BODY COMPOSITION
Lean Body Mass Calculator
Estimate lean body mass using Boer, James, Hume, and Peters formulas from height and weight.
Result
Formula comparison
- Boer
- 60.9 kg(23.9% fat)
- James
- 62.1 kg(22.3% fat)
- Hume
- 57.1 kg(28.6% fat)
- Peters
- 39.9 kg(50.1% fat)
Estimated Body Composition
Average lean body mass and estimated fat mass together explain total bodyweight.
How to use it
- Enter total body weight and body fat percentage. If you do not know your body fat %, run the Body Fat Percentage Calculator first.
- LBM = total weight minus fat mass. A 185 lb person at 20% body fat has 148 lbs of lean mass. This is your metabolically active tissue — it drives BMR, protein needs, and strength potential.
- Use LBM × 1.0–1.2 for daily protein target during a fat loss phase. This is more accurate than using total body weight because it accounts for the actual muscle mass at risk of catabolism.
- During a successful cut, LBM should decrease by no more than 20–25% of total weight lost. Losing 10 lbs while maintaining 7.5+ lbs of LBM indicates effective fat-focused cutting. Larger LBM losses signal too steep a deficit or insufficient protein.
- Re-run monthly and compare LBM across months: rising LBM while weight holds or drops = muscle gain or successful recomposition. Stable LBM while body fat drops = textbook fat loss.
Questions people usually ask
What is lean body mass?
Lean body mass (LBM) is your total body weight minus fat mass. It includes muscle, bone, organs, water, and connective tissue — everything except stored body fat.
Which formula should I trust most?
All four formulas (Boer, James, Hume, Peters) are validated estimates. The average provides a reasonable middle ground. For precision, use DEXA or hydrostatic weighing.
Why do results differ between formulas?
Each formula was derived from different study populations and uses different regression coefficients. Variation of 1-3 kg between formulas is normal.
How is this different from body fat percentage calculators?
Body fat calculators typically require circumference measurements (Navy method). LBM calculators estimate fat-free mass from height and weight only — simpler inputs but less precise.
Is this tool free and private to use?
Yes. AI Fit Hub tools are free, no-signup browser tools. Inputs stay in your browser unless you choose to share a URL.
Formula
See the math
The underlying formula with variables defined, derivation shown, and a worked example with units.
Related Resources
Learn the decision before you act
Every link here is tied directly to Lean Body Mass Calculator. Use the explanation, formula, examples, and benchmarks to pressure-test the calculator output from first principles.
How To Use
5 STEPSHow to Use Lean Body Mass Calculator
Calculate your lean body mass with our guide. Learn how to use the LBM calculator to track progress, optimize training, and set realistic body composition.
ReadGuide
6 MIN READHow to Measure Body Fat at Home: 4 Methods Compared
How to measure body fat at Home: 4 Methods Compared: compare the Navy tape method, smart scales, calipers, and visual estimation for home body fat measurement.
ReadFormula
4 VARIABLESKatch-McArdle Formula
Katch-McArdle BMR = 370 + 21.6·LBM_kg. Uses lean body mass instead of total mass — more accurate when reliable body-fat data is available.
ReadFormula
5 VARIABLESNatural Muscle Gain Potential Formula
Natural max FFMI ≈ 25 (men), 21 (women). Helms 2018 + Kouri 1995. Muscle gain rate: 0.5-1.0% bodyweight/month novice, 0.25-0.5% intermediate, <0.1% near ceiling.
ReadFormula
5 VARIABLESBody Fat Percentage Formula (US Navy)
US Navy circumference-based body fat. Men: 86.010·log10(waist−neck) − 70.041·log10(height) + 36.76. Women: 163.205·log10(waist+hip−neck) − 97.684·log10(height) − 78.387.
ReadFormula
3 VARIABLESFFMI Formula (Kouri-adjusted)
FFMI = LBM_kg / (height_m)² + 6.1·(1.8 − height_m). The +6.1·(1.8−h) is Kouri's normalization to a 1.8 m reference. Natural ceiling ~25 (male), ~22 (female).
ReadRelated deep dive
All articles →Read further
Long-form context behind the calculator output.
- Pillar·11 min
Body Composition: Bod Pod vs DEXA vs Calipers
Body composition for athletes: the sport-specific lean-mass and body-fat targets the literature actually supports, and the measurement-method error bands.
Read - Pillar·12 min
FFMI Limits: What the Natty-Boundary Data Shows
The FFMI 25 ceiling is statistically informed but soft. Four confounders flip an honest natty into looks-juicy, and trajectory beats snapshot.
Read - Article·7 min
How to Measure Body Fat at Home
Measure body fat at home: skinfold calipers, bioelectrical impedance, Navy method, and DEXA reality-check — accuracy, error bands, and method choice.
Read
Continue With Related Tools
Estimate body fat percentage with the U.S. Navy circumference method.
Open →TDEE calculator: estimate your total daily energy expenditure using the Mifflin-St Jeor equation and activity multipliers.
Open →Macro calculator: convert your calorie target into daily protein, carbs, and fat grams based on your goal and split preference.
Open →Estimate required calorie deficit and timeline pacing for a bodyweight change target.
Open →