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Skinfold Body Fat Calculator
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.
When to use
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.
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)
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.
Inputs
| Parameter | Type | Unit | Required | Description |
|---|---|---|---|---|
sex | enum: male | female | yes | Sex category used to select the formula coefficients. | |
age_years | number | years | yes | Age in years (the equations were developed on adults aged 18–61 for men and 18–55 for women). Range: ≥ 18, ≤ 90 |
method | enum: jackson_pollock_3 | jackson_pollock_7 | default jackson_pollock_3 | Which generalized equation to use; the 7-site version needs all seven skinfolds. | |
chest_mm | number | mm | no | Diagonal fold midway between the anterior axillary line and the nipple (men 3-site, 7-site). Range: ≥ 1, ≤ 80 |
abdomen_mm | number | mm | no | Vertical fold 2 cm to the right of the umbilicus (men 3-site, 7-site). Range: ≥ 1, ≤ 80 |
thigh_mm | number | mm | no | Vertical fold on the front of the thigh midway between hip and knee (all equations). Range: ≥ 1, ≤ 80 |
triceps_mm | number | mm | no | Vertical fold on the back of the upper arm midway between shoulder and elbow (women 3-site, 7-site). Range: ≥ 1, ≤ 80 |
suprailiac_mm | number | mm | no | Diagonal fold just above the iliac crest at the anterior axillary line (women 3-site, 7-site). Range: ≥ 1, ≤ 80 |
midaxillary_mm | number | mm | no | Vertical fold on the midaxillary line at the level of the xiphoid (7-site only). Range: ≥ 1, ≤ 80 |
subscapular_mm | number | mm | no | Diagonal fold just below the inferior angle of the scapula (7-site only). Range: ≥ 1, ≤ 80 |
weight_kg | number | kg | no | Optional body weight, used to report fat mass and lean mass. Range: > 0, ≤ 400 |
Outputs
| Output | Type | Unit | Description |
|---|---|---|---|
body_density | number | g/cm³ | Body density from the Jackson-Pollock equation. |
body_fat_percent | number | % | Siri equation: 495 / density − 450. |
category | string | 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+). | |
sum_of_skinfolds_mm | number | mm | Sum of the sites used by the selected equation. |
sites_used | string | Skinfold sites entered into the equation. | |
fat_mass_kg | number | kg | weight_kg × body_fat_percent / 100 (only when weight_kg is given). |
lean_mass_kg | number | kg | weight_kg − fat_mass_kg (only when weight_kg is given). |
Example
Male, 30 y, 3-site: chest 10, abdomen 20, thigh 15 mm, 80 kg: {"sex":"male","age_years":30,"method":"jackson_pollock_3","chest_mm":10,"abdomen_mm":20,"thigh_mm":15,"weight_kg":80} → {"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}
Female, 28 y, 7-site (sum 98 mm): {"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} → {"sum_of_skinfolds_mm":98,"body_density":1.05275,"body_fat_percent":20.2,"category":"Athletes"}
GET 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
Machine access
- API:
GET https://tttkmbb.com/api/v1/calculate/skinfold-body-fat(query parameters) orPOSTwith a JSON body{"inputs": {...}} - Schema: https://tttkmbb.com/api/v1/calculators/skinfold-body-fat · Markdown: https://tttkmbb.com/fitness/skinfold-body-fat.md · JSON definition: https://tttkmbb.com/fitness/skinfold-body-fat.json
- MCP: server
https://tttkmbb.com/mcp, toolrun_calculator with calculator_id="skinfold-body-fat" - OpenAPI operationId:
estimate_body_fat_skinfold - Freshness:
static. Authentication: none. Rate limit: fair use (see rate limits).
Sources
- Jackson AS, Pollock ML (1978) Generalized equations for predicting body density of men, Br J Nutr 40:497-504 (peer_reviewed)
- Jackson AS, Pollock ML, Ward A (1980) Generalized equations for predicting body density of women, Med Sci Sports Exerc 12:175-181 (peer_reviewed)
- American Council on Exercise – Percent body fat norms (reference)
FAQ
Which sites do I need?
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.
How accurate are skinfold estimates?
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.
Why the Siri equation?
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.
Related calculators
- Body Fat Calculator — Tape-measure (US Navy) estimate when no calipers are available.
- FFMI Calculator — Fat-free mass index from the resulting body-fat percentage.
- Lean Body Mass Calculator — Formula-based lean mass without body-fat measurement.