BODY COMPOSITION
Lean Body Mass Calculator
Estimate lean body mass using Boer, James, Hume, and Peters formulas from height and weight.
Try a preset
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.
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