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FFMI Calculator

Computes the fat-free mass index (FFMI = lean mass / height²) and the height-normalized FFMI (FFMI + 6.1 × (1.8 − height in m)) from weight, height and body-fat percentage, and interprets it with the bands derived from Kouri et al. (1995), whose steroid-free athletes did not exceed a normalized FFMI of about 25.

When to use

You want to judge muscularity independent of height, compare lean mass between people, or track a strength-training or bodybuilding progression.

Do not use when: You have no body-fat estimate (use body-fat or skinfold-body-fat first) or want a weight classification (use bmi); the interpretation bands come from a male athlete sample and results are informational.

Formula

lean_mass_kg = weight_kg × (1 − body_fat_percent / 100); ffmi = lean_mass_kg / (height_cm / 100)²; normalized_ffmi = ffmi + 6.1 × (1.8 − height_cm / 100)

Kouri et al. (1995) measured 157 male athletes and found that no steroid non-user exceeded a normalized FFMI of 25.0, while users ranged higher; the bands below 25 are the customary reading of that sample, not clinical categories. Women carry less muscle, so their typical normalized FFMI is about 4–5 units lower. Informational.

Inputs

Parameter	Type	Unit	Required	Description
`weight_kg`	number	kg	yes	Body weight in kilograms. Range: > 0, ≤ 400
`height_cm`	number	cm	yes	Height in centimetres. Range: ≥ 100, ≤ 250
`body_fat_percent`	number	%	yes	Body-fat percentage from calipers, DEXA, BIA or a tape-measure estimate. Range: ≥ 2, ≤ 70

Outputs

Output	Type	Unit	Description
`lean_mass_kg`	number	kg	weight_kg × (1 − body_fat_percent / 100).
`fat_mass_kg`	number	kg	weight_kg × body_fat_percent / 100.
`ffmi`	number	kg/m²	lean_mass_kg / height_m².
`normalized_ffmi`	number	kg/m²	ffmi + 6.1 × (1.8 − height_m): adjusted to a reference height of 1.80 m.
`interpretation`	string		Band of the normalized FFMI: < 18 below average, 18–20 average, 20–22 above average, 22–23 excellent, 23–25 superior, > 25 rarely attained without anabolic steroids (male norms).

Example

80 kg, 180 cm, 15 % body fat: {"weight_kg":80,"height_cm":180,"body_fat_percent":15} → {"lean_mass_kg":68,"fat_mass_kg":12,"ffmi":20.99,"normalized_ffmi":20.99,"interpretation":"Above average (20–22)"}

95 kg, 175 cm, 12 % body fat: {"weight_kg":95,"height_cm":175,"body_fat_percent":12} → {"lean_mass_kg":83.6,"ffmi":27.3,"normalized_ffmi":27.6,"interpretation":"Rarely attained without anabolic steroids (> 25)"}

GET https://tttkmbb.com/api/v1/calculate/ffmi?weight_kg=80&height_cm=180&body_fat_percent=15

Machine access

API: GET https://tttkmbb.com/api/v1/calculate/ffmi (query parameters) or POST with a JSON body {"inputs": {...}}
Schema: https://tttkmbb.com/api/v1/calculators/ffmi · Markdown: https://tttkmbb.com/fitness/ffmi.md · JSON definition: https://tttkmbb.com/fitness/ffmi.json
MCP: server https://tttkmbb.com/mcp, tool run_calculator with calculator_id="ffmi"
OpenAPI operationId: calculate_ffmi
Freshness: static. Authentication: none. Rate limit: fair use (see rate limits).

Sources

Kouri EM, Pope HG, Katz DL, Oliva P (1995) Fat-free mass index in users and nonusers of anabolic-androgenic steroids, Clin J Sport Med 5:223-228 (peer_reviewed)

FAQ

Why normalize to 1.80 m?

FFMI still rises slightly with height in the Kouri data; the 6.1 × (1.8 − height) term removes that trend so people of different heights can be compared on one scale.

Is a normalized FFMI above 25 proof of steroid use?

No. It was the upper limit of the non-users in one 1995 sample; rare genetics, measurement error and an underestimated body-fat percentage can all push the number above 25.

Do the bands apply to women?

They were derived from men; women's values run several units lower, so use them only for relative tracking.

Related calculators

Skinfold Body Fat Calculator — Caliper-based body-fat percentage to feed into the FFMI.
Body Fat Calculator — Tape-measure body-fat estimate.
BMI Calculator — Weight-based index for comparison.