Your Weight Is Two Numbers, Not One
Step on a scale and you get one number — but your body is really two: fat mass and lean body mass (everything else: muscle, bone, organs, skin and water). Two people can weigh 80 kg with completely different health profiles depending on that split.
Lean body mass (LBM) drives most of your resting calorie burn, protects you in illness, and is the tissue you want to preserve while losing fat. This calculator estimates yours with three published formulas — Boer, James and Hume — and, if you know your body fat percentage, the more accurate direct method.
Boer · James · Hume · body-fat method
? What is lean body mass, and which formula should I trust?
Lean body mass (LBM) is everything you weigh that is not fat: muscle, bone, organs, skin and water. It drives most of your resting calorie burn and is the tissue you want to keep (or grow) while losing fat.
Boer is the most validated against DEXA scans and is used in clinical settings. James and Hume are older alternatives that give slightly different numbers. If you know your body fat percentage from a measurement, the direct method (weight × (1 − body fat%)) is the most trustworthy — trust it over any formula.
Please check your entries: weight and height within realistic ranges.
Estimates for typical adult body sizes — not medical advice. Formulas lose accuracy at very high or low body weights. Direct measurement (DEXA scan) remains the gold standard; if you have a measured body fat value, prefer the direct method shown above.
How to Use the Lean Body Mass Calculator
- Select sex — all three formulas differ by sex.
- Enter weight and height (kg/cm or lb/in).
- Optionally enter body fat % if you have a measurement (calipers, smart scale, DEXA).
- Click Calculate My Lean Body Mass.
- Compare the formulas. If you entered body fat %, prefer the direct method — it beats every formula.
The Three LBM Formulas
Men: LBM = 0.407×W + 0.267×H − 19.2
Women: LBM = 0.252×W + 0.473×H − 48.3
James: men 1.10×W − 128×(W/H)² | women 1.07×W − 148×(W/H)²
Hume: men 0.32810×W + 0.33929×H − 29.5336 | women 0.29569×W + 0.41813×H − 43.2933
Direct: LBM = W × (1 − body fat%)
W = weight (kg), H = height (cm)
Worked example: man, 80 kg, 180 cm. Boer: 0.407×80 + 0.267×180 − 19.2 = 32.56 + 48.06 − 19.2 = 61.4 kg — about 77% of total weight, an athletic range.
Typical Lean Mass Percentages
| Range | Men | Women |
|---|---|---|
| Athletic | 80–85%+ | 75–80%+ |
| Fit / average active | 75–80% | 70–75% |
| Average | 70–75% | 65–70% |
| Below average | Under 70% | Under 65% |
These are rough guides from formula estimates, not clinical cutoffs.
Why Track Lean Body Mass Instead of Just Weight
Dieters use LBM to check that weight loss is coming from fat, not muscle — if LBM drops, protein or training needs fixing. Lifters track LBM gains as the true measure of muscle-building progress. Adults over 40 watch LBM because muscle declines 3–8% per decade after 30 unless trained against.
Remember: lean mass is not the same as muscle mass. Skeletal muscle is roughly 40–50% of LBM — the rest is bone, organs, water and skin.
All Three Formulas, One Person: Why the Answers Differ
Run the same man — 80 kg, 180 cm — through all three formulas and you get three different answers. None of them is a mistake; each equation was fitted to a different study population in a different decade, so each carries that population’s average build.
| Formula | Lean body mass | Implied body fat |
|---|---|---|
| James (1976) | 62.7 kg | 21.6% |
| Boer (1984) | 61.4 kg | 23.2% |
| Hume (1966) | 57.8 kg | 27.8% |
The spread is about 5 kg on the same body. Treat any single formula result as the middle of a range, not a measurement — which is exactly why this calculator shows all three, and why a measured body fat percentage beats every one of them.
FFMI: Lean Mass Adjusted for Height
The fat-free mass index (FFMI) puts lean mass on the same footing BMI puts total weight: FFMI = LBM ÷ height², with height in meters. Our example man’s Boer estimate gives 61.4 ÷ 1.80² = an FFMI of about 19.0. For context, an average untrained man scores around 19, a well-trained lifter 20–22, and values near 25 are considered the practical ceiling without performance-enhancing drugs. Women run lower — roughly 15 for an average build and around 22 at the trained extreme. FFMI is the fairest way to compare muscularity between people of different heights.
Frequently Asked Questions
What is lean body mass?
Lean body mass (LBM) is your total weight minus fat mass. It includes muscle, bone, organs, skin and water — everything that is not fat.
How is lean body mass calculated?
The Boer formula is most validated: men LBM = 0.407×weight(kg) + 0.267×height(cm) − 19.2; women LBM = 0.252×weight(kg) + 0.473×height(cm) − 48.3. If body fat % is known, LBM = weight × (1 − body fat%) is more accurate.
What is the difference between lean body mass and muscle mass?
Lean body mass includes ALL non-fat tissue (muscle, bone, organs, water). Muscle mass — specifically skeletal muscle you train — is only about 40–50% of lean body mass.
What is a good lean body mass percentage?
Roughly 75–85% for men and 70–80% for women indicates an athletic-to-fit range from formula estimates. Higher generally means more muscle relative to fat.
Which LBM formula is most accurate?
The Boer formula tracks DEXA scans best across normal body sizes and is used clinically. The James and Hume formulas are older alternatives that give slightly different values.
Can I increase my lean body mass?
Yes — progressive resistance training plus adequate protein (roughly 1.6–2.2 g/kg/day) builds skeletal muscle, the changeable part of lean mass. Bone and organ mass barely change.
What is FFMI and what is a good FFMI?
FFMI (fat-free mass index) is lean body mass divided by height in meters squared — a height-adjusted muscularity score. Around 19 is average for men, 20–22 indicates a well-trained build, and ~25 is the commonly cited natural ceiling. Women average around 15, with ~22 at the trained extreme.
Does lean body mass include water?
Yes. Total body water is roughly 50–60% of an adult’s weight, and all of it sits in non-fat tissue — muscle cells, organs and blood — so it counts as lean mass. That is why hydration shifts LBM readings by a kilogram or two, and why consistent morning measurements track better than random ones.
Final Thoughts
Weight tells you how much you are; lean body mass tells you what you are made of. Estimate it with the Boer formula, verify it against a body fat measurement when you can, and track the trend over months — that trend is the truest progress metric you own.
LBM Known? Complete the Picture
See your body fat percentage and ideal weight range next.