water-rich tissue, not fat. Dosing a very heavy patient by total weight can overshoot the right amount by 20 to 30%, so anesthesia and chemotherapy doses are often scaled to lean mass instead.
