{"success":true,"version":"v1","request":{"tool":"get_calculator_schema","calculator_id":"ffmi"},"result":{"entity_type":"calculator","id":"ffmi","calculator_id":"ffmi","canonical_url":"https://tttkmbb.com/fitness/ffmi","name":"FFMI Calculator","title":"FFMI Calculator – Fat-Free Mass Index and Height-Normalized FFMI (Kouri et al. 1995)","category":"fitness","category_name":"Sports & Fitness","tool_name":"calculate_ffmi","featured_mcp_tool":false,"description":"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.","use_when":"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.","inputs":[{"name":"weight_kg","label":"Weight","type":"number","unit":"kg","required":true,"max":400,"exclusive_min":0,"description":"Body weight in kilograms.","example":80},{"name":"height_cm","label":"Height","type":"number","unit":"cm","required":true,"min":100,"max":250,"description":"Height in centimetres.","example":180},{"name":"body_fat_percent","label":"Body fat","type":"number","unit":"%","required":true,"min":2,"max":70,"description":"Body-fat percentage from calipers, DEXA, BIA or a tape-measure estimate.","example":15}],"outputs":[{"name":"lean_mass_kg","label":"Lean (fat-free) mass","type":"number","unit":"kg","decimals":1,"description":"weight_kg × (1 − body_fat_percent / 100)."},{"name":"fat_mass_kg","label":"Fat mass","type":"number","unit":"kg","decimals":1,"description":"weight_kg × body_fat_percent / 100."},{"name":"ffmi","label":"FFMI","type":"number","unit":"kg/m²","decimals":2,"description":"lean_mass_kg / height_m²."},{"name":"normalized_ffmi","label":"Normalized FFMI","type":"number","unit":"kg/m²","decimals":2,"description":"ffmi + 6.1 × (1.8 − height_m): adjusted to a reference height of 1.80 m."},{"name":"interpretation","label":"Interpretation","type":"string","decimals":4,"description":"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)."}],"input_schema":{"type":"object","properties":{"weight_kg":{"description":"Body weight in kilograms. Unit: kg.","type":"number","maximum":400,"exclusiveMinimum":0,"examples":[80],"x-unit":"kg"},"height_cm":{"description":"Height in centimetres. Unit: cm.","type":"number","minimum":100,"maximum":250,"examples":[180],"x-unit":"cm"},"body_fat_percent":{"description":"Body-fat percentage from calipers, DEXA, BIA or a tape-measure estimate. Unit: %.","type":"number","minimum":2,"maximum":70,"examples":[15],"x-unit":"%"}},"additionalProperties":false,"required":["weight_kg","height_cm","body_fat_percent"]},"output_schema":{"type":"object","properties":{"lean_mass_kg":{"description":"weight_kg × (1 − body_fat_percent / 100). Unit: kg.","type":"number","x-unit":"kg"},"fat_mass_kg":{"description":"weight_kg × body_fat_percent / 100. Unit: kg.","type":"number","x-unit":"kg"},"ffmi":{"description":"lean_mass_kg / height_m². Unit: kg/m².","type":"number","x-unit":"kg/m²"},"normalized_ffmi":{"description":"ffmi + 6.1 × (1.8 − height_m): adjusted to a reference height of 1.80 m. Unit: kg/m².","type":"number","x-unit":"kg/m²"},"interpretation":{"description":"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).","type":"string"}}},"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)","method":"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.","sources":[{"name":"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","url":"https://pubmed.ncbi.nlm.nih.gov/7496846/","type":"peer_reviewed","retrieved_at":"2026-09-24"}],"freshness":{"type":"static","max_age_seconds":null,"note":"Deterministic formula with fixed constants; results never go stale. Inputs supplied by the caller determine the output."},"examples":[{"name":"80 kg, 180 cm, 15 % body fat","inputs":{"weight_kg":80,"height_cm":180,"body_fat_percent":15},"expected":{"lean_mass_kg":68,"fat_mass_kg":12,"ffmi":20.99,"normalized_ffmi":20.99,"interpretation":"Above average (20–22)"},"url":"https://tttkmbb.com/api/v1/calculate/ffmi?weight_kg=80&height_cm=180&body_fat_percent=15"},{"name":"95 kg, 175 cm, 12 % body fat","inputs":{"weight_kg":95,"height_cm":175,"body_fat_percent":12},"expected":{"lean_mass_kg":83.6,"ffmi":27.3,"normalized_ffmi":27.6,"interpretation":"Rarely attained without anabolic steroids (> 25)"},"url":"https://tttkmbb.com/api/v1/calculate/ffmi?weight_kg=95&height_cm=175&body_fat_percent=12"}],"faq":[{"q":"Why normalize to 1.80 m?","a":"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."},{"q":"Is a normalized FFMI above 25 proof of steroid use?","a":"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."},{"q":"Do the bands apply to women?","a":"They were derived from men; women's values run several units lower, so use them only for relative tracking."}],"tags":["ffmi","fat free mass index","normalized ffmi","muscularity index","lean mass index"],"related":[{"calculator_id":"skinfold-body-fat","reason":"Caliper-based body-fat percentage to feed into the FFMI."},{"calculator_id":"body-fat","reason":"Tape-measure body-fat estimate."},{"calculator_id":"bmi","reason":"Weight-based index for comparison."}],"links":{"html":"https://tttkmbb.com/fitness/ffmi","markdown":"https://tttkmbb.com/fitness/ffmi.md","json":"https://tttkmbb.com/fitness/ffmi.json","api":"https://tttkmbb.com/api/v1/calculate/ffmi","schema":"https://tttkmbb.com/api/v1/calculators/ffmi","openapi":"https://tttkmbb.com/openapi.json","mcp":"https://tttkmbb.com/mcp"},"version":"v1","updated_at":"2026-09-24"},"timestamp":"2026-09-24T03:44:30Z"}