ForHosting KIT · Health & fitness

Jackson-Pollock 4-site body fat calculator

Turn four caliper skinfolds into a body fat percentage: a 30-year-old man with 12, 20, 15 and 18 mm at the triceps, abdomen, hip and thigh sums 65 mm and comes out at 19.3356% with the Jackson-Pollock four-site equation. Free in your browser.

  • Free
  • Runs in your browser
  • No sign-up
  • Your data stays on your device
Run — free How it works & API
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Run — free

Worked example

If you enter
Sex
Male
Age
30
Triceps
12
Abdomen
20
Suprailiac
15
Thigh
18
You get
Triceps mm
12
Abdomen mm
20
Suprailiac mm
15
Thigh mm
18

The Jackson-Pollock four-site skinfold method estimates body fat percentage from caliper measurements at the triceps, abdomen, suprailiac, and thigh plus chronological age and sex. This capability sums those four thicknesses in millimetres, applies the sex-specific body-density polynomial published for the four-site protocol, and converts density to percent fat with the classic Siri equation.

What the Jackson-Pollock 4-site method measures and how density becomes percent fat

Skinfold calipers estimate subcutaneous fat thickness at standardized anatomical landmarks. The Jackson-Pollock four-site protocol uses the same four sites for both sexes: the vertical triceps fold on the posterior upper arm, the vertical abdominal fold near the umbilicus, the diagonal suprailiac fold above the iliac crest, and the vertical anterior midline thigh fold. Each reading is entered in millimetres; the engine sums them into Σ and combines Σ with age in years inside a sex-specific quadratic density equation.

For males, body density equals one point one zero nine three eight minus zero point zero zero zero eight two six seven times Σ plus zero point zero zero zero zero zero one six times Σ squared minus zero point zero zero zero two five seven four times age.

For females, body density equals one point zero nine six zero nine five minus zero point zero zero zero six nine five two times Σ plus zero point zero zero zero zero zero one one times Σ squared minus zero point zero zero zero zero seven one four times age. Percent fat follows Siri: four hundred ninety-five divided by body density, minus four hundred fifty. That sequence is the entire algorithm—no lookup tables, no population percentiles, and no activity multipliers.

Treat the result as an educational and field estimate: dual-energy X-ray absorptiometry, hydrostatic weighing, and multi-compartment models remain reference methods when clinical decisions require measured composition rather than a caliper-based prediction.

What you can do with it

Field caliper logging for athletes

Enter four skinfold millimetre readings, age, and sex after a training block to get a deterministic Jackson-Pollock 4-site body-fat percent for the athlete log.

FAQ

Which skinfold sites does the Jackson-Pollock 4-site calculator use?

Triceps, abdomen, suprailiac, and thigh, each in millimetres. Send sex, age in years, and the four positive thicknesses; the engine sums them and applies the sex-specific JP4 density equation.

How is body fat percent calculated from the four sites?

Body density is computed from the sum of the four skinfolds and age with published male or female JP4 coefficients. Percent fat uses the Siri equation: (495 / body density) − 450.

What happens if a skinfold or age is zero or negative?

Age and all four skinfolds must be strictly positive finite numbers; invalid or non-positive inputs are rejected and not billed.

Is this the same as the US Navy body fat calculator?

No. The US Navy method uses height and circumference measurements. This capability implements Jackson-Pollock four-site skinfolds only and has its own SEO landing and type.

Does the four-site method work for both women and men?

Yes. Both sexes use the same four skinfold sites; sex only selects which body-density equation is applied to the sum and age before the Siri step turns density into percent fat.

Related tools

Covert Bailey body fat calculatorHealth & fitnessGallagher body fat calculatorHealth & fitnessJackson-Pollock 3-site body fat for womenHealth & fitnessSkinfold body fat calculatorHealth & fitnessRelative fat mass calculatorHealth & fitnessSkinfold sum body density calculatorHealth & fitnessParrillo 9-site body fat calculatorHealth & fitnessRMR from fat-free mass calculatorHealth & fitness

For developers — API access

Everything on this page is available programmatically. This section is for teams who want to wire it into their own systems; everyone else can just use the tool above.

API endpoint

POSThttps://api.kit.forhosting.com/health/bodyfat-jp4

Prefer to automate it? One authenticated POST creates the task; the result comes back by webhook or a signed link. The same capability also runs here on the web, by email and from Telegram — and soon from our app too.

Call it from your stack

curl -X POST https://api.kit.forhosting.com/health/bodyfat-jp4 \
  -H "Authorization: Bearer $KIT_KEY" \
  -H "Content-Type: application/json" \
  -d '{"sex":"male","age":30,"triceps":12,"abdomen":20,"suprailiac":15,"thigh":18}'

Example request

{
  "sex": "male",
  "age": 30,
  "triceps": 12,
  "abdomen": 20,
  "suprailiac": 15,
  "thigh": 18
}

Example response

{
  "task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
  "type": "health.bodyfat_jp4",
  "status": "queued",
  "_links": {
    "result": "/tasks/tsk_…/result"
  }
}

The API is asynchronous: the call returns a task_id immediately and the result arrives by webhook. Polling is capped at 1 req/s per task.

Pricing

Per request$0.002

Published price — no tokens, no invented credits. A failed task is never charged.

Errors

HTTPCodeMeaning
401unauthorizedMissing or invalid API key.
402insufficient_balanceYour balance doesn't cover the task price.
404unknown_typeThat task type doesn't exist.
429rate_limitedToo many requests. Use the webhook instead of polling.

Technical notes

The path is fully offline and deterministic: no network, no random seed, and no clock, so homework checkers, coaching apps, and prepaid API clients receive identical JSON for the same inputs every time. Non-positive age or skinfold values, missing fields, and non-finite numbers fail with invalid_input and are not billed. Use the free browser tool for field checks and lab demos, or call the API at $0.002 per successful item when pipelines need a stable Jackson-Pollock 4-site contract without re-implementing coefficients.

What the Jackson-Pollock 4-site method measures and how density becomes percent fat

The response echoes sex and age, the four site thicknesses, their sum, body density, body fat percent, the method tag jackson_pollock_4_site, a formula string, and the precision used for rounding.

Input contract, validation rules, and reading the response fields

Each skinfold must be a strictly positive finite millimetre value within the documented per-site and sum ceilings so pathological totals cannot drive density outside a usable range. Do not send inches or caliper dial units other than millimetres without converting first. POST a JSON object with required sex, age, triceps, abdomen, suprailiac, and thigh. Sex accepts male or female and common short forms such as m, f, man, and woman after case folding; aliases gender and sex_at_birth are also recognized. Age is a positive finite number of years, with aliases age_years and years. Aliases include triceps_mm, abdomen_mm or abdominal, suprailiac_mm, and thigh_mm so forms and spreadsheets can reuse familiar labels. Optional precision, with aliases decimals and dp, chooses zero through twelve decimal places for rounded outputs and defaults to four. Missing fields, empty strings, non-finite numbers, zero or negative age, and zero or negative skinfolds raise invalid_input naming the offending rule; those failures do not consume prepaid balance. On success you receive body_density with at least six decimal places of reported precision so small density changes remain visible, body_fat_percent for display and logging, sum_mm for audit trails, and method always set to jackson_pollock_4_site so downstream code can branch without sniffing shapes. Do not confuse this endpoint with the US Navy circumference body-fat calculator or with three-site or seven-site Jackson-Pollock variants: those use different landmarks and coefficients and live under separate product pages.

Where a Jackson-Pollock 4-site API earns its keep and how pricing works

Coaches and sports-science labs use a Jackson-Pollock four-site body fat calculator when athletes already follow a caliper protocol and need a reproducible density-to-percent conversion without a spreadsheet copy of the polynomials. Exercise-physiology courses assign skinfold labs and need an oracle that rejects non-positive folds instead of inventing a silent zero-percent result. Keep clinical expectations honest—this is not a diagnostic medical device—and pair caliper technique training with the numbers so site placement errors are not mistaken for formula bugs. On this page the work runs entirely in your browser; through the API each successful lookup costs $0.002, validation failures are not charged, and nothing is stored after the response returns. That split matches the rest of the KIT tier-A catalog: free interactive exploration and a priced, identical automation path. If your workflow already stores body-fat percent and only needs lean mass or basal metabolic rate, use the dedicated fat-mass or Katch-McArdle endpoints rather than re-deriving composition here.

Integration use cases

Exercise-physiology lab and homework checks

Students verify body density and Siri percent fat against textbook JP4 coefficients with stable JSON suitable for grading scripts.

Fitness apps and CI fixtures

Apps and test suites call a pure prepaid endpoint so client UIs and server goldens share the same skinfold-to-percent contract.

Technical questions

What does the Jackson-Pollock 4-site body fat calculator cost?

The browser tool is free. API requests use prepaid KIT balance at $0.002 per successful call; invalid input is not billed.

Read the full KIT documentation →