What this calculator does
Fat-free mass index measures how much lean tissue a person carries relative to their height, the same way body mass index relates total weight to height. It is used mainly to compare muscularity between people of different heights, since a taller person naturally carries more lean mass without necessarily being more muscular for their frame.
FFMI is calculated from lean, or fat-free, mass rather than total body weight, so it needs a body fat percentage or fat-free mass figure as a starting point, typically from a body composition measurement rather than scale weight alone. A normalised version adjusts the raw figure to what it would be at a reference height of 1.8 metres, making comparisons between people of different heights more direct.
The formula
FFMI divides fat-free mass in kilograms by height in metres squared, the same structure as BMI but using lean mass instead of total weight. Normalised FFMI adds a height correction, 6.1 times the difference between 1.8 metres and the person's actual height, so a shorter person's raw FFMI is adjusted down slightly and a taller person's is adjusted up.
| Term | Meaning |
|---|---|
| FFMI | Fat-free mass (kg) divided by height (m) squared. |
| Normalised FFMI | FFMI adjusted to a reference height of 1.8 m, for comparing people of different heights. |
| Fat-free mass | Total body weight minus fat mass: muscle, bone, organs and water. |
The inputs explained
| Field | What to enter |
|---|---|
| Lean (fat-free) mass (kg) | Lean, or fat-free, mass in kilograms. This comes from a body composition measurement (DEXA, bioelectrical impedance or skinfold calipers, applied to total weight and body fat percentage), not scale weight on its own. |
| Height (cm) | Height in centimetres. |
When to use it
Tracking muscle gain over time
Because FFMI is built from lean mass rather than total weight, it separates muscle gained from fat gained or lost over the same period, which scale weight alone cannot do.
Comparing muscularity across heights
Normalised FFMI adjusts for the fact that taller people carry more lean mass by default, giving a fairer comparison between two people of noticeably different heights.
Assessing where a physique sits within a typical range
FFMI is sometimes used informally to gauge how a level of muscularity compares with typical ranges seen in the general population versus trained or elite athletes.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How FFMI changes with lean mass at a fixed height
A fixed 175 cm height, across a range of fat-free mass figures.
| Fat-free mass | FFMI | Normalized FFMI (to 1.8 m) |
|---|---|---|
| 50 kg | 16.3 | 16.6 |
| 55 kg | 18.0 | 18.3 |
| 60 kg | 19.6 | 19.9 |
| 65 kg | 21.2 | 21.5 |
| 70 kg | 22.9 | 23.2 |
| 80 kg | 26.1 | 26.4 |
How height changes FFMI at a fixed lean mass
A fixed 65 kg of fat-free mass, across a range of heights.
| Height | FFMI | Normalized FFMI (to 1.8 m) |
|---|---|---|
| 160 cm | 25.4 | 26.6 |
| 170 cm | 22.5 | 23.1 |
| 175 cm | 21.2 | 21.5 |
| 180 cm | 20.1 | 20.1 |
| 190 cm | 18.0 | 17.4 |
| 200 cm | 16.3 | 15.0 |
Questions
What is a typical FFMI range?
Roughly 18 to 20 is a common untrained range, with 20 to 22 typical of consistently trained individuals and figures above 23 to 25 associated with a high degree of muscularity. These bands are rough and vary by source, not a diagnostic threshold.
Why does FFMI need a body fat percentage first?
Because fat-free mass is total weight minus fat mass, and fat mass can only be found from an estimate of body fat percentage, whether from calipers, bioelectrical impedance or a DEXA scan. A more accurate body fat estimate gives a more accurate FFMI.
What does normalised FFMI correct for?
It adjusts the raw FFMI figure to what it would read at a standard 1.8 m height, so two people of different heights with the same muscularity read closer to the same normalised number.
Is a high FFMI always a good thing?
Not automatically. It reflects muscularity, not overall health, and very high figures achieved rapidly are sometimes associated with performance-enhancing drug use in sports science literature. Treat it as one descriptive measure, not a target to chase.
For the body fat percentage FFMI is calculated from, see the body fat percentage calculator. For total weight relative to height instead of lean mass, see BMI.