Lean Body Mass Calculator

Lean body mass (LBM) is the single most important number in body composition that most people never think about. It is the part of your weight that is NOT fat — muscle, bone, organs, connective tissue, and the water inside them. Your LBM determines your strength, basal metabolic rate, protein requirements, recovery capacity, and tolerance for many medications. Two 80-kilogram people can look completely different on a DEXA scanner: a trained lifter at 12% body fat carries roughly 70 kg of LBM, while a sedentary office worker at 30% body fat carries only 56 kg. Same scale weight, completely different metabolic profile. That is why every serious dietitian, strength coach, and clinician tracks LBM rather than total weight, and why this calculator gives you five clinical formulas plus a direct body-fat derivation in one place.

No predictive LBM formula is perfect. Each was derived by fitting a regression equation to a reference body composition measurement (usually DEXA or hydrostatic weighing) in a particular population. Boer was validated on adult Western Europeans. James used a WHO cohort spanning multiple countries. Peters was built for pediatric nuclear medicine. Yu was calibrated against DEXA in Asian adults. The 1-3 kg spread you see across our five formulas is normal and reflects population differences in body shape. The trick is to pick the formula that matches your demographic best (Boer for most adults, Peters for children, Yu for Asian heritage), use it consistently, and watch the trend across weeks and months. A single DEXA scan at a sports performance center costs roughly $50-$150 and lets you calibrate the formula of your choice against ground truth.

LBM is far more than an aesthetic metric. In anesthesia, propofol and many neuromuscular blockers are dosed by lean body mass because they distribute primarily through non-adipose tissue. In oncology, chemotherapy dosing increasingly uses LBM to avoid over-dosing sarcopenic patients. In renal medicine, creatinine clearance calculations rely on lean mass estimates. And in fitness, every modern macro plan starts from LBM because protein requirements scale with lean tissue, not fat. Whether you are a lifter chasing FFMI, a dietitian setting macros for clients, or a clinician running a bedside dosing check, the Boer formula has become the de facto language. This calculator gives you Boer plus four cross-checks side-by-side, so you can see the consensus number rather than relying on any single equation.

The Five Formulas Explained

How to Use This Calculator (5 Steps)

1. 1. Pick Your Sex: LBM formulas differ for male and female because women carry more essential fat and less muscle on average for any given height and weight.
2. 2. Choose a Formula: Start with Boer if you are an adult, Peters if you are under 18, or Yu if you are of East or South Asian heritage. We calculate all formulas automatically so you can compare.
3. 3. Enter Your Basics: Age, weight, and height. Toggle between kilograms or pounds and centimeters, inches, or feet-and-inches — the calculator auto-converts when you change units.
4. 4. Optional - Add Body Fat %: If you have a recent DEXA, BIA, or skinfold reading, enter your body fat percentage to unlock the direct derivation method (the most accurate option when available).
5. 5. Calculate, Compare, Track: See your LBM in kg and lbs, fat mass, FFM index, daily protein at 1.6-2.2 g/kg LBM, and a side-by-side card comparison of every formula. Re-measure every 4-8 weeks to track real progress.

Use Cases & Internal Tools

Pro Tips for Accurate Tracking

• • Calibrate once with DEXA: If accuracy matters, get a single DEXA scan to calibrate your at-home LBM formula.
• • Boer is your default: For adult tracking, Boer is the modern gold standard. Use the other formulas as cross-checks.
• • Pick by demographic: Peters for children, Yu for Asian adults. Right formula avoids systematic 2-4 kg over- or under-prediction.
• • Use the BF% method when you can: A recent DEXA or 7-site skinfold reading lets the body-fat derivation outperform every regression formula.
• • Measure first thing in the morning: After bathroom, before eating, drinking, or exercising. Consistency matters more than absolute starting condition.
• • Every 4-8 weeks, not weekly: LBM moves slowly. Measuring more often than monthly produces noise rather than signal.
• • Trust the trend, not the value: Whether Boer LBM is 64 or 66 kg today matters less than the direction across months.

Whatever your goal — lifting, dieting, clinical dosing, or long-term health tracking — lean body mass is the metric that actually tracks your body composition. Bookmark this calculator, measure under identical conditions every 4-8 weeks, and watch the trend across months. Real change happens slowly. Use this tool to verify it is happening.