Free Lean Body Mass Calculator
Calculate your lean body mass, fat mass, and body composition using multiple proven formulas. Understand your body beyond the scale.
What Is Lean Body Mass?
Lean body mass (LBM), also called fat-free mass (FFM), is your total body weight minus all fat tissue. It represents everything in your body that isn't stored fat:
- Skeletal muscle — The muscle you can see and control
- Bones — Your entire skeleton
- Organs — Heart, liver, kidneys, brain, etc.
- Skin — Your body's largest organ
- Blood — Including plasma and blood cells
- Body water — Approximately 60% of lean mass
- Connective tissue — Tendons, ligaments, cartilage
Lean Body Mass Formula
LBM = Total Body Weight − Fat Mass Or if you know your body fat percentage:
LBM = Weight × (1 − Body Fat %) Example: 180 lbs × (1 − 0.20) = 144 lbs lean mass
Why Lean Body Mass Matters
Understanding your lean body mass is valuable for several reasons:
Metabolism
Lean mass is metabolically active — it burns calories even at rest. More lean mass = higher basal metabolic rate (BMR).
Accurate Calorie Needs
The Katch-McArdle formula uses LBM to calculate BMR more accurately than weight-based formulas, especially for lean or overweight individuals.
Protein Targets
Many experts recommend protein based on lean body mass (1-1.2g per lb of LBM) rather than total body weight.
Progress Tracking
Scale weight doesn't tell the full story. Tracking LBM helps you see if you're losing fat while preserving (or building) muscle.
Lean Body Mass Formulas Explained
Several formulas can estimate lean body mass. Each has strengths and limitations:
Navy Method (Most Accurate for Home Use)
Uses circumference measurements to estimate body fat, then calculates LBM from total weight:
Navy Method — Men
BF% = 86.010 × log10(waist − neck) − 70.041 × log10(height) + 36.76 Navy Method — Women
BF% = 163.205 × log10(waist + hip − neck) − 97.684 × log10(height) − 78.387 Then: LBM = Weight × (1 − BF%)
Accuracy: ±3-4% when measurements are taken correctly. Most accurate option without professional equipment.
Boer Formula
Estimates LBM directly from height and weight:
Boer Formula
Men: LBM = (0.407 × weight in kg) + (0.267 × height in cm) − 19.2 Women: LBM = (0.252 × weight in kg) + (0.473 × height in cm) − 48.3 Best for: Quick estimates when measurements aren't available. Less accurate for very lean or overweight individuals.
James Formula
James Formula
Men: LBM = (1.1 × weight in kg) − 128 × (weight/height)² Women: LBM = (1.07 × weight in kg) − 148 × (weight/height)² Height in meters
Hume Formula
Hume Formula
Men: LBM = (0.32810 × weight in kg) + (0.33929 × height in cm) − 29.5336 Women: LBM = (0.29569 × weight in kg) + (0.41813 × height in cm) − 43.2933 These formulas are based on population averages and may be less accurate for individuals with unusual body compositions (very muscular, very lean, or obese).
Lean Body Mass vs. Muscle Mass
A common misconception is that lean body mass equals muscle mass. They're related but different:
| Lean Body Mass | Skeletal Muscle Mass |
|---|---|
| Everything that isn't fat | Only voluntary muscles |
| Includes bones, organs, water | Excludes non-muscle tissue |
| 60-90% of body weight | 30-45% of body weight (fit individuals) |
| Estimated from body fat % | Requires DEXA or MRI to measure |
| Changes with hydration | More stable measurement |
Skeletal muscle typically makes up about 40-50% of lean body mass in fit individuals. The rest is bones (~15%), organs (~10-15%), and water/other tissues.
What's a Good Lean Body Mass?
There's no single "ideal" lean body mass — it varies based on height, frame size, genetics, and goals. Here are general ranges:
LBM as Percentage of Total Weight
| Category | Men (LBM %) | Women (LBM %) |
|---|---|---|
| Elite Athletes | 88-95% | 80-88% |
| Fitness Enthusiasts | 82-88% | 75-82% |
| Average Adults | 75-82% | 68-75% |
| Overweight | 65-75% | 58-68% |
How to Preserve Lean Body Mass During Fat Loss
When losing weight, you want to lose fat while keeping as much lean mass as possible. Without the right approach, up to 25% of weight lost can be muscle. Here's how to minimize that:
High Protein Intake
Eat 0.8-1.2g protein per pound of body weight. This is the single most important factor for preserving muscle during a deficit.
Resistance Training
Lift weights 3-4x per week. This signals your body to preserve muscle tissue. Focus on compound movements.
Moderate Deficit
Keep your deficit at 500 calories or less (15-25% of TDEE). Larger deficits lead to more muscle loss.
Slow Rate of Loss
Aim for 0.5-1% of body weight per week. Faster loss correlates with more muscle loss.
Adequate Sleep
Get 7-9 hours per night. Poor sleep increases cortisol and shifts weight loss toward muscle instead of fat.
Diet Breaks
After 8-12 weeks of dieting, take a 1-2 week maintenance phase to restore hormones and prevent adaptation.
How to Build Lean Body Mass
Building lean mass (primarily muscle) requires:
1. Progressive Resistance Training
Train each muscle group 2-3x per week with progressive overload — gradually increasing weight, reps, or volume over time. Focus on compound movements: squats, deadlifts, bench press, rows, overhead press.
2. Caloric Surplus
You need excess energy to build tissue. Aim for 200-400 calories above maintenance. A smaller surplus minimizes fat gain; a larger surplus maximizes muscle gain potential but adds more fat.
3. Adequate Protein
Consume 0.8-1g protein per pound of body weight. Spread intake across 3-5 meals with 25-40g protein each for optimal muscle protein synthesis.
4. Recovery
Muscle grows during rest, not during training. Prioritize sleep, manage stress, and don't overtrain. Most people benefit from 3-5 training days per week with adequate rest between sessions.
Realistic Muscle Gain Expectations
| Training Level | Monthly Gain (Men) | Monthly Gain (Women) |
|---|---|---|
| Beginner (Year 1) | 1-1.5 lbs/month | 0.5-0.75 lbs/month |
| Intermediate (Years 2-3) | 0.5-1 lb/month | 0.25-0.5 lbs/month |
| Advanced (Years 4+) | 0.25-0.5 lb/month | 0.1-0.25 lbs/month |
Lifetime natural muscle gain potential is approximately 40-50 lbs for men and 20-25 lbs for women above their untrained baseline.
Using LBM to Calculate Calorie Needs
The Katch-McArdle formula uses lean body mass to calculate BMR, making it more accurate for very lean or overweight individuals:
Katch-McArdle BMR Formula
BMR = 370 + (21.6 × Lean Body Mass in kg) Example: Someone with 145 lbs (65.8 kg) of lean mass:
BMR = 370 + (21.6 × 65.8) = 1,791 calories/day This formula is particularly useful because it accounts for body composition. Two people weighing 180 lbs will have very different metabolic rates if one has 20% body fat and the other has 35%.
After calculating BMR, multiply by your activity factor to get TDEE:
- Sedentary (desk job): BMR × 1.2
- Light activity: BMR × 1.375
- Moderate activity: BMR × 1.55
- Very active: BMR × 1.725
- Extremely active: BMR × 1.9
Frequently Asked Questions
Everything you need to know about lean body mass
What is lean body mass?
Lean body mass (LBM) is your total body weight minus fat mass. It includes muscle, bones, organs, skin, blood, and water — everything that isn't stored fat. LBM is also called fat-free mass (FFM) and typically makes up 60-90% of total body weight depending on body fat levels.
How do I calculate lean body mass?
Lean body mass is calculated by subtracting fat mass from total body weight: LBM = Total Weight - Fat Mass. If you know your body fat percentage, use: LBM = Weight × (1 - Body Fat %). Various formulas like Boer and James can estimate LBM from height and weight alone.
What's a good lean body mass?
There's no universal 'ideal' lean body mass — it depends on your height, frame size, and goals. Athletes may have LBM of 80-90% of total weight, while average adults range from 65-85%. Focus on maintaining or building lean mass through resistance training rather than targeting a specific number.
Why is lean body mass important?
Lean body mass is metabolically active — it determines your basal metabolic rate (BMR). More lean mass means you burn more calories at rest. Knowing your LBM helps with: calculating accurate calorie needs, setting protein targets, tracking body recomposition, and assessing fitness progress beyond scale weight.
What's the difference between lean body mass and muscle mass?
Lean body mass includes ALL non-fat tissue: muscles, bones, organs, skin, and body water. Muscle mass is just one component of LBM. Skeletal muscle typically makes up about 40-50% of lean body mass in fit individuals. LBM is easier to estimate; accurate muscle mass requires DEXA or MRI scans.
How much lean body mass can I gain?
Natural muscle gain potential varies: beginners can gain 1-2 lbs of muscle per month, intermediate lifters 0.5-1 lb/month, and advanced lifters even less. Lifetime natural muscle gain potential is roughly 40-50 lbs for men and 20-25 lbs for women above untrained baseline.
Which lean body mass formula is most accurate?
The Navy method with circumference measurements is most accurate for home use (±3-4%). The Boer formula is a good quick estimate using only height and weight. For best accuracy, get a DEXA scan (±1-2%). Different formulas may suit different body types.
How do I maintain lean body mass while losing fat?
To preserve lean mass during fat loss: 1) Keep protein high (0.8-1.2g per pound bodyweight), 2) Lift weights 3-4x per week, 3) Use moderate calorie deficit (500 cal max), 4) Get adequate sleep (7-9 hours), 5) Lose weight slowly (0.5-1% body weight per week).
How does lean body mass affect calorie needs?
Lean body mass is the primary driver of your basal metabolic rate (BMR). The Katch-McArdle formula calculates BMR directly from LBM: BMR = 370 + (21.6 × LBM in kg). This is why muscular people burn more calories at rest than those with the same weight but more fat.
Can I increase lean body mass while in a calorie deficit?
Yes, this is called body recomposition. It works best for: beginners, those returning to training after a break, people with higher body fat (>20% men, >30% women), and those using a small deficit. Most people should focus on either fat loss OR muscle gain for best results.
References
- Boer P. Estimated lean body mass as an index for normalization of body fluid volumes in man. Am J Physiol. 1984;247(4 Pt 2):F632-6. PubMed
- James WPT. Research on obesity. London: Her Majesty's Stationery Office. 1976.
- Hume R. Prediction of lean body mass from height and weight. J Clin Pathol. 1966;19(4):389-91. PubMed
- Hodgdon JA, Beckett MB. Prediction of percent body fat for U.S. Navy men from body circumferences and height. Naval Health Research Center. 1984.
- Katch FI, McArdle WD. Prediction of body density from simple anthropometric measurements. Human Biology. 1973;45(3):445-455.