# 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.

- Calculator id: `skinfold-body-fat` · Category: Sports & Fitness (`fitness`) · Tool name: `estimate_body_fat_skinfold`
- Canonical page: https://tttkmbb.com/fitness/skinfold-body-fat · This document: https://tttkmbb.com/fitness/skinfold-body-fat.md · JSON definition: https://tttkmbb.com/fitness/skinfold-body-fat.json

## Purpose

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.

## Input

| Parameter | Type | Unit | Required | Description |
| --- | --- | --- | --- | --- |
| `sex` | enum: male \| female |  | required | Sex category used to select the formula coefficients. |
| `age_years` | number | years | required | Age in years (the equations were developed on adults aged 18–61 for men and 18–55 for women). (min 18, max 90) |
| `method` | enum: jackson_pollock_3 \| jackson_pollock_7 |  | optional, default "jackson_pollock_3" | Which generalized equation to use; the 7-site version needs all seven skinfolds. |
| `chest_mm` | number | mm | optional | Diagonal fold midway between the anterior axillary line and the nipple (men 3-site, 7-site). (min 1, max 80) |
| `abdomen_mm` | number | mm | optional | Vertical fold 2 cm to the right of the umbilicus (men 3-site, 7-site). (min 1, max 80) |
| `thigh_mm` | number | mm | optional | Vertical fold on the front of the thigh midway between hip and knee (all equations). (min 1, max 80) |
| `triceps_mm` | number | mm | optional | Vertical fold on the back of the upper arm midway between shoulder and elbow (women 3-site, 7-site). (min 1, max 80) |
| `suprailiac_mm` | number | mm | optional | Diagonal fold just above the iliac crest at the anterior axillary line (women 3-site, 7-site). (min 1, max 80) |
| `midaxillary_mm` | number | mm | optional | Vertical fold on the midaxillary line at the level of the xiphoid (7-site only). (min 1, max 80) |
| `subscapular_mm` | number | mm | optional | Diagonal fold just below the inferior angle of the scapula (7-site only). (min 1, max 80) |
| `weight_kg` | number | kg | optional | Optional body weight, used to report fat mass and lean mass. (> 0, max 400) |

## Output

| Field | 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). |

## 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.

## Data Sources

- Jackson AS, Pollock ML (1978) Generalized equations for predicting body density of men, Br J Nutr 40:497-504 — https://pubmed.ncbi.nlm.nih.gov/718832/ (peer_reviewed, retrieved 2026-09-24)
- Jackson AS, Pollock ML, Ward A (1980) Generalized equations for predicting body density of women, Med Sci Sports Exerc 12:175-181 — https://pubmed.ncbi.nlm.nih.gov/7402053/ (peer_reviewed, retrieved 2026-09-24)
- American Council on Exercise – Percent body fat norms — https://www.acefitness.org/resources/everyone/tools-calculators/percent-body-fat-calculator/ (reference, retrieved 2026-09-24)

Data freshness: `static`. Deterministic formula with fixed constants; results never go stale. Inputs supplied by the caller determine the output.

## API

- `GET https://tttkmbb.com/api/v1/calculate/skinfold-body-fat?sex=…&age_years=…`
- `POST https://tttkmbb.com/api/v1/calculate/skinfold-body-fat` with JSON body `{"inputs": {…}}`
- Response: unified envelope (`success`, `request`, `result.values`, `result.units`, `sources`, `freshness`, `timestamp`, `next_actions`, `links`); see https://tttkmbb.com/docs/response-format.md
- Schema: https://tttkmbb.com/api/v1/calculators/skinfold-body-fat · OpenAPI operationId `estimate_body_fat_skinfold` in https://tttkmbb.com/openapi.json
- Authentication: none. Rate limit: fair use, see https://tttkmbb.com/docs/rate-limits.md.

## MCP

- Server: `https://tttkmbb.com/mcp` (Streamable HTTP, JSON-RPC 2.0, no auth)
- Tool:  `run_calculator` with `{"calculator_id": "skinfold-body-fat", "inputs": {…}}`

## Example

- 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}` → `{"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): 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}` → `{"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
```

## Limitations

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. 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. All values are computed from the formula above; no measurement or live data is involved.

## 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

- [Body Fat Calculator](https://tttkmbb.com/health/body-fat.md) — Tape-measure (US Navy) estimate when no calipers are available.
- [FFMI Calculator](https://tttkmbb.com/fitness/ffmi.md) — Fat-free mass index from the resulting body-fat percentage.
- [Lean Body Mass Calculator](https://tttkmbb.com/health/lean-body-mass.md) — Formula-based lean mass without body-fat measurement.