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Lean Body Mass Calculator

Estimate lean body mass (LBM) using Boer, Hume, James, and Janmahasatian formulas with side-by-side comparison.

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About this tool

The Lean Body Mass Calculator estimates the weight of your fat-free tissue — muscle, bone, organs, water — using four widely cited clinical equations. Boer (1984) and Hume (1966) are the most cited general-population formulas using weight, height, and sex. James (1976) is an older WHO-cited equation that tends to give slightly higher estimates. Janmahasatian (2005) is BMI-aware and preferred when BMI is elevated. If you know your body fat percentage (e.g. from the Body Fat Navy tool), the calculator also shows a more accurate direct LBM: weight × (1 − BF%/100). LBM is useful for setting protein targets and tracking muscle changes during recomposition. This calculator is for educational purposes only and is not a substitute for professional medical advice. Always consult a qualified healthcare provider before making decisions about your health.

Why use it

  • Four validated clinical formulas — Boer, Hume, James, Janmahasatian — calculated in one place.

  • Direct method using body fat % when measured input is available, which is the most accurate.

  • BMI-aware Janmahasatian equation handles obese subjects more reliably than purely linear formulas.

  • Side-by-side comparison shows how much LBM estimates vary across methodologies.

  • Useful for setting evidence-based protein targets (1.6-2.2 g/kg of lean mass).

  • Privacy-first: anthropometric data never leaves your browser.

How to use

  1. Choose your unit system (kg/cm or lb/in).
  2. Select biological sex — all four formulas have sex-specific coefficients.
  3. Enter your current weight and height accurately.
  4. Optionally enter a measured body fat % to get a direct, more accurate LBM number.
  5. Compare the four formula results side by side to see the typical 1-3 kg variance between equations.
  6. Use the formula average for general planning, or the direct method when body fat % is known.

When it helps

  • Setting a daily protein target based on lean mass rather than total weight.
  • Tracking muscle preservation during a fat-loss phase.
  • Estimating drug dosing for medications that scale by lean mass (e.g., anesthesia).
  • Comparing predictive LBM with a DEXA, BIA, or caliper measurement.
  • Setting a realistic 'lean mass target' before starting a recomposition program.
  • Assessing sarcopenia risk in older adults (LBM declines ~1% per year after 30).

Examples

30-year-old male, 80 kg, 180 cm, no BF%

Input

Sex: male, Weight 80 kg, Height 180 cm

Expected result

Boer ≈ 61.7 kg; Hume ≈ 57.8 kg; James ≈ 64.0 kg; Janmahasatian ≈ 61.8 kg; average ≈ 61.3 kg

28-year-old female, 65 kg, 165 cm, BF% 24

Input

Sex: female, Weight 65 kg, Height 165 cm, BF 24%

Expected result

Direct LBM = 65 × 0.76 = 49.4 kg (most accurate); formula average ≈ 47.9 kg

45-year-old male, 110 kg (obese), 178 cm

Input

Sex: male, Weight 110 kg, Height 178 cm

Expected result

Boer ≈ 73.3 kg; Janmahasatian ≈ 71.5 kg (preferred at high BMI); Hume ≈ 71.6 kg

Tips

  • When BMI is over 30, weight the Janmahasatian estimate higher than Boer or Hume — it was specifically derived for obese subjects.
  • Pair this tool with the Body Fat (Navy) calculator to get a measured BF% for the direct LBM method.
  • Use LBM (not total weight) to anchor protein targets — typically 1.6 to 2.2 g per kg LBM.
  • Track LBM trends over months, not weeks — formula noise is roughly ±1 kg.
  • If your goal is muscle gain, expect realistic LBM gains of 0.25-0.5 kg/month for trained lifters.

Frequently Asked Questions

How accurate is this calculator?
Predictive formulas are typically within ±2-3 kg of DEXA-measured LBM for adults of normal-to-overweight BMI. The direct method using a measured body fat % is the most accurate.
Should I rely on this without a doctor?
For everyday fitness goals, yes — but for clinical contexts such as drug dosing, malnutrition assessment, or sarcopenia diagnosis, work with a qualified clinician using DEXA or BIA.
Are my measurements stored anywhere?
No. Calculations run entirely in your browser. No height, weight, or body fat data is transmitted to any server.
Which formula should I trust most?
For BMI under 30, Boer is the most commonly cited general-population formula. For BMI over 30, Janmahasatian was specifically derived for obese subjects and is more accurate. The direct method (weight × (1 − BF%/100)) is best when BF% is measured.
Can I use LBM to set my protein target?
Yes — current sports nutrition research supports 1.6-2.2 g protein per kg of lean body mass for muscle preservation and growth, particularly during fat loss.
Why do the four formulas disagree?
Each was derived from a different population sample (military, hospital, elderly, etc.) and uses different inputs. Disagreement of 1-3 kg is normal — the average is a reasonable consensus number.
Does LBM include water weight?
Yes. Lean body mass includes all non-fat tissue: muscle, bone, organs, blood, and intracellular and extracellular water — typically ~73% of LBM is water.
How often will my LBM change?
For trained lifters in caloric surplus, 0.25-0.5 kg/month of true LBM gain is realistic. For older adults, ~1% per year of LBM is naturally lost without resistance training.

Explore the category

Body Fat % Calculator (Navy Method) Estimate body fat percentage using the US Navy circumference measurement method for men and women. BMI Calculator Body mass index calculator Ideal Weight Calculator Calculate ideal body weight using five established medical formulas (Hamwi, Devine, Robinson, Miller, BMI-based) for any height. TDEE Calculator Calculate your Total Daily Energy Expenditure (TDEE) based on weight, height, age, sex, and activity level using the Mifflin-St Jeor equation. Macro Calculator Calculate daily protein, carbohydrate, and fat targets based on your TDEE, goal (cut/bulk/maintain), and diet preference. Heart Rate Zone Calculator (Karvonen) Calculate personalized heart rate training zones using the Karvonen formula based on resting and maximum heart rate.

Glossary

Lean mass vs fat mass
Lean mass is everything in the body that is not fat — muscle, bone, organs, and water. Fat mass is total adipose tissue. Total weight = lean + fat.
Bioimpedance
Bioelectrical impedance analysis (BIA) sends a tiny electrical current through the body and estimates lean mass from the electrical resistance; varies with hydration.
DEXA scan
Dual-energy X-ray absorptiometry — gold-standard imaging method for body composition; distinguishes bone, lean mass, and fat mass with sub-1% precision.
Body composition
Distribution of fat, lean mass, bone, and water — a more meaningful health metric than scale weight alone.
BMI vs BFP
BMI (weight ÷ height²) ignores composition; BFP (body fat percentage) is fat mass ÷ total mass and reflects composition directly.
Sarcopenia
Age-related loss of muscle mass and strength; LBM tracking helps detect it early in older adults.