Why this matters
Your total weight tells you almost nothing useful on its own because it combines muscle, bone, organs, fat, and water into a single number. Two people who weigh 80 kilograms can have vastly different body compositions: one might be 15 percent body fat with significant muscle mass, while the other might be 30 percent body fat with relatively little lean tissue. Lean body mass — the weight of everything in your body except fat — is the number that actually drives your resting metabolic rate, predicts drug clearance rates, and determines how many calories you burn at rest.
This calculator estimates lean body mass using three peer-reviewed formulas — Boer (1984), James (1976), and Hume (1966) — each of which uses your weight, height, and biological sex to produce an estimate. The Boer formula is generally preferred in modern clinical practice because it was derived from a more diverse and rigorous dataset. The calculator also derives your estimated fat mass (total weight minus lean mass) and body fat percentage, giving you a three-part body composition snapshot from just three inputs.
Formula comparison
| Formula | Year | Variables | Clinical preference |
|---|---|---|---|
| Boer | 1984 | Weight, height, sex | Preferred in modern practice |
| James | 1976 | Weight, height, sex | Older, still widely cited |
| Hume | 1966 | Weight, height, sex | Historical reference |
How to use it
Select your biological sex.
Enter your weight and height.
Choose a formula: Boer (recommended), James, or Hume.
Read your lean body mass, estimated fat mass, and body fat percentage.
Testing your result
Run all three formulas on the same inputs and compare the results — you will typically see variations of one to three kilograms between them, which reflects the different regression coefficients each study derived. The Boer result is usually the most reliable for average adults. If you have access to a DEXA scan or bioimpedance measurement, compare it against the calculator's estimate — regression formulas are accurate to within roughly plus or minus three to five kilograms for most people. Track your lean body mass over time by taking measurements monthly under the same conditions (same time of day, same scale) and watching whether the trend moves in the right direction as you train.
Common mistakes
Confusing lean body mass with fat-free mass — LBM includes a small amount of essential fat (about 3 percent in men, 12 percent in women) that fat-free mass excludes.
Using the calculator for competitive bodybuilders or elite athletes, whose body compositions fall outside the range of the original regression datasets.
Interpreting the body fat percentage as a clinical diagnosis rather than an estimate — DEXA or hydrostatic weighing provides definitive measurements.
Edge cases and limitations
The formulas assume an average adult body composition and become less accurate at the extremes — very high body fat percentages, very low body fat percentages, or significant height outside the normal adult range. They were developed primarily on Caucasian populations and may have reduced accuracy for other ethnic groups. Pregnant women and individuals with significant edema or amputations should not use these formulas. The calculator is useful for tracking relative changes over time even if the absolute number has a few kilograms of error, because the error is systematic and cancels out in before-and-after comparisons. All computation is local to your browser.
Real-world use cases
A personal trainer setting a baseline lean mass measurement for a new client and tracking it over a twelve-week program.
A clinical professional estimating lean body mass for drug dosing calculations in chemotherapy or anesthesia.
A fitness enthusiast combining LBM data with a calorie calculator to set a precise maintenance or deficit target.
Frequently asked questions
Q: What is the difference between formulas?
A: Boer (1984), James (1976) and Hume (1966) use slightly different regression coefficients. Boer is generally preferred in modern clinical practice.
Q: Is LBM the same as fat-free mass?
A: Almost. LBM includes a small amount of essential fat (3% in men, 12% in women); fat-free mass excludes all fat. The difference is small for most uses.
Q: How accurate are these formulas?
A: Estimates within plus or minus 3-5 kg for average adults. DEXA, hydrostatic weighing or bioimpedance give better accuracy.
Q: Why does LBM matter?
A: LBM drives resting metabolic rate — more muscle means more calories burned at rest. It is also used to dose medications like chemotherapy.
Q: Are my numbers stored?
A: No — all math is local.
Start using it now
Estimate your lean body mass with the Lean Body Mass Calculator. For complementary body composition tools, try the Body Fat Calculator or the Ideal Weight Calculator. Pair your LBM with a Calorie Calculator to set precise nutrition targets, or check fat distribution patterns with the Waist-to-Hip Ratio Calculator.
