Lean Body Mass Calculator (from Body Fat %)
Calculate lean body mass and fat mass from your total weight and body-fat percentage. Runs entirely in your browser.
Body composition
- Lean body mass
- 64.0 kg
- Fat mass
- 16.0 kg
- Lean mass %
- 80.0 %
Lean body mass = total weight × (1 − body-fat fraction). It includes muscle, bone, organs, and water. Useful for setting protein targets and tracking changes in composition rather than scale weight alone. Not medical advice.
About this tool
Lean body mass (LBM) is everything in your body that is not fat — muscle, bone, organs, connective tissue, and water. If you know your total weight and your body-fat percentage (from a DEXA scan, calipers, a smart scale, or a hydrostatic test), the split is straightforward arithmetic: fat mass is your weight times the body-fat fraction, and lean mass is what remains. This calculator does that in kilograms or pounds. LBM is a more useful number than scale weight for several things: it is the basis many guidelines use for protein and calorie targets, and tracking it over time tells you whether you are losing fat while preserving muscle (the goal of most diets) versus losing both. The accuracy of the result depends entirely on the accuracy of the body-fat measurement you feed it, since the math itself is exact. It is informational, not medical advice. Everything runs in your browser.
How to use it
- Enter your total body weight in kg or lb.
- Enter your measured body-fat percentage.
- Read your lean body mass and fat mass.
- Track LBM over time to see whether you are preserving muscle while losing fat.
Frequently asked questions
- How is lean body mass calculated?
- Fat mass = total weight × (body-fat % ÷ 100); lean body mass = total weight − fat mass. For an 80 kg person at 20% body fat, fat mass is 16 kg and lean mass is 64 kg. The math is exact; accuracy depends on the body-fat measurement.
- What counts as lean body mass?
- Everything that is not fat: skeletal muscle, bone, organs, connective tissue, and body water. Note that LBM includes essential fat in some definitions; this calculator uses the common total-minus-fat definition (sometimes called fat-free mass).
- Why use lean mass instead of total weight?
- Because total weight hides what changed. Two people at the same weight can have very different muscle and fat. Tracking LBM shows whether a diet is costing you muscle, and many protein and calorie formulas are scaled to lean mass for better accuracy.
- How do I measure body-fat percentage?
- Methods range from convenient-but-rough (smart scales, handheld BIA) to more accurate (skinfold calipers done well, DEXA, hydrostatic weighing, Bod Pod). Use the most reliable figure you have; the calculator is only as accurate as that input.
- Does this work in pounds?
- Yes. Pick kg or lb and the lean and fat masses are returned in the same unit. The percentage math is unit-independent.
- Is this medical advice?
- No. It is an informational body-composition tool. For clinical assessment or a weight-management plan, consult a qualified professional.