Lean Body Mass Calculator
Calculate your lean body mass, fat mass, and lean mass index from your weight and body fat percentage.
Results
Visualization
How It Works
Lean Body Mass (LBM) is everything in your body that is not fat: muscles, bones, organs, water, and connective tissue. Tracking lean mass alongside fat mass gives a much clearer picture of fitness progress than total weight alone, especially when building muscle while losing fat.
The Formula
Variables
- Weight — Total body weight in kilograms
- Body Fat % — Percentage of total weight that is fat
- Lean Mass — Weight of all non-fat tissue (kg)
- Fat Mass — Weight of body fat (kg)
- LMI — Lean Mass Index, analogous to BMI but for lean tissue only
- Height — Height in meters, used for LMI calculation
Worked Example
A person weighing 70 kg with 20% body fat and 170 cm height: Fat mass = 70 * 0.20 = 14.0 kg. Lean mass = 70 - 14 = 56.0 kg. LMI = 56.0 / (1.70^2) = 56.0 / 2.89 = 19.4, which is 'Above average'.
Practical Tips
- If you are losing weight but your lean mass stays the same, you are successfully losing fat while preserving muscle.
- Eating sufficient protein (1.6-2.2 g per kg of body weight) helps preserve lean mass during a calorie deficit.
- Lean Mass Index above 17 for men and above 15 for women is generally considered a sign of good muscular development.
- Water retention can temporarily inflate lean mass readings. Measure consistently at the same time and hydration level.
- Resistance training is the most effective way to increase lean body mass over time.
Frequently Asked Questions
What is a good lean mass percentage?
For men, 75-85% lean mass (15-25% body fat) is typical and healthy. For women, 68-80% lean mass (20-32% body fat) is normal. Athletes may have lean mass percentages above 85% for men and 80% for women.
How do I know my body fat percentage to use this calculator?
You can estimate body fat using the Navy method (our Body Fat Calculator), bioelectrical impedance scales, skinfold calipers, or more precise methods like DEXA scans. Even an approximate value gives useful lean mass insights.
Can I gain lean mass while losing fat?
Yes, this is called body recomposition. It is most achievable for beginners, those returning to exercise after a break, and people with higher body fat. It requires adequate protein intake and progressive resistance training, typically at a small calorie deficit or maintenance.
What is Lean Mass Index and why does it matter?
LMI is like BMI but only counts lean tissue. It helps assess muscular development independent of fat. A higher LMI indicates more muscle relative to height. It is useful for tracking strength training progress without body fat confounding the picture.
How often should I measure lean body mass?
Every 4-8 weeks is ideal for tracking changes. Lean mass changes slowly with consistent training and nutrition. More frequent measurements mostly capture water weight fluctuations rather than true tissue changes.