Lean Body Mass Calculator
Lean body mass (LBM) is everything in your body except fat: muscle, bone, organs and water. Enter your height, weight and sex to estimate your lean mass, fat mass and lean percentage. Switch between the Boer, James and Hume formulas, or enter a measured body fat percentage for a direct split, in metric or imperial.
Formula
Worked example
A 75 kg, 178 cm man (Boer): 0.407 × 75 + 0.267 × 178 − 19.2 = 30.5 + 47.5 − 19.2 ≈ 58.8 kg lean, leaving about 16.2 kg of fat. The James and Hume formulas give roughly 60.7 kg and 60.4 kg for the same person, so the choice of formula moves the answer by a kilogram or two.
What lean body mass means
Lean body mass is your total weight minus your body fat. It includes muscle, bone, connective tissue, organs and the water in your body. Tracking lean mass is useful because it is the metabolically active part of you, it burns most of your calories at rest, and because preserving it during weight loss is a key goal for long-term health and strength. Lean mass usually sits between about 60% and 90% of total body weight.
Boer, James and Hume formulas
When you do not have a measured body fat reading, this calculator estimates lean mass from height, weight and sex using one of three published equations. The Boer formula (1984) is the most widely cited and is the default. The James formula (1976) uses the ratio of weight to height squared, which tends to behave differently at the extremes. The Hume formula (1966) is another classic clinical equation. All three return slightly different numbers because they were fitted to different populations, so the calculator shows all three side by side when you use the Boer method, letting you see the spread rather than trusting a single figure.
Using a measured body fat percentage
If you have a body fat percentage from a DEXA scan, a bioimpedance scale, hydrostatic weighing or skinfold calipers, switch the method to "from measured body fat %". The calculator then splits your weight directly: lean mass equals weight times one minus the fat fraction, and fat mass is the remainder. This is more personal than any height and weight formula because it reflects your actual body composition rather than a population average, so it is the preferred input when you have a reliable reading.
Why lean mass matters for dosing and nutrition
Several practical figures scale with lean mass rather than total weight. Some medication doses are calculated on lean body mass to avoid over-dosing based on fat tissue, and protein needs are sometimes set per kilogram of lean mass. Knowing your approximate lean mass helps put those guidelines into context, though clinical decisions should always use measured values rather than a formula estimate.
Lean body mass formulas at a glance
| Formula | Men | Women |
|---|---|---|
| Boer (1984) | 0.407W + 0.267H − 19.2 | 0.252W + 0.473H − 48.3 |
| James (1976) | 1.1W − 128(W/H)² | 1.07W − 148(W/H)² |
| Hume (1966) | 0.32810W + 0.33929H − 29.5336 | 0.29569W + 0.41813H − 43.2933 |
| Body fat % | W × (1 − fat%/100) | W × (1 − fat%/100) |
W = weight in kg, H = height in cm. All return lean mass in kg.
Frequently asked questions
How is lean body mass different from muscle mass?
Lean body mass is broader: it includes muscle plus bone, organs and water. Muscle mass is just the skeletal muscle portion, which is typically around half of lean body mass.
Which formula should I use, Boer, James or Hume?
Boer is the most widely used default and works well for most adults. James can read differently for very lean or very heavy people because it uses weight over height squared. Hume is another established clinical equation. This calculator shows all three so you can see the range; if you have a measured body fat percentage, that method is more accurate than any of them.
How accurate are these formulas?
They are well-regarded population estimates, but they predict an average for your height and weight rather than measuring your body. People who are very muscular or carry unusual fat distributions may differ. For a precise measurement, methods like DEXA scanning, hydrostatic weighing or bioelectrical impedance measure body composition directly.
Can I calculate lean mass from my body fat percentage?
Yes. Switch the method to "from measured body fat %" and enter your reading. Lean mass equals your weight multiplied by one minus the fat fraction, and fat mass is the rest. This reflects your actual body composition, so it is preferred when you have a reliable measurement.
Should drug doses use lean body mass?
Some are. Certain medications are dosed on lean body mass to avoid overestimating from fat tissue. This is a clinical decision made with measured values; use this estimate for general understanding only.