What is Lean Body Mass?
**Lean Body Mass (LBM)** is the weight of everything in your body *except* fat. It includes your skeletal muscle, bones, organs, skin, body water, and blood.
LBM vs. Muscle Mass
Many people use these terms interchangeably, but they are different. LBM includes bone and water. **Skeletal Muscle Mass** is a subset of LBM. However, since your bone mass and organ weight stay relatively constant, changes in LBM usually reflect changes in muscle mass.
The 3 Formulas Explained
Boer Formula (1984)
Generally considered the most accurate formula for individuals with a normal to overweight BMI. It is the primary method used in this calculator.
James Formula (1976)
Widely known but has a mathematical flaw at high BMIs (usually > 35), where it can underestimate LBM or even produce paradoxical results.
Hume Formula (1966)
An older formula often used in clinical settings for drug dosing calculations (e.g., determining chemotherapy dosages).
Why LBM Matters for Diet
Your LBM is the primary driver of your basal metabolism. Fat tissue burns very few calories at rest, whereas lean tissue (especially muscle and organs) is metabolically active.
- Metabolism: A higher LBM means a higher BMR (Metabolic Rate), allowing you to eat more while maintaining weight.
- Protein Needs: Your protein intake should be based on your lean mass, not your total weight. Calculate your needs with our Macro Calculator.
- Health Longevity: Preserving LBM as you age (Sarcopenia prevention) is minimal for bone density and mobility.
Frequently Asked Questions
Is Lean Body Mass the same as Muscle Mass?
What is a "good" Lean Body Mass percentage?
Why is the James formula result strange?
Tools & Data Verified by the EverydayCalculators Medical Research Team.
Last updated: December 2025.