Fat-Free Mass Index Calculator
Calculate fat-free mass index, compare a height-normalised estimate and explore how body-fat input uncertainty changes FFMI.
Body composition index workspace
Supplied body-fat input; no natural-status judgement
Your fat-free mass profile
20.99 kg/m² · raw FFMI
Height-normalised scenario: 20.99 kg/m²
Full result table
| Body-fat scenario (%) | Fat-free kg | Raw FFMI (kg/m²) | Normalised FFMI |
|---|---|---|---|
| 13 | 69.6 | 21.481481 | 21.481481 |
| 15 | 68 | 20.987654 | 20.987654 |
| 17 | 66.4 | 20.493827 | 20.493827 |
Body-fat input quality affects every result. The height-normalised number is a research-derived adjustment originally studied in male athletes, not a universal diagnostic standard. FFMI does not prove natural status, steroid use or a muscle-building limit.
Let’s work it out.
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How to use Fat-Free Mass Index Calculator
- Enter current body weight, height and a measured or estimated body-fat percentage.
- Fat-free mass is a kilogram amount; FFMI divides that amount by squared height.
- Alternative body-fat percentages show how a ±2 percentage-point input change affects the index.
Worked example
80 kg at 15% body fat gives 68 kg fat-free mass. At 1.80 m, FFMI and height-normalised FFMI both equal 20.987654 kg/m².
Formula, accuracy & limitations
Fat-free mass = weight × (1 − body-fat%/100). FFMI = fat-free kg / height(m)². The optional Kouri-style normalisation is FFMI + 6.1 × (1.8 − height in metres).
Body-fat input quality affects every result. The height-normalised number is a research-derived adjustment originally studied in male athletes, not a universal diagnostic standard. FFMI does not prove natural status, steroid use or a muscle-building limit.
These outputs use the stated model and your supplied inputs. A mathematically consistent model can still differ from personal measurements. Scenario curves and formula comparisons are not statistical confidence intervals.
Source and method context ↗Supply body fat from a known method
Enter current body weight, height and a measured or estimated body-fat percentage. Keep track of the source, because a scan, skinfold estimate and circumference estimate can disagree. This calculator uses the number supplied and does not decide which measurement method is correct.
Separate mass from the height index
Fat-free mass is a kilogram amount; FFMI divides that amount by squared height. The normalised value adds a height adjustment around 1.80 metres. Compare the two index values in their shared kg/m² units, rather than putting body mass and an index on the same numerical scale.
Interpret the sensitivity comparison cautiously
Alternative body-fat percentages show how a ±2 percentage-point input change affects the index. These are chosen arithmetic scenarios. A high score is not evidence of drug use, and a single threshold should not be used to judge an individual training history or health status.
Audit the denominator and the body-fat assumption
At 80 kg and 15% body fat, 12 kg is assigned to fat and 68 kg to the fat-free remainder. Height 1.80 m has a square of 3.24 m², so 68 divided by 3.24 gives 20.987654 kg/m². The normalisation term is zero at that reference height. If the body-fat input rises while weight stays fixed, calculated fat-free mass and FFMI fall. That arithmetic is not evidence that actual muscle disappeared between two differently measured estimates.
Frequently asked questions
Is FFMI the same as lean body mass?
Fat-free mass is an amount of mass. FFMI scales that amount to squared height, making it a different quantity from the existing lean-body-mass calculator.
Why show a height-normalised result?
It provides a research-derived comparison around 1.80 m. The adjustment has population limits and is displayed separately instead of silently replacing raw FFMI.
Does a score above 25 prove steroid use?
The calculator makes no such inference. Measurement error, body size, population differences and individual variation prevent that conclusion from a score alone.
What if my body-fat estimate changes?
FFMI changes even when body weight stays constant. Use the sensitivity view to see that effect before interpreting a small difference as new muscle.
