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Bra size calculator

An 80 cm underbust with a 94 cm bust is a US 32DDD, a UK 32E and a European 80B. Enter both tape readings in centimetres or inches to get band and cup in all three systems, plus sister sizes. Free in your browser.

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Run — free How it works & API

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Worked example

If you enter

Underbust: 80
Bust: 94

You get

Underbust cm: 80 cm
Bust cm: 94
Underbust in: 31.4960629921
Bust in: 37.0078740157

Finding a bra size from a tape measure should be arithmetic, not folklore. This bra size calculator takes the two readings that actually determine the garment: the snug underbust around the ribcage and the fullest bust measurement, both in centimetres or inches. It converts those numbers with the exact 2.54 centimetre inch, places the US and UK band on the nearest even inch of the underbust, places the European band on the nearest five centimetres, and reads the cup from the raw difference — one inch per US or UK letter, two centimetres per European letter.

How band and cup are computed from two tape readings

A wearable bra size is a band plus a cup letter. The band tracks the ribcage, so this calculator never adds the vintage four or five inches that older shop charts used to inflate the underbust; it uses the true-to-measure rule and takes the nearest even inch of the underbust as the US and UK band and the nearest multiple of five centimetres as the European band. The cup is not taken from the rounded band.

It is taken from the raw difference between the fullest bust and the snug underbust, which is the volume the cup has to hold. On the US and UK inch ladder each inch of that difference is one letter: zero inches is AA, one inch is A, two is B, three is C, four is D, then the regional letter spellings diverge (US DD and DDD, UK DD and E, and so on through M).

On the European centimetre ladder each two centimetres is one letter and ten centimetres maps to AA, so a fourteen centimetre difference is a European B even when the same pair of readings is a US DDD. That mismatch is not a bug: the two systems really do step the cup at different physical intervals, and publishing both labels is more honest than forcing one index onto every region.

Sister sizes keep cup volume while moving the band two inches: one cup up with a smaller band, one cup down with a larger band, omitted when the letter would leave the AA-through-M table. Measure the underbust firmly on an exhale, standing, tape level; measure the bust at the fullest point without cinching. Use the same unit for both numbers, and send unit as in when the tape is marked in inches — the default unit is centimetres.

Where a measurement bra size calculator earns its keep

Fitting rooms, lingerie boutiques, and mail-order size finders need a single deterministic answer from two tape readings so staff and customers are not arguing over a plus-four chart on one wall and a centimetre chart on the other. Wardrobe and inventory apps can store the returned US, UK, and EU labels next to a profile and later convert an already-known size with a dedicated converter when the shopper changes region.

Exam and tutorial authors who teach clothing math can pin the worked eighty-by-ninety-four centimetre case and the thirty-two-by-thirty-six inch case in tests without standing up a spreadsheet. Failed validation does not consume balance. Use a size-system converter when the input is already a label such as 34C; use this calculator when the input is two tape numbers.

What you can do with it

Home or fitting-room tape check

Enter the snug underbust and the fullest bust in centimetres or inches and read the US, UK, and EU size plus sister sizes before buying or altering a bra.

Storefront and inventory profiles

Capture two measurements in a size-finder form and store the returned size, band, cup, and regional labels on a customer profile for later conversion or restock.

FAQ

How do I calculate bra size from underbust and bust?

Send the snug underbust and the fullest bust in the same unit (cm by default, or in).

Does this use the old plus-four band method?

No. The band is true-to-measure: nearest even inch of the underbust for US and UK, nearest 5 cm for EU. Cup letters come from the raw bust-minus-underbust difference.

Why can the US cup and the EU cup disagree on the same tape readings?

US and UK cups step by one inch; EU cups step by two centimetres. The calculator reports each system's native ladder instead of forcing a single index onto every region.

What are sister sizes?

They are sizes that keep the same cup volume while the band moves two inches: one cup up with a smaller band, or one cup down with a larger band. For a US 32DDD, the sister sizes are 30G and 34DD.

How should I take the two measurements?

Measure the underbust firmly around the ribcage on an exhale, standing, with the tape level. Measure the bust at the fullest point without pulling the tape tight, and use the same unit for both readings.

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

POSThttps://api.kit.forhosting.com/life/bra-size

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/life/bra-size \
  -H "Authorization: Bearer $KIT_KEY" \
  -H "Content-Type: application/json" \
  -d '{"underbust":80,"bust":94}'

const res = await fetch("https://api.kit.forhosting.com/life/bra-size", {
  method: "POST",
  headers: {
    "Authorization": `Bearer ${process.env.KIT_KEY}`,
    "Content-Type": "application/json"
  },
  body: JSON.stringify({
    "underbust": 80,
    "bust": 94
  })
});
const { task_id } = await res.json();

import os, requests

res = requests.post(
    "https://api.kit.forhosting.com/life/bra-size",
    headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
    json={
    "underbust": 80,
    "bust": 94
},
)
task_id = res.json()["task_id"]

<?php
$res = file_get_contents("https://api.kit.forhosting.com/life/bra-size", false, stream_context_create([
  "http" => [
    "method"  => "POST",
    "header"  => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
    "content" => '{"underbust":80,"bust":94}',
  ],
]));
$task = json_decode($res, true);

body := bytes.NewBufferString(`{"underbust":80,"bust":94}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/life/bra-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

{
  "underbust": 80,
  "bust": 94
}

Example response

{
  "task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
  "type": "life.bra_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

Per request$0.002

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

Limits

`min_underbust_cm`	50
`max_underbust_cm`	150
`min_bust_cm`	50
`max_bust_cm`	180
`min_underbust_in`	20
`max_underbust_in`	60
`min_bust_in`	20
`max_bust_in`	70
`max_cup_index`	13

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

When the bust reading is smaller than the underbust the task fails with invalid_input instead of inventing a negative cup.

Input contract, response shape, and error behaviour

Both readings must share that unit. Underbust must lie in fifty to one hundred fifty centimetres or twenty to sixty inches; bust must lie in fifty to one hundred eighty centimetres or twenty to seventy inches. Numbers are rounded to ten decimal places so goldens stay stable across runtimes. POST a JSON body with two required finite positive numbers: underbust, the ribcage reading, and bust, the fullest chest reading. Optional unit is the closed set cm or in and defaults to cm; aliases centimetre, inches, and the like are accepted in the solver. Field aliases under_bust, ribcage, and band_measurement map onto underbust; overbust, chest, and over_bust map onto bust. Missing fields, empty strings, non-finite values, non-positive measurements, unknown units, and values outside those ranges fail with invalid_input naming the field. On success the body echoes the inputs, reports both centimetre and inch conversions, the centimetre and inch differences, the primary US size as size plus band and cup, the UK and EU labels, the two cup indices, sister_sizes as a short US list, method true_to_measure, and a formula string restating the rounding rules. Pair the response with your own brand chart if you need a specific maker's vanity sizing; this endpoint returns the measurement size only and does not emit product recommendations or fit prose beyond the formula field.

Integration use cases

Tutorials and clothing-math fixtures

Pin deterministic band and cup results for known measurement pairs in lessons, goldens, and LMS checks without a spreadsheet or a vintage plus-four chart.

Technical questions

What happens if the bust measurement is smaller than the underbust?

The task fails with invalid_input. A smaller bust would imply a negative cup volume, so the request is rejected and is not billed.

What does the bra size 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 →

Run — free