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Fat-free mass index is the ratio of lean mass to height squared and it gives a height-adjusted view of muscularity that is harder to judge with weight alone. Tracking this index helps lifters, coaches, and clinicians compare physiques with a like for like yardstick.
This calculator accepts body weight, standing height, and either body fat percentage or a direct lean mass reading. It delivers the raw FFMI, an optional normalised value referenced to 1.8 metres, and the matching benchmark band for male or female profiles. Each output includes lean and fat mass in both metric and imperial units so you can cross-check measurements from calipers, impedance devices, or lab scans.
A practical example is a strength athlete who logs weight in pounds and a DEXA lean mass report. Switching the override on bypasses body fat estimation, updates FFMI in real time, and keeps the classification aligned with the selected sex. Clipboard, CSV, DOCX, and JSON exports preserve the snapshot for training logs or consultations.
Measurements remain estimates and large deviations in hydration, measurement method, or growth stage can shift interpretation. Use consistent equipment, measure at similar times of day, and pair the number with broader health context.
This tool provides informational estimates and does not substitute professional advice.
FFMI quantifies lean mass per square metre of height so people of different stature can be compared on a common muscularity scale. Lean mass may be derived from body weight and an estimated body fat percentage or supplied directly from a scan. The calculator presents both the raw FFMI and a normalised figure that adjusts everyone to a reference height of 1.8 m using the coefficient described by Kouri and colleagues.
Inputs accept kilograms or pounds for mass and centimetres, metres, or inches for height. Percentages are clamped to realistic adult ranges and lean mass overrides may not exceed body weight, preventing inconsistent combinations. When the normalisation switch is on, benchmark comparisons use the adjusted FFMI; otherwise they use the raw figure.
| Symbol | Meaning | Unit/Datatype | Source |
|---|---|---|---|
| Body weight after unit conversion | kg | Input | |
| Body fat percentage | percent | Input or derived by measurement | |
| Fat-free mass (lean mass) | kg | Derived or direct override | |
| Standing height | m | Input | |
| Fat-free mass index | kg·m−2 | Derived | |
| Normalised FFMI at the 1.8 m reference | kg·m−2 | Derived | |
| Reference height | 1.8 m | Constant | |
| Normalisation coefficient | 6.3 | Kouri et al. (1995) |
Worked example:
Weight = 82 kg, height = 178 cm, body fat = 15%. Height converts to 1.78 m.
For a male profile this sits in the “Above average” band, suggesting solid muscular development without approaching the elite ceiling.
| FFMI range (male) | Label | Implication |
|---|---|---|
| 0.0 to 17.0 | Below average | Low lean mass for adult males; focus on consistent training and nutrition. |
| 17.0 to 19.0 | Average | Typical recreational baseline with balanced muscle mass. |
| 19.0 to 21.0 | Above average | Noticeable muscularity from regular resistance training. |
| 21.0 to 23.0 | Excellent | Well-developed physique consistent with long-term structured lifting. |
| 23.0 to 25.0 | Elite competitive | Upper natural range; typical of advanced competitive athletes. |
| ≥ 25.0 | Uncommon (requires verification) | Cross-check measurements; values above 25 often indicate pharmacological support or measurement error. |
| FFMI range (female) | Label | Implication |
|---|---|---|
| 0.0 to 14.0 | Below average | Below typical lean mass for adult females; emphasise progressive strength work. |
| 14.0 to 16.0 | Average | General population baseline with balanced composition. |
| 16.0 to 18.0 | Above average | Consistent training yields visible muscularity while maintaining balance. |
| 18.0 to 20.0 | Excellent | Strong physique characteristic of well-trained athletes. |
| 20.0 to 22.0 | Elite competitive | High-level conditioning; maintain regular monitoring. |
| ≥ 22.0 | Uncommon (requires verification) | Validate inputs; readings beyond 22 are rare without exceptional genetics or pharmacological aid. |
When normalisation is enabled the classification logic uses FFMInorm so that tall and short individuals are compared on equal footing. With the toggle off, categories reflect the raw FFMI.
Mass inputs and overrides are stored in kilograms after conversion; height is stored in centimetres and metres as needed. FFMI values display to two decimals in tables and one decimal in the headline. Lean and fat masses round to two decimals, while classification labels rely on unrounded calculations. Rounding uses symmetric nearest ties (half up) at the stored precision.
| Field | Type | Min | Max | Step/Pattern | Notes |
|---|---|---|---|---|---|
| Weight value | number | 1 | 400 | 0.1 | Values outside the range are clamped and unit conversions preserve magnitude. |
| Weight unit | select | — | — | kg | lb | Switching units automatically converts the numeric value. |
| Height value | number | 50 | 250 | 0.1 | Values outside the adult range snap to the nearest bound. |
| Height unit | select | — | — | cm | m | in | Conversions retain the underlying centimetre value. |
| Body fat percentage | number | 1 | 70 | 0.1 | Field is inactive when lean mass override is enabled. |
| Sex | select | — | — | male | female | Determines which benchmark table is applied. |
| Use known lean mass | switch | false | true | boolean | When true, the body fat field is ignored. |
| Lean mass value | number | 1 | 200 | 0.1 | Only active when the override switch is on; must not exceed weight. |
| Lean mass unit | select | — | — | kg | lb | Appears with the override input. |
| Normalise to 1.8 m | switch | false | true | boolean | Applies the Kouri adjustment before classification. |
All calculations are direct algebra with constant-time complexity and negligible memory overhead. Even repeated unit switches or rapid input changes remain instantaneous in modern browsers.
Inputs stay in the browser, and exports are generated locally. No network requests, storage, or analytics are triggered, so measurements remain on your device.
Example: 180 lb, 70 in, 14 % body fat delivers an FFMI near 22 and a normalised FFMI just above 22 with a male profile.
The outcome is a clear FFMI snapshot with supporting context you can reuse in training logs, athlete check-ins, or consultation notes.
It expresses lean mass relative to height, letting you compare progress over time and against typical ranges for your sex. Use the classification badge as a quick reference, but pair it with strength, performance, and health markers.
Normalisation adjusts everyone to a 1.8 m baseline so tall and short individuals can be compared more fairly. It is especially useful when you track multiple athletes of different heights or want to align with the Kouri research conventions.
Increase lean mass while managing body fat. Raise lean body mass through progressive resistance training, sufficient protein, and sleep, or reduce body fat at steady pace so the lean-to-height ratio improves. The calculator shows your current position so you can plan incremental changes.
Yes. Switch on the lean mass override and input the scan value in kilograms or pounds. The body fat percentage is ignored and the tool uses your direct measurement for higher fidelity.
Weight toggles between kilograms and pounds. Height accepts centimetres, metres, or inches. Lean mass shares the same mass units. All conversions are automatic.
No. Calculations, clipboard copies, CSV files, DOCX exports, and JSON bundles are generated in your browser and stay on your device.
Once the page and its assets are loaded, the calculator runs without a network connection. Exports and copies continue to work offline.
The package does not declare pricing or licence terms. Treat the outputs as informational guidance for personal or coaching use.