How Measured Size works

Measured Size turns two measurements into a bra size, translates that size into each brand's own labels, and says how sure it is. This page sets out the method, what it doesn't do, the brands it covers, and how apps and AI agents can ask it.

• Her size from her measurements
• Sister sizes
• Translated into each brand's labels
• How sure it is, and when it refuses
• What it doesn't do
• Brands covered
• For apps and AI agents: the API and MCP server
• Citing Measured Size
• Licence
• Corrections
• Changelog

Her size from her measurements

Two measurements with a soft tape: the underbust, snug around the ribcage just under the bust, after breathing out, and the bust, around the fullest part with the tape flat. Inches or centimetres, decimals allowed. Centimetres are turned into inches, to the hundredth, before anything else.

• Band: the underbust, to the nearest even inch. Bands 28 to 54 are sized. Most stores make nothing below 28, so a band of 26 or 24 is given as the band 28 sister size that holds the same cup volume, and says so. Past 54, Measured Size says it can't size the band, rather than guess.
• Cup: one US cup letter for each inch the bust is bigger, counted from halfway between the underbust and the band. The US letters run AA, A, B, C, D, DD, DDD, G, H, I, J, K, L, M, N, O. Past O, it says so rather than guess.

This is Measured Size's base size. The same code works it out in the Chrome extension, the bra size calculator and the API, and no AI model decides a size at any point.

Sister sizes

One band up with one cup letter smaller, or one band down with one cup letter bigger: the same cup volume on a looser or tighter band. Measured Size offers a sister size one band away, never two.

Translated into each brand's labels

Her size is the same everywhere. Brands label it differently, so Measured Size keeps her size and translates it, rather than sizing her again for each brand.

• Cup letters. Past D, many brands use their own letters. A US DDD is a UK E, and SKIMS's F is a US DDD. Each brand's letters come from its own published charts.
• Bands a brand doesn't make. Where a brand's bands stop short of hers, Measured Size says "Not carried here" and names the brand's last band, rather than offering a size that won't fit.
• Letter sizes (S, M, L). From the brand's own letter chart where it publishes one. Where it doesn't, the usual letter: the one most brand charts on file give her size, with how many agree.
• A brand's own method. Measured Size's own size is the default. A brand's own sizing method replaces it only where real-fit evidence shows it fits better.
• Other countries. The UK size always. EU and French sizes only where their tables give her whole size, band and cup.

How sure it is, and when it refuses

Every size comes with a badge, and the badge rates the size shown. On a listing, the extension also reads the listing's own chart and stock, so its answers can rate higher than the API's, which answer for a brand in general.

Where the honest answer is no size, it says why instead:

What it doesn't do

• It doesn't know her shape. Breast shape, root width, where the fullness sits and how she likes a bra to feel all change which size fits. Two people with the same measurements can wear different sizes.
• It doesn't judge styles. A plunge, a balconette and a sports bra can fit the same size differently. The API answers for a brand in general (its band and cup bras as an underwire style); a listing's own chart can differ.
• It isn't a fitting, and it doesn't guarantee a fit. A tape taken differently gives a different size, and a fitter can see what a tape can't. The badge says how much Measured Size knows, not how a bra will feel.
• The API doesn't read listings or stock. The extension does that, on the listing she has open.

Brands covered

159 brands, each researched from its own published sizing. The research notes themselves are not published. "Own" cup letters lists the first places a brand's letters differ from US letters, and "ends at" its last cup letter on file; "letter chart" says whether the brand's own S, M, L chart is on file. Pour Moi is listed twice because Measured Size gives its sizes two ways: on its own US store, pourmoiclothing.com, in the US letters that store prints, and on every other store, its UK store included, in Pour Moi's UK letters.

For apps and AI agents: the API and MCP server

The same engine, free, with no key and no account. Every answer carries the engine's badge and its reason: pass them on with the size, and pass on a refusal and its reason rather than a guess. Nothing sent is stored or logged; the only record is a count of requests per day for each endpoint. Each address can make 120 requests a minute.

• Address: https://api.measuredsize.com
• OpenAPI description: https://api.measuredsize.com/openapi.json (and a copy at https://measuredsize.com/openapi.json)
• MCP server: https://api.measuredsize.com/mcp, Streamable HTTP, no sign-in

Her size from her measurements

curl "https://api.measuredsize.com/v1/size?underbust=32&bust=38&unit=in"

{
  "size": "32DDD",
  "confidence": {
    "of": "this size on a band and cup listing at a store Measured Size has no brand data for",
    "rating": "Likely fits",
    "badge": "Possible Match",
    "refused": false,
    "why": "Standard fit, no adjustment from the calculated size. Your size here is 32DDD."
  },
  "sister_sizes": {
    "band_down": "30G",
    "band_up": "34DD",
    "at_band_28": null
  },
  "international": {
    "uk": "32E",
    "eu": null,
    "fr": null
  },
  "usual_letter": {
    "size": "M",
    "plus_size": null,
    "rating": "Uncertain, check size chart",
    "badge": "Limited Data",
    "why": "7 of the 11 brand size charts Measured Size has that list your 32DDD put it in an M."
  }
}

Her size at one brand

curl "https://api.measuredsize.com/v1/brand-size?brand=Curvy%20Kate&underbust=32&bust=38"

{
  "brand": {
    "id": "curvy_kate",
    "name": "Curvy Kate"
  },
  "your_size": "32DDD",
  "band_and_cup": {
    "size": "32E",
    "rating": "Likely fits",
    "badge": "Possible Match",
    "refused": false,
    "why": "Standard fit, no adjustment from the calculated size. Your size here is 32E.",
    "band_edge": null
  },
  "letter_sizes": null
}

A known size at one brand, at another

Only where it can be done honestly: the label is read back to Measured Size's base size through the first brand's recorded labels, then given in the second brand's. Rated no higher than Possible Match, since a label says less than measurements do. A letter size, or a label the first brand's record doesn't give, is refused with the reason.

curl -X POST "https://api.measuredsize.com/v1/translate" -H "Content-Type: application/json" -d '{"from_brand":"SKIMS","size":"32F","to_brand":"Bravissimo"}'

{
  "from": {
    "size": "32F",
    "base_size": "32DDD"
  },
  "to": {
    "size": "32E",
    "rating": "Likely fits",
    "badge": "Possible Match",
    "rating_from_measurements": "Likely fits",
    "why": "Standard fit, no adjustment from the calculated size. Your size here is 32E.",
    "band_edge": null
  },
  "why": "32F at SKIMS is Measured Size's 32DDD. A size label says less than her measurements do, so a translated size is rated no higher than Possible Match."
}

The brands on file

curl "https://api.measuredsize.com/v1/brands?q=wacoal"

Every endpoint takes GET with query parameters, or POST with a JSON body (the brand list GET only), and answers with api_version. Errors say what was wrong: 400 for a missing or non-numeric input, 404 for a brand not on file, 422 for measurements the extension would not size (with its own message), 429 when an address passes its limit (with Retry-After).

MCP

Four tools: bra_size_from_measurements, bra_size_at_brand, translate_bra_size and list_bra_brands, all read-only. The server speaks both the 2026-07-28 protocol and the earlier initialize handshake.

• Claude: in Claude's connector settings, choose Add custom connector and paste https://api.measuredsize.com/mcp. No sign-in is needed.
• Claude Code: claude mcp add --transport http measured-size https://api.measuredsize.com/mcp
• ChatGPT: with developer mode on, add a custom MCP server with the address https://api.measuredsize.com/mcp and no authentication.
• Any other MCP client:

{
  "mcpServers": {
    "measured-size": {
      "type": "http",
      "url": "https://api.measuredsize.com/mcp"
    }
  }
}

Citing Measured Size

When an answer or this page's summary is used or quoted, name Measured Size and link here: Measured Size, "How Measured Size works", https://measuredsize.com/how-it-works.html. Quote the badge with any size. The licence below requires that credit.

Licence

The published summary tables (the brand list and each brand's labels for each base size, as shown on the method page and returned by the API) are licensed under CC BY 4.0: credit "Measured Size" and link to https://measuredsize.com/how-it-works.html. The code, the research notes, the page readers and the full brand data are private and not licensed.

Corrections

Brands, and anyone else, can send a correction to support@measuredsize.com, with the brand's published size chart or a link to it. A correction is checked against the brand's own published sizing before anything changes, and recorded below.

Changelog

• 8 October 2026: The brand table names each brand's last cup letter on file, and says where each of Pour Moi's two rows applies. The API's brand list leaves out a cup letter or range not on file.
• 5 October 2026: The bra size calculator can be added to a phone's home screen, where it works with no connection, and can remember your measurements on your phone if you ask.
• 5 October 2026: The fit API and MCP server, version 1.0.0, for apps and AI agents, and this page.
• 1 October 2026: The free bra size calculator. The site moves to measuredsize.com, under the name Measured Size.
• 30 September 2026: Measured Size 1.0.0 for Chrome, packaged for the Chrome Web Store.