What is estimated
Fat-free mass index from body weight, body-fat percentage, and height, plus the original 1.80 m normalization.
FFMI Calculator
Fat-free mass index
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Reference range
How this result is calculated
A clear calculation path based on your inputs.
Inputs usedReview the information used for this result.
Full calculation and sourcesReview the equation, normalized inputs, assumptions, and version.
Versioned calculationFormula v1.0.0
Method
Fat-free mass is derived first, then divided by height squared; no muscularity category is imposed.
Important boundary
A body-fat estimate can dominate the error. The often-cited value of 25 came from a small 1995 male-athlete cohort and is not a universal natural limit.
Derive fat-free massWeight x (1 - body-fat fraction).
80 kg x 0.82 = 65.6 kgScale to heightDivide fat-free mass by height squared.
65.6 / 1.80^2 = 20.25Show named normalizationApply the published correction to 1.80 m separately.
At 1.80 m the correction is zero.Interpretation
kg/m2 is fat-free mass scaled to height squared.
Changing the body-fat estimate changes FFMI even when weight and height stay fixed.
The calculator does not label a result as natural, enhanced, healthy, or unhealthy.
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Related calculators and resources
Method and test recordFormula v1.0.0
- Recorded scope
- health-body-composition
- Publisher
- CalculatorGeek
- Recorded review date
- 2026-10-04
- Next source review
- 2027-10-04
- Definition fixtures
- 4 configured scenarios
- Published examples
- 4 shown below
Recorded method
Independently derive fat-free mass, divide by height squared, apply the published normalization, and verify metric-imperial parity and body-fat sensitivity.
These records describe the published model and reference tests. A test-case count is not a certification of every possible input or an independent specialist review. Editorial policy
Known limitations
- FFMI inherits error from the body-fat estimate.
- Fat-free mass includes more than skeletal muscle.
- The normalized equation and 25 observation were derived from a limited male-athlete cohort.
- FFMI cannot prove drug use, health, training status, or a biological ceiling.
Method sources
- Kouri et al. (1995): FFMI in users and nonusers of anabolic-androgenic steroids
- VanItallie et al. (1990): Height-normalized fat-free mass and fat mass indices
Latest model update
2026-10-04 - Initial CalculatorGeek shared-system implementation with source-backed method, fixtures, content, and result journey.
Reference inputs and expected results
Up to 12 examples from the configured definition fixtures are shown. Expected values use the stated output units; invalid inputs are intended to be rejected.
| Case | Inputs | Expected result |
|---|---|---|
| 1.80 m reference | Body weight: 80 kg; Standing height: 180 cm; Estimated body fat: 18 percent | Derived fat-free mass: 65.6 kg; FFMI: 20.24691358 kg/m2; Normalized FFMI at 1.80 m: 20.24691358 kg/m2 |
| imperial parity | Body weight: 176.36981 lb; Standing height: 70.866142 in; Estimated body fat: 18 percent | FFMI: 20.2469 kg/m2 (allowed numeric tolerance: 0.002) |
| normalization below 1.80 m | Body weight: 70 kg; Standing height: 170 cm; Estimated body fat: 20 percent | Derived fat-free mass: 56 kg; FFMI: 19.37716263 kg/m2; Normalized FFMI at 1.80 m: 20.00716263 kg/m2 |
| invalid body fat | Body weight: 80 kg; Standing height: 180 cm; Estimated body fat: 90 percent | Reject invalid input: body_fat_percent |
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On this page
What the FFMI Calculator answers
Calculate FFMI from derived fat-free mass and height, with the original normalized FFMI shown separately.
The calculator avoids unsupported muscularity categories and explains why body-fat measurement error carries into the result.
Start with the Health calculators hub when you need to choose a different body-measurement method.
How to use the FFMI Calculator
- Enter body weight and standing height.
- Enter the body-fat estimate that applies to that weight.
- Record how body fat was measured or estimated.
- Read raw and normalized FFMI without treating a cohort observation as a universal cutoff.
For a complementary body-measurement workflow, use the Lean Body Mass Calculator. Compare height-weight lean-body-mass equations.
Formula, variables and method
Fat-free mass = weight x (1 - body-fat percentage / 100).
FFMI = fat-free mass (kg) / height (m)^2.
Normalized FFMI = FFMI + 6.3 x (1.80 - height in m). This normalization comes from the cited 1995 male-athlete study and is shown as a named historical method.
To compare this method with a different input model, open the Body Fat Calculator. Estimate body-fat percentage with a separate method.
Worked examples and boundary checks
80 kg, 1.80 m, 18% body fat
Fat-free mass is 65.6 kg. FFMI is 65.6 / 3.24 = 20.25 kg/m2, displayed as 20.2. Normalized FFMI is the same because height is 1.80 m.
Measurement-sensitivity check
At the same weight and height, changing body fat from 18% to 20% lowers derived fat-free mass by 1.6 kg and lowers FFMI accordingly.
How to interpret the result
FFMI describes estimated fat-free mass relative to height. It is not a direct muscle index and has no universal diagnostic category.
- FFMI inherits error from the body-fat estimate.
- Fat-free mass includes more than skeletal muscle.
- The normalized equation and 25 observation were derived from a limited male-athlete cohort.
- FFMI cannot prove drug use, health, training status, or a biological ceiling.
For another relevant cross-check, continue with the Relative Fat Mass Calculator. Estimate adult body fat from height and waist.
Frequently asked questions
What body-fat percentage should I enter?
Use the estimate tied to the entered weight and record its method; different methods are not interchangeable.
What is normalized FFMI?
It is the correction used in the cited 1995 study to express FFMI at a reference height of 1.80 m.
Does FFMI above 25 prove steroid use?
No. The cited observation came from a small, specific cohort and cannot prove drug use in an individual.
Is FFMI the same as muscle mass index?
No. Fat-free mass includes water, organs, bone, connective tissue, and muscle.
Can I compare two FFMI results?
Only cautiously, using the same body-fat method and similar measurement conditions.
Primary sources and evidence limits
- Kouri et al. (1995): FFMI in users and nonusers of anabolic-androgenic steroids
- VanItallie et al. (1990): Height-normalized fat-free mass and fat mass indices
Sources were checked 2026-10-04. They establish the published method or measurement context; they do not turn an anthropometric estimate into a diagnosis, direct body-composition measurement, or treatment recommendation.