Body surface area calculator
Get body surface area with the Boyd formula from 1935: someone 170 cm tall weighing 70 kg has 1.834670289827 m². Its mass exponent shifts with weight, so results differ slightly from other formulas. Centimetres, metres, inches, kilograms or pounds all work. Free in your browser.
- Free
- Runs in your browser
- No sign-up
- Your data stays on your device
Run — free
Worked example
- Height
- 170
- Weight
- 70
- Height unit
- cm
- Weight unit
- kg
- Height (cm)
- 170 m2
- Weight (kg)
- 70
- Weight g
- 70,000
- Bsa
- 1.8346702898
A Boyd body surface area calculator turns standing height and body weight into the square-metre estimate used in paediatric dosing literature, burn assessment teaching, and physiological scaling when a weight-dependent exponent is preferred over a simple square root. Enter height and weight (centimetres and kilograms by default, or metres, inches, and pounds with unit labels); the tool converts inputs when needed, applies the classic Boyd expression with height in centimetres and mass in grams, and returns BSA in m² with normalized centimetres and kilograms, the formula name, equation year, and ordered substitution steps.
What Boyd body surface area means
Body surface area is a geometric estimate of the outer surface of the human body expressed in square metres. The Boyd formula, published in nineteen thirty-five, uses a constant times height in centimetres raised to three tenths times body mass in grams raised to a weight-dependent exponent: zero point seven two eight five minus zero point zero one eight eight times the base-ten logarithm of weight in grams.
Equivalently, after converting kilograms to grams by multiplying by one thousand, the solver evaluates that single real number as a scaling factor for paediatric chemotherapy teaching examples, cardiac index denominators, and burn-area percentages when absolute body size matters more than body-mass index alone. Unlike BMI, which is mass over height squared and reports density-like risk bands, Boyd BSA grows with both height and weight under a logarithmic mass adjustment and is reported in square metres rather than kilograms per square metre.
Teachers and automated graders care about the non-positive rejection as much as the successful numeric path: a student who enters zero kilograms has left the domain of the formula because the logarithm of non-positive mass is undefined, and a silent fallback to a default adult would hide that mistake. The check path recomputes the log, exponent, powers, and product with the same fixed rounding used for published fields, so a successful response always ships with steps you can assert in tests or worksheets.
Classic adult reference values near one hundred seventy centimetres and seventy kilograms produce roughly one point eight three five square metres under Boyd, close to but not identical with Mosteller or Du Bois on the same anthropometrics, which is exactly why separate landings exist for each named formula.
What you can do with it
Pharmacy and nursing education
Students enter height and weight from case vignettes and compare hand-written Boyd BSA against a deterministic m² result before dose-scaling exercises that cite the 1935 formula.
Pre-flight before mg/m² workflows
Pipelines compute Boyd BSA first, then branch to drug-specific dosing only when height and weight pass positive validation and the published formula name is Boyd.
FAQ
What is the Boyd body surface area formula?
Height and weight must be positive; the result is reported in square metres.
What happens if height or weight is missing, non-numeric, zero, or negative?
The geometric model is undefined (and log of non-positive mass is undefined).
Which units are accepted?
Height defaults to centimetres (also m or in). Weight defaults to kilograms (also lb). Values are converted to cm and kg, then weight to grams, before the formula.
Is this the same as Mosteller or Du Bois BSA?
No. This capability implements only the Boyd 1935 formula. Mosteller and Du Bois use different expressions and coefficients and have their own calculators.
Is the Boyd body surface area calculator free?
The browser tool is free and runs locally.
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/bsa-boyd \
-H "Authorization: Bearer $KIT_KEY" \
-H "Content-Type: application/json" \
-d '{"height":170,"weight":70}'const res = await fetch("https://api.kit.forhosting.com/health/bsa-boyd", {
method: "POST",
headers: {
"Authorization": `Bearer ${process.env.KIT_KEY}`,
"Content-Type": "application/json"
},
body: JSON.stringify({
"height": 170,
"weight": 70
})
});
const { task_id } = await res.json();import os, requests
res = requests.post(
"https://api.kit.forhosting.com/health/bsa-boyd",
headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
json={
"height": 170,
"weight": 70
},
)
task_id = res.json()["task_id"]<?php
$res = file_get_contents("https://api.kit.forhosting.com/health/bsa-boyd", false, stream_context_create([
"http" => [
"method" => "POST",
"header" => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
"content" => '{"height":170,"weight":70}',
],
]));
$task = json_decode($res, true);body := bytes.NewBufferString(`{"height":170,"weight":70}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/health/bsa-boyd", 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": 170,
"weight": 70
}Example response
{
"task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
"type": "health.bsa_boyd",
"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_weight_kg | 500 |
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
Missing fields, non-numeric strings, and non-positive height or weight are rejected with invalid_input so incomplete forms never invent a surface area.
How to call the API and what comes back
Send a JSON body with two required finite positive numbers: height (aliases height_cm, h, stature, ht) and weight (aliases weight_kg, w, mass, wt). Optional height_unit defaults to cm and accepts cm, m, or in; optional weight_unit defaults to kg and accepts kg or lb. After conversion, height must stay within three hundred centimetres and weight within five hundred kilograms so intermediate powers remain finite on the Worker and in the browser. On success the response echoes rounded height and weight as submitted, height_unit and weight_unit as normalized labels, height_cm and weight_kg after conversion, weight_g as kilograms times one thousand, bsa and bsa_m2 as the same rounded square-metre value, unit as the string m2, formula as the Boyd expression string, formula_name as Boyd, equation_year as nineteen thirty-five, message as a short English summary, and steps as an ordered array of substitution lines including log ten of mass and the weight exponent. Failed validation—missing fields, non-finite numbers, non-positive height or weight, unsupported unit labels, or values outside absolute limits after conversion—returns invalid_input and is never charged. Pricing is a flat $0.002 per successful request with unit item and zero marginal unit price.
Practical tips, limits, and related searches
Keep anthropometrics inside the published absolute range after conversion so the weight-dependent exponent and the two power terms cannot overflow into a non-finite BSA that the solver must reject. Prefer sending numbers rather than pre-formatted strings when you control the client; string numbers are accepted only when they match a strict finite numeric pattern. Prefer this capability when you already have height and weight and only need the Boyd estimate; if your source is a word problem, extract the two anthropometrics and their units first. When teaching, pair examples: one hundred seventy centimetres and seventy kilograms yields approximately one point eight three five m² under Boyd; one hundred sixty centimetres and fifty-five kilograms yields a smaller adult value; deliberate zero weight or negative height demonstrates the hard invalid_input path; and a five-foot-seven adult entered as sixty-seven inches with one hundred fifty-four pounds shows unit conversion into the same centimetre-kilogram-to-gram pipeline. This landing targets people searching for a body surface area calculator, Boyd formula, or BSA from height and weight with the nineteen thirty-five expression; it is not a Mosteller or Du Bois calculator, not a BMI tool, and not a clinical dosing system that applies milligrams per square metre to a drug. Failed tasks that return invalid_input do not consume prepaid balance, which keeps exploratory scripts cheap when learners deliberately set weight to zero. Round-trip checks are easy: recompute zero point zero zero zero three two zero seven times height to the three-tenths times grams raised to the Boyd exponent by hand and confirm it equals the published bsa within the same twelve-decimal rounding, and confirm that changing only the sign of an input flips the response from success to invalid_input.
Integration use cases
Unit tests for clinical calculators
Generate known-good adult and paediatric Boyd BSA fixtures plus deliberate zero, missing, or non-numeric rejections for coding kata expected values.