FFMI & lean body mass calculator

Fat-free mass index (FFMI) estimates how much lean tissue you carry relative to height. Combined with lean body mass (LBM), it helps athletes and lifters track muscular development without treating BMI as the whole story. This free calculator needs weight, height, and a body-fat percentage.

From body fat % to LBM and FFMI

Fat mass = body weight × (body-fat% ÷ 100). Lean body mass is everything else: muscle, bone, organs, and water in fat-free tissue. FFMI = LBM ÷ height in meters squared—the same shape as BMI, but applied to lean mass only. A taller athlete with the same LBM will show a lower FFMI than a shorter athlete.

Example: 80 kg at 15% body fat → fat mass 12 kg and LBM 68 kg. At 1.75 m height, FFMI ≈ 68 / 1.75² ≈ 22.2. Garbage-in still applies: if body-fat % comes from a cheap scale that swings five points day to day, FFMI will swing with it. Prefer a consistent method (Navy tape, calipers, or DEXA when available).

Kouri normalized FFMI

The height-normalized adjustment adds 6.1 × (1.8 − height_m) to FFMI so shorter and taller people are compared on a more even footing. Literature popularized these numbers for adult men; women’s typical ranges sit lower. FreeNutri still shows the band as an orientative label only.

Bands on normalized FFMI (below average / average / above / high / near natural-limit range) are educational scaffolding—not a steroid test, not a police report, and not a diagnosis. Plenty of natural lifters sit in “high” ranges with good genetics and years of training; measurement error alone can move you across a boundary.

How to use FFMI without obsession

Track the same way monthly, not daily. Pair FFMI with strength progress (see the one-rep max tool), protein intake, and how clothes fit. If fat loss is the goal, rising FFMI with stable or falling weight often means you preserved lean mass—more useful than chasing a single “perfect” number.

Get body-fat % from the free Navy calculator when you lack a lab reading, then return here. Cross-check with BMI for general weight-for-height context. None of these replace clinical evaluation for eating disorders, hormonal disease, or unexplained weight change.

Units and common mistakes

Enter kilograms or pounds and centimeters or inches; the tool converts before math. Mixing a pound weight with a centimeter height left on the wrong unit selector is the classic error—double-check dropdowns when the FFMI looks absurd.

Hydration, glycogen, and recent training swell LBM on the scale without new muscle protein. Judge trends across weeks. Educational only—not medical advice. Free forever, no signup required.

Keep composition goals realistic

Building lean mass needs progressive training, enough protein, and usually a mild surplus or at least maintenance calories. Aggressive cuts can drop scale weight while FFMI stalls if you lose lean tissue. Use FreeNutri’s TDEE and protein calculators beside this tool.

If normalized FFMI lands in a “high” or “near natural-limit” label, treat it as a prompt to verify body-fat accuracy—not as proof of anything illicit. Coaches and clinicians use fuller context: photos, strength standards, labs, and history.