Snug and level, directly under the bust.
Loose and level, across the fullest point.
Enter both measurements to continue.
A schematic fit diagram, scaled to your cup size — not a photographic or anatomical rendering.
The formula behind the result
Band size comes from your underbust measurement, rounded to the nearest even number. Cup size comes from the gap between your bust and that band — every inch of difference moves the letter one step up the scale.
cup difference = round(bust, in) − band
This direct-measurement method is what modern bra-fitting guidance recommends, replacing the older "add four inches" approach that tends to overestimate the band.
The science
Why this method, and not the old one
For decades, the standard advice was to measure the underbust, then add four or five inches to arrive at a band size. That shortcut dates back to an era when bras were built with less structured elastic and heavier boning, and it has stuck around in a lot of size charts long after the reasoning behind it stopped applying.
Modern stretch fabrics and engineered elastic mean a band no longer needs that extra allowance to hold its shape. Fit specialists and most contemporary sizing guides now recommend measuring the underbust directly and rounding it to the nearest whole number, which is exactly what this calculator does. The practical effect is usually a smaller, snugger band paired with a larger cup letter than the old method would suggest — the same volume, described more precisely.
Underbust + 4″
Adds a fixed allowance regardless of fabric or brand, often producing a band that's too loose to support properly.
Underbust, rounded
Used by this calculator. Matches how most modern elastic and construction actually behave.
Bust − band = cup index
Cup letter was never an absolute size — it's always a relationship between two numbers, not one measurement alone.
Snugger band, bigger cup
Switching methods typically moves someone into a smaller band and a couple of cup letters larger than they expected.
Full reference
Complete band and cup table
Already know your band and cup difference? Cross-reference it here instead of recalculating.
| Cup diff. | Letter | 30 | 32 | 34 | 36 | 38 | 40 | 42 | 44 |
|---|
Read down the "Cup diff." column to your calculated gap, then across to your band for the full size. See the complete bra size chart for petite and plus ranges too.
Going international
Quick size conversion
The same body translates into different-looking labels depending on where you're shopping. Here's a fast reference for a mid-range size; for a full interactive version, use the international bra size converter.
| Region | Band unit | Example (34D equivalent) |
|---|---|---|
| US / Canada | Inches | 34D |
| UK / Australia / India | Inches | 34D |
| France / Spain | Centimetres | 75D |
| Germany / Italy | Centimetres | 75D |
| Japan / South Korea | Centimetres (JIS) | ≈80B (approx., different formula) |
Getting an accurate reading
Before you measure
Wear a thin, unpadded bra
Or measure without one — padding and moulded cups distort the bust reading specifically.
Use a soft measuring tape
A fabric or vinyl tape gives a more accurate read than a rigid ruler.
Stand naturally
Arms relaxed at your sides, breathing normally — not exaggeratedly straight or slouched.
Measure again periodically
Weight change, pregnancy, and time can all shift both numbers, so it's worth re-measuring every so often.
What throws the result off
Common measuring mistakes
Pulling the tape too tight
Both readings should sit against you naturally — pulling either tape tight understates the true number.
Measuring at the wrong height
The bust reading should sit at the fullest point, which isn't always exactly at nipple level for every body.
Raising your arms
Lifting your arms while measuring shifts tissue and shrinks the bust reading compared to how you'll actually wear a bra.
Using an old measurement
A number from a year or more ago may no longer reflect your current size, especially after any significant body change.
By the numbers
The sizing system, in facts
Speak the language
Glossary of bra-fitting terms
- Band
- The elastic strip that circles the ribcage, carrying most of a bra's support.
- Cup
- The pocket that holds and shapes the bust; its letter describes a gap in inches, not an absolute volume.
- Gore
- The centre panel between the two cups; it should sit flat against the breastbone in a well-fitted bra.
- Sister size
- A different band-and-cup combination that holds roughly the same cup volume as your own size.
- Underwire
- The rigid wire along the base of the cup that defines and supports its shape.
- Projection
- How far the cup extends outward from the chest wall, independent of overall cup size.
- Ptosis
- The clinical term for natural breast position relative to the inframammary fold, used in fit and support discussions.
- JIS sizing
- Japan's centimetre-based sizing standard, which uses its own formula rather than adapting the inch-based system.
Questions
Bra size calculator FAQs
Your underbust measurement (snug, directly under the bust) and your bust measurement (loose, across the fullest point). Those two numbers are all the formula needs.
It's an accurate starting point based on the standard band-and-cup formula, but individual brands cut differently, so treat the result as a strong first estimate rather than a guarantee.
Some charts still use the older method of adding four inches to the underbust for the band. This calculator uses the more accurate direct-measurement method that current fitting guidance recommends.
Yes — toggle to centimetres for the input, and the result reflects standard international band-and-cup conventions. See the international quick-conversion table above for more.
Try the sister size shown alongside your main result — it holds a very similar volume and may fit better depending on the brand.
The older method added four or five inches to the underbust to estimate the band, which tends to run large. The direct-measurement method used here rounds the underbust itself, which most modern fitting guidance considers more accurate.
Yes, the formula scales the same way from AA through the higher end of the cup scale — the math doesn't change, only the numbers do.
Cup letter is a relative measurement — the same letter holds different volumes on different band sizes. See our breast volume calculator for an estimated volume figure.
A simple schematic fit diagram, similar to a dress form used in garment pattern-making — it illustrates roughly where the band and cups sit at your calculated size, not a photographic or anatomical rendering.
Measuring twice and averaging the results is a reasonable way to catch a one-off reading error, especially if the two attempts differ noticeably.
Band and cup describe a garment measurement, not overall body size — someone can have any combination of frame size and cup size, since the two are calculated independently.
Yes — use the print button under your result to get a clean, storable copy without the surrounding page content.
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