{"success":true,"version":"v1","request":{"tool":"get_calculator_schema","calculator_id":"skinfold-body-fat"},"result":{"entity_type":"calculator","id":"skinfold-body-fat","calculator_id":"skinfold-body-fat","canonical_url":"https://tttkmbb.com/fitness/skinfold-body-fat","name":"Skinfold Body Fat Calculator","title":"Skinfold Body Fat Calculator – Jackson-Pollock 3-Site and 7-Site Caliper Equations with Siri Conversion","category":"fitness","category_name":"Sports & Fitness","tool_name":"estimate_body_fat_skinfold","featured_mcp_tool":false,"description":"Estimates body density and body-fat percentage from caliper skinfolds with the generalized 3-site and 7-site equations of Jackson & Pollock (1978, men) and Jackson, Pollock & Ward (1980, women), converts density to fat percentage with the Siri equation and classifies the result with ACE categories.","use_when":"You have skinfold caliper readings (chest/abdomen/thigh for men, triceps/suprailiac/thigh for women, or all seven sites) and want body-fat percentage and fat/lean mass.","do_not_use_when":"You only have tape measurements (use body-fat), the person is under 18, pregnant, or very lean or obese beyond the equations' calibration range; results are informational, not a clinical measurement.","inputs":[{"name":"sex","label":"Sex","type":"enum","required":true,"values":[{"value":"male","label":"Male","aliases":["m","man","men"]},{"value":"female","label":"Female","aliases":["f","woman","women"]}],"description":"Sex category used to select the formula coefficients.","example":"male"},{"name":"age_years","label":"Age","type":"number","unit":"years","required":true,"min":18,"max":90,"description":"Age in years (the equations were developed on adults aged 18–61 for men and 18–55 for women).","example":30},{"name":"method","label":"Equation","type":"enum","required":false,"default":"jackson_pollock_3","values":[{"value":"jackson_pollock_3","label":"Jackson-Pollock 3-site (men: chest, abdomen, thigh; women: triceps, suprailiac, thigh)","aliases":["3-site","3 site","jp3"]},{"value":"jackson_pollock_7","label":"Jackson-Pollock 7-site (chest, midaxillary, triceps, subscapular, abdomen, suprailiac, thigh)","aliases":["7-site","7 site","jp7"]}],"description":"Which generalized equation to use; the 7-site version needs all seven skinfolds.","example":"jackson_pollock_3"},{"name":"chest_mm","label":"Chest skinfold","type":"number","unit":"mm","required":false,"min":1,"max":80,"description":"Diagonal fold midway between the anterior axillary line and the nipple (men 3-site, 7-site).","example":10},{"name":"abdomen_mm","label":"Abdominal skinfold","type":"number","unit":"mm","required":false,"min":1,"max":80,"description":"Vertical fold 2 cm to the right of the umbilicus (men 3-site, 7-site).","example":20},{"name":"thigh_mm","label":"Thigh skinfold","type":"number","unit":"mm","required":false,"min":1,"max":80,"description":"Vertical fold on the front of the thigh midway between hip and knee (all equations).","example":15},{"name":"triceps_mm","label":"Triceps skinfold","type":"number","unit":"mm","required":false,"min":1,"max":80,"description":"Vertical fold on the back of the upper arm midway between shoulder and elbow (women 3-site, 7-site).","example":12},{"name":"suprailiac_mm","label":"Suprailiac skinfold","type":"number","unit":"mm","required":false,"min":1,"max":80,"description":"Diagonal fold just above the iliac crest at the anterior axillary line (women 3-site, 7-site).","example":15},{"name":"midaxillary_mm","label":"Midaxillary skinfold","type":"number","unit":"mm","required":false,"min":1,"max":80,"description":"Vertical fold on the midaxillary line at the level of the xiphoid (7-site only).","example":12},{"name":"subscapular_mm","label":"Subscapular skinfold","type":"number","unit":"mm","required":false,"min":1,"max":80,"description":"Diagonal fold just below the inferior angle of the scapula (7-site only).","example":14},{"name":"weight_kg","label":"Weight","type":"number","unit":"kg","required":false,"max":400,"exclusive_min":0,"description":"Optional body weight, used to report fat mass and lean mass.","example":80}],"outputs":[{"name":"body_density","label":"Body density","type":"number","unit":"g/cm³","decimals":5,"description":"Body density from the Jackson-Pollock equation."},{"name":"body_fat_percent","label":"Body fat","type":"number","unit":"%","decimals":1,"description":"Siri equation: 495 / density − 450."},{"name":"category","label":"Category","type":"string","decimals":4,"description":"ACE body-fat category for the given sex (men: essential 2–5, athletes 6–13, fitness 14–17, average 18–24, obese 25+; women: 10–13, 14–20, 21–24, 25–31, 32+)."},{"name":"sum_of_skinfolds_mm","label":"Sum of skinfolds","type":"number","unit":"mm","decimals":1,"description":"Sum of the sites used by the selected equation."},{"name":"sites_used","label":"Sites used","type":"string","decimals":4,"description":"Skinfold sites entered into the equation."},{"name":"fat_mass_kg","label":"Fat mass","type":"number","unit":"kg","decimals":1,"description":"weight_kg × body_fat_percent / 100 (only when weight_kg is given)."},{"name":"lean_mass_kg","label":"Lean mass","type":"number","unit":"kg","decimals":1,"description":"weight_kg − fat_mass_kg (only when weight_kg is given)."}],"input_schema":{"type":"object","properties":{"sex":{"description":"Sex category used to select the formula coefficients.","type":"string","enum":["male","female"],"examples":["male"]},"age_years":{"description":"Age in years (the equations were developed on adults aged 18–61 for men and 18–55 for women). Unit: years.","type":"number","minimum":18,"maximum":90,"examples":[30],"x-unit":"years"},"method":{"description":"Which generalized equation to use; the 7-site version needs all seven skinfolds.","type":"string","enum":["jackson_pollock_3","jackson_pollock_7"],"default":"jackson_pollock_3","examples":["jackson_pollock_3"]},"chest_mm":{"description":"Diagonal fold midway between the anterior axillary line and the nipple (men 3-site, 7-site). Unit: mm.","type":"number","minimum":1,"maximum":80,"examples":[10],"x-unit":"mm"},"abdomen_mm":{"description":"Vertical fold 2 cm to the right of the umbilicus (men 3-site, 7-site). Unit: mm.","type":"number","minimum":1,"maximum":80,"examples":[20],"x-unit":"mm"},"thigh_mm":{"description":"Vertical fold on the front of the thigh midway between hip and knee (all equations). Unit: mm.","type":"number","minimum":1,"maximum":80,"examples":[15],"x-unit":"mm"},"triceps_mm":{"description":"Vertical fold on the back of the upper arm midway between shoulder and elbow (women 3-site, 7-site). Unit: mm.","type":"number","minimum":1,"maximum":80,"examples":[12],"x-unit":"mm"},"suprailiac_mm":{"description":"Diagonal fold just above the iliac crest at the anterior axillary line (women 3-site, 7-site). Unit: mm.","type":"number","minimum":1,"maximum":80,"examples":[15],"x-unit":"mm"},"midaxillary_mm":{"description":"Vertical fold on the midaxillary line at the level of the xiphoid (7-site only). Unit: mm.","type":"number","minimum":1,"maximum":80,"examples":[12],"x-unit":"mm"},"subscapular_mm":{"description":"Diagonal fold just below the inferior angle of the scapula (7-site only). Unit: mm.","type":"number","minimum":1,"maximum":80,"examples":[14],"x-unit":"mm"},"weight_kg":{"description":"Optional body weight, used to report fat mass and lean mass. Unit: kg.","type":"number","maximum":400,"exclusiveMinimum":0,"examples":[80],"x-unit":"kg"}},"additionalProperties":false,"required":["sex","age_years"]},"output_schema":{"type":"object","properties":{"body_density":{"description":"Body density from the Jackson-Pollock equation. Unit: g/cm³.","type":"number","x-unit":"g/cm³"},"body_fat_percent":{"description":"Siri equation: 495 / density − 450. Unit: %.","type":"number","x-unit":"%"},"category":{"description":"ACE body-fat category for the given sex (men: essential 2–5, athletes 6–13, fitness 14–17, average 18–24, obese 25+; women: 10–13, 14–20, 21–24, 25–31, 32+).","type":"string"},"sum_of_skinfolds_mm":{"description":"Sum of the sites used by the selected equation. Unit: mm.","type":"number","x-unit":"mm"},"sites_used":{"description":"Skinfold sites entered into the equation.","type":"string"},"fat_mass_kg":{"description":"weight_kg × body_fat_percent / 100 (only when weight_kg is given). Unit: kg.","type":"number","x-unit":"kg"},"lean_mass_kg":{"description":"weight_kg − fat_mass_kg (only when weight_kg is given). Unit: kg.","type":"number","x-unit":"kg"}}},"formula":"S = sum of skinfolds (mm). Men 3-site: D = 1.10938 − 0.0008267·S + 0.0000016·S² − 0.0002574·age; women 3-site: D = 1.0994921 − 0.0009929·S + 0.0000023·S² − 0.0001392·age; men 7-site: D = 1.112 − 0.00043499·S + 0.00000055·S² − 0.00028826·age; women 7-site: D = 1.097 − 0.00046971·S + 0.00000056·S² − 0.00012828·age; body_fat_percent = 495 / D − 450 (Siri)","method":"Generalized equations from Jackson & Pollock (308 men, 18–61 y) and Jackson, Pollock & Ward (249 women, 18–55 y), validated against hydrostatic weighing with a standard error of about 3.5–4 % body fat. Accuracy depends on caliper technique: take each fold on the right side, twice, and use the mean. Informational, not medical advice.","sources":[{"name":"Jackson AS, Pollock ML (1978) Generalized equations for predicting body density of men, Br J Nutr 40:497-504","url":"https://pubmed.ncbi.nlm.nih.gov/718832/","type":"peer_reviewed","retrieved_at":"2026-09-24"},{"name":"Jackson AS, Pollock ML, Ward A (1980) Generalized equations for predicting body density of women, Med Sci Sports Exerc 12:175-181","url":"https://pubmed.ncbi.nlm.nih.gov/7402053/","type":"peer_reviewed","retrieved_at":"2026-09-24"},{"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":"2026-09-24"}],"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":"Male, 30 y, 3-site: chest 10, abdomen 20, thigh 15 mm, 80 kg","inputs":{"sex":"male","age_years":30,"method":"jackson_pollock_3","chest_mm":10,"abdomen_mm":20,"thigh_mm":15,"weight_kg":80},"expected":{"sum_of_skinfolds_mm":45,"body_density":1.0677,"body_fat_percent":13.6,"category":"Athletes","fat_mass_kg":10.9,"lean_mass_kg":69.1},"url":"https://tttkmbb.com/api/v1/calculate/skinfold-body-fat?sex=male&age_years=30&method=jackson_pollock_3&chest_mm=10&abdomen_mm=20&thigh_mm=15&weight_kg=80"},{"name":"Female, 28 y, 7-site (sum 98 mm)","inputs":{"sex":"female","age_years":28,"method":"jackson_pollock_7","chest_mm":8,"midaxillary_mm":10,"triceps_mm":16,"subscapular_mm":12,"abdomen_mm":18,"suprailiac_mm":14,"thigh_mm":20},"expected":{"sum_of_skinfolds_mm":98,"body_density":1.05275,"body_fat_percent":20.2,"category":"Athletes"},"url":"https://tttkmbb.com/api/v1/calculate/skinfold-body-fat?sex=female&age_years=28&method=jackson_pollock_7&chest_mm=8&midaxillary_mm=10&triceps_mm=16&subscapular_mm=12&abdomen_mm=18&suprailiac_mm=14&thigh_mm=20"}],"faq":[{"q":"Which sites do I need?","a":"Men 3-site: chest, abdomen, thigh; women 3-site: triceps, suprailiac, thigh; 7-site (both sexes): chest, midaxillary, triceps, subscapular, abdomen, suprailiac, thigh. A missing required site raises MISSING_PARAMETER."},{"q":"How accurate are skinfold estimates?","a":"About ±3.5 % body fat against hydrostatic weighing when taken by a trained tester; untrained measurement adds error, and the equations are less accurate for very lean or obese people."},{"q":"Why the Siri equation?","a":"Siri (1961) assumes fat density 0.9 and fat-free density 1.1 g/cm³; the Brozek alternative (457 / D − 414.2) differs by only a few tenths of a percent at typical densities."}],"tags":["skinfold body fat","jackson pollock","caliper body fat","3-site skinfold","7-site skinfold","body density"],"related":[{"calculator_id":"body-fat","reason":"Tape-measure (US Navy) estimate when no calipers are available."},{"calculator_id":"ffmi","reason":"Fat-free mass index from the resulting body-fat percentage."},{"calculator_id":"lean-body-mass","reason":"Formula-based lean mass without body-fat measurement."}],"links":{"html":"https://tttkmbb.com/fitness/skinfold-body-fat","markdown":"https://tttkmbb.com/fitness/skinfold-body-fat.md","json":"https://tttkmbb.com/fitness/skinfold-body-fat.json","api":"https://tttkmbb.com/api/v1/calculate/skinfold-body-fat","schema":"https://tttkmbb.com/api/v1/calculators/skinfold-body-fat","openapi":"https://tttkmbb.com/openapi.json","mcp":"https://tttkmbb.com/mcp"},"version":"v1","updated_at":"2026-09-24"},"timestamp":"2026-09-24T08:01:51Z"}