Estimate your lean (fat-free) body mass three ways — using the Boer, James, and Hume formulas — and see your fat mass and FFMI.
Results update live as you type.
Average Lean Body Mass
Total weight
180 lbs
on the scale
Lean mass
141 lbs
~78%
Fat mass
39 lbs
~22%
FFMI
21.0
fat-free mass index
| Formula | Lean Mass | Notes |
|---|---|---|
| Boer (1984) | — | Most widely used |
| James (1976) | — | Adjusts for obesity |
| Hume (1966) | — | Original clinical eq. |
| Average | — | Used as the headline |
The Formulas
Lean Body Mass (LBM) is your total body weight minus fat — bones, muscle, organs, blood, and water. Because it cannot be measured directly without a DEXA scan or hydrostatic test, clinicians use validated equations. We compute three of the most widely cited — Boer, James, and Hume — and report the average as your headline result.
Boer · James · Hume (male)
About This Tool
A lean body mass calculator — also called a fat-free mass calculator or LBM estimator — works out how much of your total body weight is everything except fat: skeletal muscle, bones, organs, connective tissue, and the water inside them. Lean body mass is what most lifters and athletes actually want to track, since it is muscle and bone that drive performance, not the total number on the scale.
Because true LBM can only be measured with a DEXA scan, BodPod, or hydrostatic weighing, clinicians rely on predictive equations based on height, weight, sex, and sometimes age. The three most respected — Boer (1984), James (1976), and Hume (1966) — give slightly different numbers because they were derived from different reference populations. Reporting all three plus their average produces a more honest picture than any single formula alone.
Subtracting LBM from total weight gives you an estimate of fat mass. From that we calculate FFMI (Fat-Free Mass Index), which normalises lean mass by height² — a useful number for comparing muscularity between people of different sizes (a natty trained male caps around FFMI 25; FFMI 22 is solid; sub-19 is below average).
This free online LBM calculator runs entirely in your browser. Use it to set protein targets, track muscle preservation during a cut, or compare yourself against population norms. For absolute precision, get a DEXA scan once a year and trust the equations between scans.
3 Validated Formulas
Boer, James, and Hume — the most widely cited LBM equations.
Metric & Imperial
Toggle between kg/cm and lbs/ft+in with auto-conversion.
Body Composition
Donut chart shows lean-mass vs fat-mass split visually.
FFMI Included
Fat-Free Mass Index normalises lean mass by height for fair comparisons.
Live Comparison Table
See all three formula results side by side — never one number alone.
100% Private
All math runs locally — no sign-up, no tracking, no data stored.
Four quick inputs give you a complete body-composition estimate.
Choose US / Imperial (lbs & ft/in) or Metric (kg & cm). The math is unit-aware.
Each formula has separate male / female constants. Choose your biological sex and enter age.
Imperial uses feet + inches; metric uses centimetres. Measure shoes off, against a wall.
Weigh yourself first thing in the morning, after bathroom, before food. Drag the slider to model targets.
The big number is your average lean body mass. The donut shows fat vs lean split.
The table shows Boer, James, and Hume side by side. Pick one as your tracking number and stay consistent.
Everything you need to know about LBM, FFMI, and the equations behind them.
Lean body mass (LBM) is your total body weight minus fat. It includes muscle, bone, organs, connective tissue, blood, and the water held inside them. Roughly 50–60% of LBM in a typical adult is skeletal muscle. Because muscle and bone (not fat) determine strength, metabolic rate, and physical performance, LBM is the number coaches, lifters, and clinicians actually care about.
Predictive equations are typically within ±5–10% of DEXA-measured LBM for an adult of average build. They are less accurate at the extremes — very lean elite athletes, very obese individuals, and people with unusually short or long limbs. For long-term tracking the consistency of the estimate matters more than absolute accuracy: use the same formula each time and watch the trend.
Boer (1984) is the most widely cited and tends to be most accurate for adults of average build. James (1976) uses a quadratic weight-to-height term and is slightly better for higher-BMI individuals. Hume (1966) is the original clinical equation and is still used for medical dosing calculations. We report all three plus their average so you can see the spread; for personal tracking pick one and stay with it.
Fat-Free Mass Index = lean mass (kg) ÷ height (m)². It normalises lean mass by height the same way BMI normalises total weight. Rough male reference: 16–17 below average, 18–20 average, 21–22 well-trained, 23–25 very muscular, 25+ unusual for natural athletes. Female norms run about 3 points lower. FFMI is a sensible muscularity score that doesn't punish tall people the way raw lean-mass numbers do.
Yes — that's exactly what these equations are for. They use only height, weight, age, and sex to estimate LBM without needing a body-fat measurement. If you already know your body fat percentage from a DEXA scan or calipers, you can calculate LBM more accurately as: LBM = weight × (1 − BF%/100).
Because each formula was derived from a different reference population (different decade, country, age range, and measurement technology). Their predictions differ by 1–4 kg for most people. Showing all three is more honest than picking one and pretending it's "the answer". The average is a reasonable single value if you need one.
No. LBM = muscle + bone + organs + connective tissue + water. Skeletal muscle mass (SMM) is just the muscle, typically 40–50% of LBM in adult men and 35–45% in women. Most calculators (including DEXA) report total LBM, not isolated muscle mass — distinguishing the two requires more sophisticated multi-compartment analysis.
For natural lifters: a male beginner can add roughly 9–11 kg (20–25 lb) of LBM in year 1, an intermediate 4–5 kg in year 2, and an advanced lifter 1–2 kg/year thereafter. Women progress at roughly half those rates. Genetic ceiling is around FFMI 25 natural for men, 22 for women. Beyond that, lean-mass increases come slowly and require precise training and nutrition.
Some LBM loss is unavoidable in a calorie deficit — typically 20–35% of total weight lost will be lean tissue. You can minimise it with adequate protein (1.6–2.2 g/kg of body weight), resistance training 3–5×/week, a moderate deficit (≤25% below TDEE), and adequate sleep. Aggressive cuts and low-protein diets can double the LBM loss percentage.
Many modern nutrition recommendations scale to LBM rather than total weight. Protein: 1.6–2.2 g/kg LBM optimises muscle protein synthesis. BMR equations like Katch-McArdle use LBM directly and are more accurate for very lean or very heavy individuals than weight-based formulas. Hydration: water needs scale with LBM, not total weight, because adipose tissue holds little water.
For most athletes it's reasonable, but the prediction error grows for those at the extremes of body composition. A 100 kg 180 cm bodybuilder with 8% body fat may have his true LBM underestimated by 3–5 kg, because the equations assume an average fat percentage at that height/weight. For high-performance athletes, DEXA or a 4-compartment model is worth the cost.
No. Some drugs (notably chemotherapy and anaesthetics) are dosed by lean body weight using clinically validated equations such as Hume or McCarron. Hospital pharmacy systems use specific approved formulas in a specific clinical context. This calculator is educational — never use it to determine medication doses.