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Lean Body Mass Calculator
Calculate your fat-free mass from weight and body fat percentage
Body Composition
kg
%
cm
What This Calculator Measures
Lean Body Mass (LBM) is your total weight minus fat mass. It includes muscle, bone, organs, water, and everything else that is not stored body fat. Tracking LBM is crucial during cutting phases to ensure you are losing fat while preserving muscle.
How It Works
The primary calculation is straightforward:
**LBM = Weight x (1 - Body Fat % / 100)**
For example, an 80 kg person at 15% body fat: LBM = 80 x 0.85 = **68 kg**.
The calculator also shows the **Boer formula** as a secondary estimate, which predicts LBM from weight and height without requiring body fat percentage:
**Boer LBM (male) = 0.407 x weight + 0.267 x height - 19.2**
Why LBM Matters
•**During a cut:** If your weight drops but LBM stays stable, you are successfully losing fat. If LBM drops significantly, you may need to increase protein or reduce your deficit.
•**For nutrition planning:** Some coaches base protein and calorie recommendations on LBM rather than total body weight, especially for overweight individuals.
•**Progress tracking:** Gaining 2 kg of LBM over 3 months while maintaining body fat is a much better indicator of progress than scale weight alone.
Limitations
•Accuracy depends entirely on the accuracy of your body fat estimate. A 3% error in body fat translates directly to an error in LBM.
•The Boer formula is a population average and does not account for individual variation in muscularity.
•Water retention, glycogen stores, and food in the gut all affect weight and can make LBM calculations appear to fluctuate day-to-day.
Frequently Asked Questions
- What is lean body mass?
- Lean body mass is everything in your body that isn't fat: muscle, bone, organs, water, and connective tissue. If you weigh 200 lbs at 20% body fat, your lean body mass is 160 lbs. It's a better indicator of your physique than total bodyweight.
- Why should I track lean body mass instead of just weight?
- Because the scale doesn't tell you what you're gaining or losing. If you gain 5 lbs, was it muscle or fat? Tracking lean body mass alongside total weight shows whether you're building muscle, losing fat, or both.
- How do I increase my lean body mass?
- Lift heavy, eat enough protein (0.8-1g per pound), and maintain a slight calorie surplus. Lean body mass increases primarily through resistance training and proper nutrition. Cardio alone won't do it.
- What's a good lean body mass for my height?
- An FFMI (Fat-Free Mass Index) of 20-22 is a well-built natural physique. For a 5'10" guy, that's roughly 155-170 lbs of lean mass. Above an FFMI of 25 is extremely rare without performance-enhancing drugs.