Muscle Calculator
Estimate your lean body mass, muscle mass, FFMI, and body fat percentage using scientifically validated formulas. Fast, private, and accurate.
Your Information
Enter your information and click Calculate
We'll estimate your lean body mass, fat mass, and FFMI
Detailed Breakdown
How to use the muscle calculator
This calculator estimates your lean body mass (LBM), fat mass, and Fat-Free Mass Index (FFMI) using validated scientific formulas. Simply enter your sex, age, height, and weight to get started. For more accurate results, you can optionally provide your body fat percentage or take body circumference measurements.
The calculator uses multiple estimation methods including the Boer, Hume, James, and Katch–McArdle formulas, plus the US Navy body fat estimation method. Results are displayed with a visual chart showing your body composition split between lean mass and fat mass.
How it works
Lean Body Mass (LBM)
Lean body mass represents everything in your body except fat — including muscle, bone, organs, water, and connective tissue. If you know your body fat percentage, we calculate LBM using:
\( LBM = W \times \left(1 - \frac{BF}{100}\right) \)
where \( W \) is weight in kg and \( BF \) is body fat percentageIf body fat percentage is unknown, we estimate LBM using one of several validated formulas based on your height, weight, and sex:
Boer Formula
Male: \( LBM = 0.407W + 0.267H - 19.2 \)
Female: \( LBM = 0.252W + 0.473H - 48.3 \)
where \( W \) is weight in kg and \( H \) is height in cmHume Formula
Male: \( LBM = 0.32810W + 0.33929H - 29.5336 \)
Female: \( LBM = 0.29569W + 0.41813H - 43.2933 \)
Fat-Free Mass Index (FFMI)
FFMI is a measure of muscle mass relative to height, similar to BMI but accounting for body composition. It's calculated as:
\( FFMI = \frac{LBM}{H^2} \)
where \( LBM \) is in kg and \( H \) is height in metersThe normalized FFMI adjusts for height differences, allowing fairer comparisons across individuals:
\( FFMI_{adj} = FFMI + 6.1 \times (1.8 - H) \)
Normalization adjusts for height to compare across people (not a diagnosis)FFMI ranges (approximate):
- 16-17: Below average muscle mass
- 18-19: Average for healthy adults
- 20-21: Above average, athletic build
- 22-23: Excellent muscle development
- 24-25: Near the natural limit for most people
- 26+: Rare without performance-enhancing substances
Tips to build muscle sustainably
Building muscle is a gradual process that requires consistency across multiple factors:
- Progressive resistance training: Lift weights or use bodyweight exercises 3-5 times per week, gradually increasing weight or reps over time
- Adequate protein intake: Aim for 1.6-2.2 grams of protein per kilogram of body weight daily, distributed across meals
- Caloric surplus (for muscle gain): Eat 200-500 calories above maintenance to support muscle growth, but avoid excessive bulk
- Quality sleep: Aim for 7-9 hours per night; muscle recovery and growth happen during sleep
- Consistency over time: Natural muscle gain is slow — expect 0.25-0.5 kg (0.5-1 lb) of muscle per month for beginners, less for advanced lifters
- Track progress: Measure your FFMI and body composition monthly rather than weekly to see meaningful trends
For evidence-based strength training guidance, visit the CDC's physical activity guidelines. Remember that BMI and similar metrics have limitations for muscular individuals — learn more from MedlinePlus.
Frequently Asked Questions
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. LBM is not the same as "muscle mass" — muscle typically makes up 40-50% of LBM in healthy adults.
What's the difference between muscle mass and lean body mass?
Muscle mass refers only to skeletal muscle tissue, while lean body mass includes everything except body fat: muscle, bones, organs, blood, and water. LBM is always higher than muscle mass alone. This calculator estimates total LBM; to estimate skeletal muscle specifically, multiply LBM by approximately 0.4-0.5.
What is FFMI and why does it matter?
Fat-Free Mass Index (FFMI) is a measure of how much muscle mass you have relative to your height. It's more useful than BMI for athletic or muscular individuals. FFMI helps track muscle-building progress and provides a realistic assessment of your physique. An FFMI above 25 is rare naturally, while 19-21 is typical for active adults.
How accurate are body fat percentage estimates?
The US Navy method used in this calculator typically has a margin of error of ±3-4% compared to gold-standard DEXA scans. Accuracy depends heavily on precise measurements. For best results, have someone else measure your circumferences, and take measurements at the same time of day consistently.
Which lean body mass formula is most accurate?
If you know your body fat percentage accurately (from a DEXA scan or hydrostatic weighing), calculating LBM directly from body fat is most accurate. Among predictive formulas, the Boer and Hume formulas are well-validated for general populations. The James formula can be less stable for very tall or short individuals.
Can I use this calculator if I'm overweight or obese?
Yes. The calculator works for all body types. However, LBM formulas that don't use body fat percentage may be less accurate at higher body fat levels. If possible, provide body fat percentage or measurements for better estimates.
What is normalized FFMI?
Normalized FFMI adjusts the standard FFMI to account for height differences, making it easier to compare muscle mass across people of different heights. The adjustment adds or subtracts points based on whether you're taller or shorter than 1.8 meters (5'11"). It's a research tool, not a diagnostic measure.
How often should I measure my lean body mass?
For meaningful progress tracking, measure monthly rather than weekly. Natural muscle gain is slow (typically 0.25-0.5 kg per month for beginners), and daily fluctuations in water weight and food can mask real changes. Consistent measurement conditions (same time of day, similar hydration) improve accuracy.
Why do different formulas give different results?
Each formula was developed from different populations and uses slightly different assumptions about body composition. Differences of 1-3 kg between formulas are normal. Use the same formula consistently over time to track changes, rather than comparing absolute values across formulas.
Is this calculator suitable for athletes?
Yes. FFMI is particularly useful for athletes and bodybuilders, as it accounts for muscle mass in a way BMI cannot. However, elite athletes may want to use DEXA scans or other clinical methods for the most precise body composition data.
Can I export or print my results?
Yes. After calculating, expand the "Detailed breakdown" section to access export and print options. The CSV export includes all your inputs, results, and method comparisons. The print-friendly version formats results for easy printing or PDF saving.
Does this calculator store my data?
No personal data is sent to any server. All calculations happen in your browser. If you enable "Remember my inputs," your preferences and last entries are saved locally on your device only and can be cleared anytime by using the Reset button or clearing your browser data.
About this calculator
The Muscle Calculator was created by OmniCalculator.Space to provide fitness enthusiasts, athletes, and health-conscious individuals with a free, accurate, and privacy-respecting tool for estimating body composition. All formulas are based on peer-reviewed research and validated estimation methods.
We combine multiple calculation methods to give you the most comprehensive view of your lean mass and muscle development. Whether you're tracking training progress, planning a body recomposition, or simply curious about your body composition, this calculator provides science-backed estimates you can trust.
For more fitness and health calculators, visit our fitness tools section or explore our full collection at OmniCalculator.Space.
⚠️ Medical Disclaimer
This calculator provides educational estimates only and is not a substitute for professional medical advice, diagnosis, or treatment. Body composition measurements and estimates have inherent limitations and should not be used for clinical decisions. Always consult with a qualified healthcare provider or registered dietitian for personalized health and fitness advice. If you have medical conditions, eating disorders, or body image concerns, seek professional guidance before making changes to diet or exercise.