{"success":true,"version":"v1","request":{"tool":"get_calculator_schema","calculator_id":"body-fat"},"result":{"entity_type":"calculator","id":"body-fat","calculator_id":"body-fat","canonical_url":"https://tttkmbb.com/health/body-fat","name":"Body Fat Calculator","title":"Body Fat Percentage Calculator – US Navy Circumference Method","category":"health","category_name":"Health","tool_name":"calculate_body_fat_percentage","featured_mcp_tool":false,"description":"Estimates body-fat percentage from height and neck, waist (and hip for women) circumferences using the US Navy method, and classifies the result with ACE categories.","use_when":"You want a tape-measure estimate of body-fat percentage and fat/lean mass without calipers or a DEXA scan.","do_not_use_when":"High accuracy is required (DEXA, hydrostatic weighing or BIA are better), or the person is a child or pregnant.","inputs":[{"name":"sex","label":"Biological sex","type":"enum","required":true,"values":[{"value":"male","label":"Male","aliases":["m","man"]},{"value":"female","label":"Female","aliases":["f","woman"]}],"description":"Biological sex used to select the formula coefficients.","example":"male"},{"name":"height_cm","label":"Height","type":"number","unit":"cm","required":true,"min":50,"max":300,"description":"Height in centimetres.","example":175},{"name":"waist_cm","label":"Waist circumference","type":"number","unit":"cm","required":true,"min":30,"max":300,"description":"Waist measured at the navel (men) or narrowest point (women).","example":85},{"name":"neck_cm","label":"Neck circumference","type":"number","unit":"cm","required":true,"min":15,"max":100,"description":"Neck measured just below the larynx.","example":38},{"name":"hip_cm","label":"Hip circumference","type":"number","unit":"cm","required":false,"min":30,"max":300,"description":"Hip at the widest point. Required for women, ignored for men.","example":95},{"name":"weight_kg","label":"Weight","type":"number","unit":"kg","required":false,"max":700,"exclusive_min":0,"description":"Optional body weight, used to report fat mass and lean mass in kg.","example":70}],"outputs":[{"name":"body_fat_percent","label":"Body fat","type":"number","unit":"%","decimals":1,"description":"Estimated body-fat percentage (US Navy formula)."},{"name":"category","label":"Category","type":"string","decimals":4,"description":"ACE body-fat category for the given sex."},{"name":"fat_mass_kg","label":"Fat mass","type":"number","unit":"kg","decimals":1,"description":"Body fat in kilograms (only when weight_kg is given)."},{"name":"lean_mass_kg","label":"Lean mass","type":"number","unit":"kg","decimals":1,"description":"Fat-free mass in kilograms (only when weight_kg is given)."}],"input_schema":{"type":"object","properties":{"sex":{"description":"Biological sex used to select the formula coefficients.","type":"string","enum":["male","female"],"examples":["male"]},"height_cm":{"description":"Height in centimetres. Unit: cm.","type":"number","minimum":50,"maximum":300,"examples":[175],"x-unit":"cm"},"waist_cm":{"description":"Waist measured at the navel (men) or narrowest point (women). Unit: cm.","type":"number","minimum":30,"maximum":300,"examples":[85],"x-unit":"cm"},"neck_cm":{"description":"Neck measured just below the larynx. Unit: cm.","type":"number","minimum":15,"maximum":100,"examples":[38],"x-unit":"cm"},"hip_cm":{"description":"Hip at the widest point. Required for women, ignored for men. Unit: cm.","type":"number","minimum":30,"maximum":300,"examples":[95],"x-unit":"cm"},"weight_kg":{"description":"Optional body weight, used to report fat mass and lean mass in kg. Unit: kg.","type":"number","maximum":700,"exclusiveMinimum":0,"examples":[70],"x-unit":"kg"}},"additionalProperties":false,"required":["sex","height_cm","waist_cm","neck_cm"]},"output_schema":{"type":"object","properties":{"body_fat_percent":{"description":"Estimated body-fat percentage (US Navy formula). Unit: %.","type":"number","x-unit":"%"},"category":{"description":"ACE body-fat category for the given sex.","type":"string"},"fat_mass_kg":{"description":"Body fat in kilograms (only when weight_kg is given). Unit: kg.","type":"number","x-unit":"kg"},"lean_mass_kg":{"description":"Fat-free mass in kilograms (only when weight_kg is given). Unit: kg.","type":"number","x-unit":"kg"}}},"formula":"Men: %fat = 495 / (1.0324 − 0.19077·log10(waist − neck) + 0.15456·log10(height)) − 450. Women: %fat = 495 / (1.29579 − 0.35004·log10(waist + hip − neck) + 0.22100·log10(height)) − 450. All lengths in cm.","sources":[{"name":"Hodgdon & Beckett (1984) Prediction of percent body fat for U.S. Navy men/women from body circumferences and height","url":"https://apps.dtic.mil/sti/citations/ADA143890","type":"peer_reviewed","retrieved_at":"2026-09-23"},{"name":"American Council on Exercise – Percent body fat norms","url":"https://www.acefitness.org/resources/everyone/tools-calculators/percent-body-fat-calculator/","type":"reference","retrieved_at":"2026-09-23"}],"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":"Male, 175 cm, waist 85, neck 38, 70 kg","inputs":{"sex":"male","height_cm":175,"waist_cm":85,"neck_cm":38,"weight_kg":70},"expected":{"body_fat_percent":16.9,"category":"Fitness","fat_mass_kg":11.9,"lean_mass_kg":58.1},"url":"https://tttkmbb.com/api/v1/calculate/body-fat?sex=male&height_cm=175&waist_cm=85&neck_cm=38&weight_kg=70"},{"name":"Female, 165 cm, waist 75, neck 33, hip 95","inputs":{"sex":"female","height_cm":165,"waist_cm":75,"neck_cm":33,"hip_cm":95},"expected":{"body_fat_percent":26.9,"category":"Average"},"url":"https://tttkmbb.com/api/v1/calculate/body-fat?sex=female&height_cm=165&waist_cm=75&neck_cm=33&hip_cm=95"}],"faq":[{"q":"How accurate is the US Navy method?","a":"Roughly ±3–4 percentage points versus hydrostatic weighing for most adults; less accurate at very high or very low body fat."},{"q":"Where exactly do I measure?","a":"Neck just below the larynx, sloping slightly downward; waist horizontally at the navel (men) or the narrowest point (women); hips at the widest point (women)."}],"tags":["body fat","body fat percentage","navy method","body composition","lean mass"],"related":[{"calculator_id":"lean-body-mass","reason":"Formula-based lean mass without circumference measurements."},{"calculator_id":"bmi","reason":"Compare with the simpler BMI screening measure."}],"links":{"html":"https://tttkmbb.com/health/body-fat","markdown":"https://tttkmbb.com/health/body-fat.md","json":"https://tttkmbb.com/health/body-fat.json","api":"https://tttkmbb.com/api/v1/calculate/body-fat","schema":"https://tttkmbb.com/api/v1/calculators/body-fat","openapi":"https://tttkmbb.com/openapi.json","mcp":"https://tttkmbb.com/mcp"},"version":"v1","updated_at":"2026-09-23"},"timestamp":"2026-09-23T23:32:17Z"}