Lean Body Mass Calculator
Calculate your lean body mass (everything that is not fat — muscle, bone, organs, and water) and fat mass from your weight and body-fat percentage. Tracking lean mass helps you tell muscle gain from fat change during a diet or training program.
How to Use
- Enter your body weight in kilograms.
- Enter your body-fat percentage — from a scale, calipers, DEXA, or the Body Fat Calculator.
- Read your lean body mass (weight × (1 − bodyfat/100)) and your fat mass.
- Re-measure periodically and compare lean mass to see whether you are keeping or building muscle.
- Use a preset to see typical splits for an average male, average female, or lean athlete.
Show Work
Formula
About the Lean Body Mass Calculator
Working on health and fitness tracking? The Lean Body Mass Calculator is a free browser tool that gives you the answer in seconds. Calculate your lean body mass (everything that is not fat — muscle, bone, organs, and water) and fat mass from your weight and body-fat percentage. Tracking lean mass helps you tell muscle gain from fat change during a diet or training program.
How it works
Put each value in its box and read the answer as you go. Because it recalculates live, you can play with the inputs to see how each one moves the result — handy for checking your own working or planning ahead. Everything happens on your device, so it is fast and private.
Want the deeper story? The Knowledge Base explains the ideas behind the tools in more detail.
Frequently Asked Questions
What exactly is lean body mass?
Lean body mass (LBM), also called fat-free mass, is your total weight minus your fat mass. It includes muscle, bone, connective tissue, organs, and body water. It is not the same as muscle mass — muscle is just one part of it — but changes in LBM over time are a reasonable proxy for muscle changes when body water is stable.
Why track lean body mass instead of just weight?
The scale alone cannot tell whether a change is fat, muscle, or water. By combining weight with body-fat percentage, you can see whether a diet is preserving muscle while losing fat, or whether a bulk is adding lean mass rather than just fat. This makes it far more useful for judging whether a program is working.
How accurate is this calculation?
The math is exact, but the result is only as good as your body-fat measurement. Bioimpedance scales can be off by several percentage points and fluctuate with hydration; calipers depend on technique; DEXA is more reliable but not always available. For tracking, use the same method under the same conditions each time so the trend is meaningful even if the absolute number is imperfect.
What is a normal lean body mass?
It varies widely with sex, height, and training. As a rough guide, lean mass makes up around 75–85% of body weight in men with average body fat and roughly 70–78% in women, who naturally carry more essential fat. There is no single 'ideal' number — the useful comparison is your own lean mass over time.
Can I use lean body mass for anything else?
Yes. Some calorie equations (such as Katch-McArdle) base BMR on lean mass, protein targets are sometimes set per kilogram of lean mass, and certain medication doses use it. This tool is informational and not medical advice; consult a clinician for any clinical use.
How do I use the Lean Body Mass Calculator?
Just type your numbers. The answer shows up right away — there is no button to press. Change anything and it updates by itself.
Do I need to install or sign up for anything?
Not at all — it runs in the browser with nothing to install and no account. After it loads once, it even works without an internet connection.
Is my information private?
Yes. Everything happens in your browser. Nothing you type is sent to a server or saved anywhere.
Common Use Cases
Judging diet quality
Confirm a fat-loss phase is preserving lean mass rather than burning muscle.
Tracking muscle gain
Watch lean mass rise during a bulk to verify you are building, not just gaining fat.
Setting protein targets
Use lean mass instead of total weight to calculate protein needs more precisely.
Estimating BMR by lean mass
Feed your LBM into lean-mass-based metabolic equations like Katch-McArdle.
Body recomposition tracking
Separate fat and lean change when weight stays flat but composition improves.
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