Estimate your lean body mass (fat-free mass) from your body weight and body-fat percentage. This is a mathematical estimate, not a muscle measurement — see terminology below for why that distinction matters.
Don't know your body-fat percentage? Estimate one first with the Body Fat Calculator — this calculator doesn't estimate lean mass from height and weight alone, to avoid presenting a weak estimate as precise.
Lean body mass generally refers to the portion of body weight that isn't stored body fat. For calculator purposes, estimated fat-free or lean mass can be calculated from body weight and body-fat percentage: Estimated fat-free mass = body weight × (1 − body-fat percentage). It includes water, organs, connective tissue, and minerals — not just muscle, and many consumer resources use "lean body mass" and "fat-free mass" interchangeably, though modern body-composition literature favors the more precise term "fat-free mass."
Equivalent: Estimated FFM = Body Weight × (1 − Body Fat %). For example, at 80 kg and 20% body fat: Fat Mass = 80 × 0.20 = 16 kg; Estimated Fat-Free Mass = 80 − 16 = 64 kg. That 64 kg is the estimated non-fat component under this method — not 64 kg of muscle.
All examples are illustrative. Every value here is an estimate, not a measurement.
| Term | What it refers to |
|---|---|
| Fat mass | Estimated weight of stored body fat |
| Fat-free mass (FFM) | Body mass excluding fat — the modern, more precise term |
| Lean mass / lean body mass | Often used interchangeably with FFM in consumer contexts, historically less precisely defined |
| Lean soft tissue | Fat-free mass minus bone mineral, as reported by some imaging methods (e.g. DXA) |
| Skeletal muscle mass | Specifically the body's skeletal muscle tissue — one component of fat-free mass, not the whole of it |
Terminology in this space has historically been inconsistent, which is part of why contemporary body-composition guidance increasingly favors "fat-free mass" over the more casually used "lean body mass." This page uses both terms, consistent with how people search for this topic, but the calculation always represents the same thing: total weight minus estimated fat mass.
Fat-free mass is broader than skeletal muscle — it also includes body water, organs, connective tissue, proteins, glycogen, minerals, and bone-related components depending on the measurement method. Skeletal muscle is only one part of it, so LBM ≠ muscle mass.
Many calculators and consumer resources use the two terms interchangeably, and this page does too for search purposes — but modern body-composition literature generally recommends "fat-free mass" as the more precise, less ambiguous term.
Body-fat percentage is one of the key inputs used to estimate fat-free mass here — the two are complementary shares of your total body weight, not separate measurements. Estimate your body-fat percentage if you don't already have one.
BMI uses only height and weight and doesn't directly measure body fat or lean mass. Two people with an identical BMI can have very different body-fat percentages, muscle mass, and fat distribution. Calculate your BMI to compare with your composition estimate.
| Measure | What it represents | How it's estimated |
|---|---|---|
| Body weight | Total body mass | Scale |
| Body fat % | Share of body mass classified as fat | BIA, skinfolds, DXA, equations |
| Fat mass | Estimated mass of fat | Weight × body-fat % |
| Fat-free mass | Body mass excluding fat | Weight − fat mass |
| Skeletal muscle mass | Skeletal muscle tissue specifically | Requires specialized assessment |
Estimates body composition from how tissue conducts a small electrical signal. Results are sensitive to hydration, recent food or exercise, and the specific device's algorithm — useful for tracking trends when conditions are kept consistent.
Estimates body fat from the thickness of skin folds at specific sites, converted with a chosen equation. Results depend heavily on technician technique and site selection.
Provides a more detailed body-composition assessment, distinguishing fat, lean soft tissue, and bone mineral content. Terminology in DXA reports can vary between providers.
Estimates body composition from body volume measured via air displacement in a sealed chamber (e.g. Bod Pod), then converts density to fat and fat-free mass.
Estimates body density from underwater weight compared to weight on land, then converts to fat and fat-free mass using standard equations.
Can assess specific tissues and compartments in detail but is not a typical consumer body-composition approach — it's generally reserved for clinical or research settings.
Calculator estimates and instrument-based measurements are not interchangeable. This page's calculation is a simple weight-and-body-fat-percentage math exercise, not a substitute for any of the methods above.
Accuracy depends on several factors: the accuracy of your entered body weight, the accuracy of your body-fat percentage (itself an estimate from whichever method produced it), measurement technique, hydration status, device or software differences, and the population the underlying method was developed on. This calculator does not claim a fixed accuracy figure — doing so without genuine validation would be misleading. Treat the result as directional, not clinical.
Changes in scale weight can involve fat, water, glycogen, lean tissue, or a mix of all of them. Scale weight alone doesn't reveal exactly what changed. Tracking body-fat percentage and estimated fat-free mass over time, under consistent measurement conditions, can add useful context — alongside, not instead of, guidance from a healthcare provider. For energy-intake context, see the TDEE Calculator, Calorie Calculator, and Protein Calculator.
An increase in calculated fat-free mass doesn't necessarily mean an equivalent increase in skeletal muscle — changes in hydration, glycogen, and other non-fat components can move the number too. This calculator can't tell you how much muscle you've gained, and it doesn't promise or predict future muscle gain.
Athletes often use lean-mass estimates for body-composition tracking, weight-class management, monitoring changes during training phases, and as an input to FFMI. A calculator estimate is useful for spotting trends over time, but it should not be treated as a laboratory measurement, and there's no single "optimal" lean-mass figure that applies universally.
Generic consumer weight-and-body-fat math may not be appropriate for every population, including:
| Body weight | Body fat % | Estimated fat mass | Estimated fat-free mass |
|---|---|---|---|
| 60 kg | 15% | 9.0 kg | 51.0 kg |
| 70 kg | 20% | 14.0 kg | 56.0 kg |
| 80 kg | 25% | 20.0 kg | 60.0 kg |
| 90 kg | 18% | 16.2 kg | 73.8 kg |
| 100 kg | 30% | 30.0 kg | 70.0 kg |
These figures are for illustration only. No row here is labeled "healthy," "ideal," or "optimal" — this page does not assign value judgments to specific body-composition numbers.
When the calculator shows "your estimated fat-free mass is X," that number represents the estimated portion of your body mass not classified as fat under this method. It does not represent skeletal muscle mass alone, bone mass alone, organ mass alone, or "metabolically active mass" alone — it's a single combined estimate of everything that isn't fat.
For energy and nutrition context: BMR, TDEE, Calorie, Macro, and Protein calculators. For fitness: One Rep Max and Running Pace calculators.
This page is produced and maintained by the CalculatorSearch content team, following the calculation methodology and terminology described above. It has not been reviewed by a physician, dietitian, or other named medical/scientific expert. If a qualified reviewer is engaged in the future, their name, credentials, and review date will be added here rather than implied.
September 8, 2026. Updated only when the calculator's methodology or content changes meaningfully.
This page summarizes widely used conventions and terminology; it does not implement or cite a specific predictive LBM equation (e.g. Boer, Hume, Janmahasatian), since doing so responsibly requires population-specific validation this calculator does not claim.
This calculator provides an estimate for informational and educational purposes. It does not diagnose medical conditions and does not replace professional medical advice or a clinical body-composition assessment. It does not prescribe calorie targets, medication, supplements, treatment, or medical weight-loss plans.
Body weight and body-fat data can be sensitive personal information. This calculator performs all calculations in your browser — no account is required, and nothing you enter is transmitted to or stored on our servers.