Bra Size Calculator

Why bra sizes differ by country

The United States and United Kingdom size bands in inches; continental Europe and Japan size them in centimeters; France adds 15 to the European band number. Cup letters are not a fixed volume either — they mark the gap between your bust and underbust measurement, and each country divides that gap into a different step size. A garment labeled the same letter can still fit differently between brands, since wire shape, cut, and fabric stretch are not standardized.

This calculator takes your underbust and bust measurements once and applies each country's own band and cup rule to them, so every number shown is derived the same way — not looked up from a fixed conversion chart.

Limits

Cup and band boundaries here follow the general public rules published by manufacturers and national fit guides, not any single brand's proprietary chart. The US/UK result also depends on which convention you select above — modern and traditional give different band and cup numbers for the same body. Because of this, your result can differ from a cross-country size chart on another site; the correspondence table further below quotes such charts directly rather than calculating them, since the two do not always agree. Results are for reference — try a physical fitting when possible.

How to find your true bra size

Measure underbust and bust in centimeters or inches, then compare the difference. For EU, France, and Japan/Korea, band comes from underbust rounded to that country's cm step, and cup from bust minus underbust. US/UK works differently — see below.

How to measure correctly

1. Wear a well-fitting, non-padded bra, or measure directly against skin.
2. Underbust: wrap the tape directly under the bust, along the ribcage, parallel to the floor. Keep it snug but not compressed, and breathe normally.
3. Bust: wrap the tape around the fullest part of the bust, keeping it level front and back, without pulling tight.
4. Record both numbers in the same unit (cm or inches).
5. Re-measure after a significant weight change, after pregnancy, or once a year — underbust and bust shift over time.

The band and cup rules, country by country

• Underbust — measured directly under the bust, around the ribcage
• Bust — measured around the fullest point of the bust
• EU, France, and Japan/Korea compute cup from bust minus underbust. US/UK computes it from bust minus the band number instead — so the convention you pick changes both numbers at once

Example: same underbust, growing bust difference (EU / France / Japan-Korea, calculated)

Cross-country size correspondence — quoted from published charts, not calculated

FAQ

Why is my US/UK size so different from my EU or Japanese size for the same body?

EU, France, and Japan/Korea calculate cup from bust minus underbust, in centimeters, and divide that gap into different-sized steps. US/UK calculates cup from bust minus the band number instead — and the band number itself depends on which convention you pick, so several things shift at once. This is also why the calculator does not try to convert your EU or JP size directly into a US/UK one; see the next question.

Why does the calculator show two different US/UK results, Modern and Traditional?

Current fit guidance measures underbust directly and rounds to the nearest even number, with nothing added. Older guides, and some tag conversion charts still in use, instead add about 4 inches to the underbust before rounding — a habit from less stretchy elastic decades ago. Since both conventions are still in real use, this calculator computes both and labels which one you are looking at, rather than picking one silently.

Why doesn't the calculator convert my EU/JP size straight into a US/UK size, or vice versa?

Published international size charts, like the one above, come from decades of industry convention rather than a clean formula — checking them shows they do not consistently match either the modern or the traditional calculation method. Rather than compute a conversion that might not agree with what is printed on an actual garment tag, this calculator only quotes such correspondences directly from named sources, and calculates each country's own size independently from your measurements.

Is French sizing really just EU plus 15?

Yes, for the band number. Brand size guides describe the French band as the European band plus 15, with the same cup letters. Only the number on the tag changes, not your actual measurements.

Does Korea use its own bra sizing standard?

Korea has a national standard, KS K 9404, for foundation garment sizing. Public sizing guides from Korean innerwear brands describe the same centimeter-based approach as Japan — underbust rounded to the nearest 5 cm, cup steps a few centimeters apart — so this calculator shows Korea using the same rule as Japan.

Cup and band boundaries for the EU, France, and Japan/Korea rows follow the public rules published by C&A, Primadonna, and Wacoal Japan referenced below, not any single brand's full chart. The US/UK row depends on which convention you choose above, and neither convention is guaranteed to match a specific manufacturer's tag. The cross-country correspondence table above is quoted directly from published charts rather than calculated, because recomputing it from a raw measurement does not reliably reproduce what is printed on real garments — expect your result here to sometimes differ from other sites' conversion charts for exactly that reason.

Sources: Wacoal Japan — bra size chart & measuring method, C&A — how to determine your bra size (band & cup table), Primadonna — French, European and American bra sizing compared, HauteFlair — bra size chart, incl. international conversion table, Little Women — why fitters no longer use the +4 inch method, Korean Agency for Technology and Standards — KS K 9404 (foundation garment sizing) listing, Bluegrain — Korean bra size measuring & band/cup guide