Deurenberg body fat from BMI
Turn a BMI you already know into a body fat estimate: a 35-year-old man with a BMI of 24.5 comes out at 21.25% body fat under the 1991 Deurenberg equation. Age and sex are the only other inputs. Free in your browser.
- Free
- Runs in your browser
- No sign-up
- Your data stays on your device
Run — free
Worked example
- Bmi
- 24.5
- Age
- 35
- Sex
- male
- Age (years)
- 35
- Sex code
- 1
- Body fat percent
- 21.25
- Precision
- 2
This Deurenberg body fat calculator estimates adult body fat percentage from three everyday inputs: body mass index in kilograms per square metre, age in years, and biological sex. The classic 1991 Deurenberg equation applies published coefficients so you get a reproducible BF% without calipers, DEXA bookings, or proprietary apps—only BMI, age, and a male or female sex term. A typical check might use BMI twenty-four point five, age thirty-five, and male sex, yielding a mid-teens body-fat estimate under the linear formula that adds BMI and age contributions then subtracts the male offset and intercept. The engine is fully deterministic: no network calls, no random sampling, and no date-dependent shortcuts.
What the Deurenberg equation estimates and what you get
Body mass index alone does not equal body fat percentage, yet population studies showed that BMI combined with age and sex can approximate percent fat for many adults. Deurenberg and colleagues published a simple linear predictor in nineteen ninety-one: body fat percent equals one point two zero times BMI plus zero point two three times age in years, minus ten point eight times a sex code, minus five point four.
The sex code is one for males and zero for females, reflecting average differences in lean mass and fat distribution between the sexes at a given BMI and age.
Where Deurenberg body fat estimates fit in practice and software
Coaches and athletes use Deurenberg estimates when only scale weight, height-derived BMI, age, and sex are available, when comparing rough composition trends without calipers, or when explaining why two people with similar BMI can differ once age and sex enter the equation. Students meet this formula in exercise physiology, nutrition, and public-health coursework when textbooks introduce BMI-based fat predictors before laboratory techniques.
Product teams call the same contract from scripts to seed deterministic test vectors for health dashboards, to normalize user-entered BMI before composition charts, or to satisfy programming-challenge statements that demand a Deurenberg result rather than a black-box app. Treat the output as an educational and planning estimate: hydration, athletic build, ethnicity, and extreme BMI ranges affect how well a population equation matches an individual, and clinical or research decisions should prefer multi-method body composition and a qualified professional when accuracy matters.
Pair this tool with BMI, fat-mass, or calorie helpers when you need related anthropometrics, keep units honest, and never mix pound-inch inputs without converting BMI to kilograms per square metre first.
What you can do with it
Quick composition from BMI logs
Estimate adult body fat when you already have BMI, age, and sex and want the published Deurenberg linear formula without calipers or a clinic visit.
Exercise physiology and nutrition labs
Let students compute Deurenberg BF% from textbook BMI-age-sex examples and compare against circumference or skinfold methods in the same lab report.
FAQ
What does the Deurenberg body fat calculator cost?
It is free in your browser on this page.
What happens if BMI, age, or sex is missing?
BMI and age must be finite positive numbers; sex must resolve to male or female (or 1/0).
Does this replace DEXA or the US Navy method?
No. It is a BMI-age-sex population estimate. Use laboratory composition or circumference protocols when those methods better match your workflow.
What units should I use?
BMI in kg/m² and age in years. Compute BMI from kilograms and metres first if you only have weight and height in other units.
Which formula does the Deurenberg calculator use?
Body fat percent equals 1.20 times BMI plus 0.23 times age, minus 10.8 times the sex code, minus 5.4. The sex code is 1 for men and 0 for women.
Related tools
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
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-deurenberg \
-H "Authorization: Bearer $KIT_KEY" \
-H "Content-Type: application/json" \
-d '{"bmi":24.5,"age":35,"sex":"male"}'const res = await fetch("https://api.kit.forhosting.com/health/bodyfat-deurenberg", {
method: "POST",
headers: {
"Authorization": `Bearer ${process.env.KIT_KEY}`,
"Content-Type": "application/json"
},
body: JSON.stringify({
"bmi": 24.5,
"age": 35,
"sex": "male"
})
});
const { task_id } = await res.json();import os, requests
res = requests.post(
"https://api.kit.forhosting.com/health/bodyfat-deurenberg",
headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
json={
"bmi": 24.5,
"age": 35,
"sex": "male"
},
)
task_id = res.json()["task_id"]<?php
$res = file_get_contents("https://api.kit.forhosting.com/health/bodyfat-deurenberg", false, stream_context_create([
"http" => [
"method" => "POST",
"header" => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
"content" => '{"bmi":24.5,"age":35,"sex":"male"}',
],
]));
$task = json_decode($res, true);body := bytes.NewBufferString(`{"bmi":24.5,"age":35,"sex":"male"}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/health/bodyfat-deurenberg", body)
req.Header.Set("Authorization", "Bearer "+os.Getenv("KIT_KEY"))
req.Header.Set("Content-Type", "application/json")
res, _ := http.DefaultClient.Do(req)Example request
{
"bmi": 24.5,
"age": 35,
"sex": "male"
}Example response
{
"task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
"type": "health.bodyfat_deurenberg",
"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
Published price — no tokens, no invented credits. A failed task is never charged.
Errors
| HTTP | Code | Meaning |
|---|---|---|
401 | unauthorized | Missing or invalid API key. |
402 | insufficient_balance | Your balance doesn't cover the task price. |
404 | unknown_type | That task type doesn't exist. |
429 | rate_limited | Too many requests. Use the webhook instead of polling. |
Technical notes
What the Deurenberg equation measures and what the response contains
This calculator implements that exact adult formula and returns the normalized BMI, age in years, resolved sex label, numeric sex_code, body_fat_percent, the method tag deurenberg_1991, a human-readable formula string, and the precision applied to rounded fields. There is no skinfold site list, no circumference protocol, no ethnicity-specific coefficient table, and no clinical diagnosis in this contract: the engine either computes a finite fat percent from positive BMI and age plus a recognized sex value, or rejects the request with invalid_input.
How inputs are read, validated, and kept reproducible
Sex accepts male, man, m, or numeric one for the male coefficient, and female, woman, f, or numeric zero for the female coefficient; mixed case and surrounding whitespace are normalized before matching. Optional leading plus signs and scientific notation on numeric strings are allowed when they parse as finite decimals; embedded spaces, hex prefixes, and non-numeric tokens are rejected with a clear message so a bad cell does not become a wrong fat percentage. Computation multiplies the published coefficients, forms BF%, and rounds without Monte Carlo sampling, external tables, or network I/O. The preferred fields are bmi for kilograms per square metre, age for years, and sex for male or female, with aliases body_mass_index and bmi_value for BMI; age_years or years for age; and gender or sex_code for sex. BMI and age must be strictly positive: zero, negative, missing, empty, or non-finite values fail with invalid_input and a message that names the offending field. An optional precision field, with aliases decimals and dp, chooses how many decimal places to expose for the numeric outputs, defaulting to two and capped at fifteen so JSON goldens stay stable across runtimes. Pair the free browser run on this page with the priced API when you need the identical mathematical result inside an automated pipeline without re-implementing Deurenberg coefficients yourself.
Where Deurenberg body fat estimates fit in practice and software
On this page the work runs entirely in your browser; through the API each successful lookup costs $0.002, failed validation is not charged, and nothing is stored after the response returns. That split—free interactive exploration and a priced, identical API—matches the rest of the KIT tier-A catalog and lets you move from a classroom demo to an automated pipeline without changing the mathematical result.
Integration use cases
Deterministic health fixtures
Generate reproducible body_fat_percent fields for unit tests, dashboards, and CI assertions on fixed BMI, age, and sex inputs.
Technical questions
Which formula is used?
Deurenberg 1991 adult equation: BF% = 1.20 × BMI + 0.23 × age − 10.8 × sex − 5.4, with sex = 1 for males and 0 for females.