Body frame size calculator
Classifies your skeletal frame as small, medium or large from height and wrist size: a 175 cm man with a 17.5 cm wrist has a ratio of 10, which is medium. It describes bone structure, not fat or muscle. Free in your browser.
- Free
- Runs in your browser
- No sign-up
- Your data stays on your device
Run — free
Worked example
- Height
- 175
- Wrist circumference in centimetres
- 17.5
- Sex
- Male
- Height (cm)
- 175
- Wrist cm
- 17.5
- R index
- 10
- Frame size
- medium
This body frame size calculator classifies skeletal frame as small, medium, or large from standing height in centimetres, wrist circumference in centimetres, and sex using the classic height-to-wrist r-index. Frame size is a simple anthropometric category that describes bone structure rather than fat or muscle: two people with the same height and weight can carry mass differently when one has a slender wrist and the other a broad one. The engine computes r equals height divided by wrist circumference, then applies sex-specific cutoffs that are fully deterministic and free of network calls, sampling, or date-dependent shortcuts. A typical check is a one hundred seventy-five centimetre man with a sixteen point five centimetre wrist, which lands in the medium band for males.
What frame size means and what you get
Body frame size is a three-way skeletal category—small, medium, or large—derived from how thick the wrist is relative to stature. Clinicians and coaches have long used wrist circumference because it is easy to measure with a tape just distal to the styloid processes, changes little with short-term fat gain or loss, and correlates with bone breadth better than mid-arm or calf girth alone.
This calculator does not estimate body fat percentage, lean mass, or ideal weight; it only answers whether the height-to-wrist ratio places the subject in the small, medium, or large band for the stated sex. Wrist circumference of zero is always rejected because division by zero is undefined and a zero wrist is not a valid measurement.
Where frame size fits in fitness, education, and software
Athletes and coaches use frame size when interpreting body weight against bone structure, when explaining why two teammates of equal height can look leaner or denser on the same scale weight, or when logging anthropometrics that should not flip every time body fat changes. Students meet height-to-wrist charts in exercise physiology and nursing coursework when textbooks introduce skeletal frame before more expensive lab methods.
Product teams call the same contract from scripts to seed deterministic test vectors for health dashboards, to normalize user-entered height and wrist before frame badges, or to satisfy programming-challenge statements that demand a small-medium-large label rather than a black-box app. Watch and bracelet retailers sometimes use frame or wrist bands as a rough sizing hint; this tool is educational and planning oriented, not a substitute for a measured wrist size chart from a manufacturer.
Treat the output as an anthropometric category, not a clinical diagnosis: if medical or research accuracy is required, prefer a measured protocol and a qualified professional. Pair this tool with lean-body-mass or BMI helpers when you need related size metrics, and keep units honest so centimetres are never mixed with inches by accident.
What you can do with it
Fitness and anthropometry logs
Classify small, medium, or large frame from height and wrist when logging bone structure that should not change with short-term fat loss.
Teaching and physiology demos
Show students how the height-to-wrist ratio maps stature and bone breadth to a three-way frame category.
FAQ
What does the body frame size calculator cost?
It is free in your browser on this page.
How is frame size calculated?
For males: small if r > 10.4, medium if 9.6 ≤ r ≤ 10.4, large if r < 9.6. For females: small if r > 11.0, medium if 10.1 ≤ r ≤ 11.0, large if r < 10.1.
What happens if wrist circumference is zero?
Wrist circumference must be strictly positive; a zero wrist makes the r-index undefined.
Does this estimate body fat or ideal weight?
No. It only classifies skeletal frame as small, medium, or large from height, wrist, and sex. Use dedicated tools for fat mass, BMI, or target weight.
Which units should I use?
Height and wrist circumference are in centimetres (cm). Measure the wrist just distal to the styloid processes with a flexible tape for a reproducible reading.
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/frame-size \
-H "Authorization: Bearer $KIT_KEY" \
-H "Content-Type: application/json" \
-d '{"height":175,"wrist":17.5,"sex":"male"}'const res = await fetch("https://api.kit.forhosting.com/health/frame-size", {
method: "POST",
headers: {
"Authorization": `Bearer ${process.env.KIT_KEY}`,
"Content-Type": "application/json"
},
body: JSON.stringify({
"height": 175,
"wrist": 17.5,
"sex": "male"
})
});
const { task_id } = await res.json();import os, requests
res = requests.post(
"https://api.kit.forhosting.com/health/frame-size",
headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
json={
"height": 175,
"wrist": 17.5,
"sex": "male"
},
)
task_id = res.json()["task_id"]<?php
$res = file_get_contents("https://api.kit.forhosting.com/health/frame-size", false, stream_context_create([
"http" => [
"method" => "POST",
"header" => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
"content" => '{"height":175,"wrist":17.5,"sex":"male"}',
],
]));
$task = json_decode($res, true);body := bytes.NewBufferString(`{"height":175,"wrist":17.5,"sex":"male"}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/health/frame-size", body)
req.Header.Set("Authorization", "Bearer "+os.Getenv("KIT_KEY"))
req.Header.Set("Content-Type", "application/json")
res, _ := http.DefaultClient.Do(req)Example request
{
"height": 175,
"wrist": 17.5,
"sex": "male"
}Example response
{
"task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
"type": "health.frame_size",
"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.
Limits
max_height_cm | 300 |
max_wrist_cm | 50 |
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
Use it free in the browser for fitness logs, clothing and watch fit notes, or physiology homework, or call the API at $0.002 per successful request when you need reproducible frame labels in apps, spreadsheets, or teaching demos that must not invent categories.
What body frame size means and what the response contains
The response returns height_cm and wrist_cm as normalized inputs, the resolved sex used for cutoffs, the continuous r_index equal to height over wrist, the categorical frame_size string, the method tag height_wrist_r_index, a human-readable formula string that restates the cutoffs, the numeric high and low cutoff pair for that sex, and the precision applied to rounded fields. There is no age term, no ethnicity table, and no population percentile in this contract: the engine either classifies a finite positive pair of anthropometrics or rejects the request with invalid_input.
How height, wrist, and sex are read and how labels stay reproducible
Sex accepts male, female, m, f, man, woman, and their plural forms after case folding so forms and spreadsheets can reuse the same contract. 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 frame label. Height and wrist must be strictly positive and within the documented ceilings of three hundred centimetres and fifty centimetres. Computation divides height by wrist once, compares r against the published high and low thresholds for the chosen sex, and returns the category without Monte Carlo sampling or external lookup tables. The preferred fields are height for centimetres, wrist for wrist circumference in centimetres, and sex for the cutoff table, with aliases height_cm, h, and stature for height; wrist_cm, wrist_circumference, and circumference for the wrist; and gender or sex_at_birth for sex. Zero, negative, or non-finite wrist values fail with invalid_input and a message that names the wrist field and the zero or non-positive rule, matching the product requirement that wrist circumference of zero is an error. An optional precision field, with aliases decimals and dp, chooses how many decimal places to expose for height_cm, wrist_cm, and r_index, defaulting to four and capped at fifteen so JSON goldens stay stable. Pair the free browser run on this page with the priced API when you need the identical classification inside an automated pipeline without re-implementing the r-index cutoffs yourself.
Where frame size fits in fitness, education, 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 test fixtures
Generate reproducible r_index and frame_size labels for unit tests, health dashboards, and CI assertions.