How to use the Lean Body Mass
- Enter your current body weight.
- Enter your body fat percentage, or use a formula estimate.
- Read lean body mass and fat mass.
- Track it alongside weight during a diet to confirm you are losing fat rather than muscle.
How the calculation works
Lean mass is simply weight × (1 − body fat fraction), so its accuracy depends entirely on the body fat figure feeding it. Fat-free mass index normalises lean mass by height squared, the same way BMI normalises weight, allowing comparison between people of different sizes: untrained men typically sit around 18 to 20, well-trained natural athletes reach 22 to 25, and figures much above that are rarely achieved without pharmacological help.
The practical use is diagnostic. In a well-run deficit, weight falls while lean mass holds within a kilogram or two; if lean mass is dropping alongside it, the deficit is too aggressive, protein is too low, or resistance training is absent. Lean mass also drives resting metabolism, which is why preserving it protects your maintenance calories and makes the weight loss easier to keep.
LBM = weight × (1 − body fat %) ; fat mass = weight − LBM ; FFMI = LBM(kg) / height(m)²Source: Standard body composition partitioning; FFMI normalisation per Kouri et al. fat-free mass index research.
Worked example
An 84 kg man at 1.78 m with 19% body fat, after twelve weeks reaching 78 kg at 13%.
- Start: LBM = 84 × 0.81 = 68.0 kg; fat = 16.0 kg.
- End: LBM = 78 × 0.87 = 67.9 kg; fat = 10.1 kg.
- Fat lost = 5.9 kg; lean lost = 0.1 kg.
- FFMI = 67.9 / 1.78² = 21.4.
Almost all six kilograms came from fat, with lean mass essentially preserved — an FFMI of 21.4 places him in the well-trained range.
Frequently asked questions
Why track lean mass instead of weight?+
Weight conflates fat, muscle and water. Lean mass tells you whether a deficit is costing you muscle, which is the difference between a successful diet and a damaging one.
How much lean mass can I keep while dieting?+
Nearly all of it, with a moderate deficit, protein around 1.6 to 2.2 g per kg of body weight, and continued resistance training. Aggressive deficits without those cost muscle quickly.
What is a good FFMI?+
Around 18 to 20 for untrained men and 22 to 25 for well-trained natural athletes. Women's values run roughly three to four points lower.
Does lean mass include water?+
Yes, and that is why it fluctuates day to day. Carbohydrate intake, sodium and hydration all shift water weight, so compare weekly averages rather than single readings.
Last reviewed August 31, 2026. We review this page whenever the underlying formula, tax year, published rate or standard changes.