Fat-free mass and height-scaled indices
Fat-free mass is estimated from the body-fat input rather than measured directly. FFMI includes all fat-free tissue, not muscle alone. Neither index diagnoses health, establishes training potential, or identifies drug use.
For the separate estimation of fat-free mass using the supported methods, the Lean Body Mass Calculator explains its method-specific inputs.
The formula
FFMI scales estimated fat-free mass by squared height. The additional height adjustment is a published cohort-derived convention referenced in the cited study. It is shown separately from the unadjusted index and is not a universal clinical correction.
Worked example
At 80 kg, 15% estimated body fat, and 1.80 m height, estimated fat-free mass is 68 kg. FFMI is 68/1.8² = 20.9877 kg/m². At this reference height, the Kouri adjustment adds zero, so both indices match.
How to use this calculator
- Enter body mass, estimated body-fat percentage using the units shown.
- Set height.
- Calculate and review fat-free mass index alongside the supporting quantities.
Body-fat uncertainty affects the index
A body-fat estimate depends on the measurement method, conditions, and individual characteristics. Small percentage-point differences can alter fat-free mass and FFMI even when body weight is unchanged. Compare results using a consistent method, and consider the underlying uncertainty rather than interpreting a decimal difference as a precise change in muscle mass.
Understand the height adjustment
The basic FFMI definition uses fat-free kilograms divided by squared height in meters. The separately displayed Kouri adjustment references a 1.80 m height and a slope of 6.1 from a particular research convention. Its interpretation is limited by the studied populations. No threshold for muscularity, health, or performance is applied on this page.
Assumptions & limitations
What this calculation assumes
- The entered body-fat percentage estimates the fat fraction of total body mass.
What to keep in mind
- Does not measure muscle, diagnose nutritional status, or infer performance or substance use.
Common questions
Is fat-free mass the same as skeletal muscle?
No. It also includes body water, organs, bone, and other non-fat tissue.
Which result should I compare?
Keep the unadjusted and height-adjusted conventions distinct, and compare values calculated with the same body-fat method.
Why do both FFMI results match at 180 cm?
The height adjustment uses a 1.80 m reference. At that height, its added correction is zero.
Sources & further reading
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