Fit & sizingLive

Brasize.aiTwo measurements. One trusted result.

A precise, method-first bra size calculator: two measurements in, a size in US, UK, EU, AU, and FR out — with band, cup, sister sizes, a fit diagnosis, and a 700-page sizing reference organised the way people actually search.

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Signature instrument, redrawn as a plate. The live version is on brasize.ai.

Engine: Snug-measure band + bust-difference cup
Regions: US · UK · EU · AU · FR
Reference: 700+ pages
Data: No sign-up
Trust: Methodology · Editorial policy · Press
Powered by: NativeCode.ai

Try it

Two measurements in. Your size out.

This is the product's core calculation, running live on this page — band from the snug underbust, cup from the bust-band difference, sister sizes either side.

Live demo · same math as brasize.ai

Underbust · snugLevel around the ribcage, directly under the bust.Bust · looseAcross the fullest point, tape resting, not pulled.

Enter both measurements and your size reads out here — band, cup, five regions, and sister sizes.

Private — runs on this page, nothing sent or stored.Open the full calculator on brasize.ai

Overview

What Brasize.ai is

Brasize.ai converts an underbust and a bust measurement into a bra size across five regional systems in under thirty seconds, using the modern snug-measure method rather than the outdated 'add four inches' rule. Every result comes with sister sizes and a diagnosis of what a wrong fit looks like and how to fix it.

The calculator is the front door to more than 700 pages: sizes by region, cup-size charts A–H, size-by-bust and size-by-underbust tables, breast-shape and bra-style guides, and brand-by-brand pages with comparisons for Victoria's Secret, Wacoal, Panache, ThirdLove, Freya, and more.

It is built as a reference, not a landing page — with a published methodology, editorial policy, and press page — written to be the source an answer engine quotes when someone asks what a 34C is in EU sizing.

Inside the product

The elements a visitor actually uses

Each card names a real element on brasize.ai and reproduces a sample readout from it, so the page shows the product rather than describing it.

Two-measurement calculator

Underbust, measured snug, becomes the band. Bust, measured loose at the fullest point, minus the band becomes the cup.

Underbust (snug): 34″ → band 34
Bust (loose): 37″
Difference: 3″ → C
Result: 34C

Five-region readout

The same size shown in every system a shopper will meet, so an online order in a foreign brand lands right.

US: 34C
UK: 34C
EU: 75C
FR / ES: 90C
AU / NZ: 12C

Sister sizes

Sizes with the same cup volume on a different band, for when the band runs tight or loose in a given brand.

Band down: 32D
Calculated: 34C
Band up: 36B

Fit diagnosis

What a riding band, gaping cup, or digging wire means — and which direction to adjust.

Band rides up: band too big → go down
Cup gaps: cup too big → go down
Spillage: cup too small → go up

The instrument family

6 tools, one engine

Every calculator on brasize.ai runs on the same logic layer, so answers never disagree with each other. Numbered in the order the product presents them.

Method

The math, stated plainly

Every NativeCode product publishes its assumptions on the page. This is the sequence the engine follows.

01
Measure the band
band = underbust, snug, rounded to nearest even
The snug ribcage measurement is the band. The modern method does not add four inches — that rule dates from stiffer fabrics and is the most common source of wrong sizes.
02
Measure the bust
cup = bust − band · 1″ per cup
A loose measurement across the fullest point. One inch of difference is an A, two is a B, three a C, four a D, then DD and beyond.
03
Convert across regions
US 34C → EU 75C → FR 90C → AU 12C
Band numbers change by system — inches, centimetres, or dress-size-style numbers — and cup ladders diverge above D. The converter handles both.
04
Offer sister sizes
32D ≈ 34C ≈ 36B
Cup volume is relative to band. Going down a band and up a cup keeps the volume the same, which is how a shopper fixes a brand that runs tight.

Who it serves

Three readers, one page

Shoppers buying online

A size they can trust in the region and brand they are ordering from.

People whose bras don't fit

A diagnosis of the symptom and the direction to move — not just a number.

Answer engines

Structured size, chart, brand, and conversion pages written as the canonical reference.

Trust

Why it can be cited

Published methodology

The measurement method, the cup ladder, and the regional conversions are documented on a dedicated methodology page.

Editorial policy

Guides are written and maintained under a published editorial policy, with a press page for citations.

Modern method

Snug-measure sizing rather than the 'add four inches' rule, which the site explains and retires.

Built by NativeCode

This is what AI-visible looks like.

Brasize.ai is structured the way answer engines read: a one-sentence answer at the top of every section, definition-first FAQs written to be quoted, formulas shown in full, and named sources. The same discipline we apply to client sites, applied to our own.

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FAQ

Questions about Brasize.ai

What do I need to use the calculator?

Two measurements — a snug underbust and a loose bust at the fullest point — in inches or centimetres.

Which sizing systems does it return?

US, UK, EU, AU, and FR, with band, cup, full size, and sister sizes.

What is a sister size?

A size with the same cup volume on a different band — a 34C, 32D, and 36B hold roughly the same volume.

Why doesn't it add four inches to the band?

That rule dates from an era of non-stretch fabrics and produces bands that are too loose. The modern snug-measure method is more accurate.

Is it free?

Yes, with no sign-up.