Lean Body Mass Calculator
Overview
The Lean Body Mass (LBM) Calculator estimates how much of your body weight consists of muscle, organs, bone, and water — excluding body fat. Using three medical formulas (Boer, James, and Hume), this tool provides a fast and accurate breakdown of your lean mass based on height, weight, and sex. It also calculates body fat mass, percent body fat, and optional FFMI (Fat-Free Mass Index) for a deeper look at body composition. Whether you're tracking fitness progress, planning nutrition goals, or evaluating fat-free mass, this lean body mass calculator offers instant, private, and science-based insights. All computations run directly in your browser to ensure full privacy and instant results.
About
About Lean Body Mass & Body Composition
Lean body mass is a foundational metric used in fitness, sports science, and clinical health assessments. It represents the total weight of everything in the body except fat — including muscle, bone, water, and internal organs. Because direct measurement requires medical imaging or specialized equipment, researchers developed mathematical formulas to estimate LBM from basic measurements like height, weight, and sex. This calculator includes three widely accepted formulas: Boer, James, and Hume. The Boer formula is frequently referenced for clinical dosing and tends to produce slightly higher values for individuals with more muscle mass. The James and Hume formulas adjust differently based on height-to-weight ratios and provide alternative perspectives on lean mass. By comparing all three, users gain a more reliable estimate. Additionally, when body fat percentage is provided, the calculator determines fat mass and offers FFMI, which normalizes lean mass relative to height. All computations run locally for privacy.
Features:
- Boer formula – widely used clinical LBM estimate
- James formula – height-adjusted lean mass calculation
- Hume formula – alternative LBM approach
- Average LBM – consensus estimate across formulas
- Body fat mass – calculated from LBM and weight
- Body fat percentage – derived or user-provided
- FFMI – fat-free mass index (kg/m²)
- Normalized FFMI – height-adjusted FFMI
- Formula comparison chart – visualize differences
- Metric and imperial units – kg/cm or lbs/ft+in support
FAQ
What is lean body mass?
Lean body mass is the total weight of your body minus fat mass. It includes muscle, bone, water, organs, and other non-fat tissues.
Which LBM formula is best?
The Boer formula is commonly used in clinical settings, but comparing all formulas (Boer, James, Hume) gives a more balanced estimate. Results vary slightly between formulas.
What is FFMI?
FFMI (Fat-Free Mass Index) shows lean mass relative to height, similar to BMI but for lean tissue. It helps assess muscle development independent of height.
Do I need my body fat percentage?
No. It's optional. If you don't provide it, the calculator derives body fat % from the average LBM estimate.
Can this replace medical testing?
No. These formulas provide estimates, not diagnostic results. For precise body composition, use DEXA, hydrostatic weighing, or consult a healthcare professional.
How accurate are these formulas?
LBM formulas are estimates with typical accuracy of ±5–10 kg. Accuracy depends on individual body composition, muscle mass, and bone density.
Why do the three formulas give different results?
Each formula uses different coefficients and assumptions about body composition. Comparing all three provides a more reliable range.
Can I use this for athletes?
Yes, but athletes with high muscle mass may have higher actual LBM than estimates. Consider using body fat % input for more accurate results.