Bra Size Calculator (underbust & bust, KS/JIS cup)

Understanding the bra size calculator — concept and principle

A bra size has two parts: a band number that describes the size of the rib cage just under the breasts, and a cup letter that describes how much larger the fullest part of the bust is than that band. This calculator takes two tape measurements in centimeters and returns the size in the Korean/Japanese (and EU) system, such as 75C, plus the matching US/UK band number. It is useful for online shopping or comparing labels from different countries.

The lower loop sets the band; the gap between the two loops sets the cup letter.

How it is calculated

Band = underbust rounded to the nearest 5 cm (70, 75, 80 …).
Difference d = bust − underbust (cm).
Cup = AA at 7.5 cm, then one letter per 2.5 cm step: A 10, B 12.5, C 15, D 17.5, E 20, F 22.5 cm. The calculator picks the step closest to d.
EU band = the same centimeter band.
US/UK band = EU band × 2 ÷ 5 + 4 (EU 70 → 32, 75 → 34, 80 → 36), the standard conversion.

The bust must be larger than the underbust, otherwise no cup can be assigned.

The principle

The band carries most of the support, so it is defined by the firm rib-cage measurement. Cup volume is not measured directly; instead the bust-minus-underbust difference is used as a proxy, because for a given band a larger difference means more breast tissue to hold. Japanese industrial standard JIS L 4006 and the Korean system use the same idea: 5 cm band steps and 2.5 cm cup steps. Because a cup letter is relative to its band, a 70D and a 75C hold similar volumes (so-called sister sizes), while a 75C and an 85C do not.

Worked example

Default inputs: underbust 73 cm, bust 88 cm.

Band: 73 ÷ 5 = 14.6, rounded to 15, so 15 × 5 = 75.
Difference: 88 − 73 = 15 cm.
Cup step: (15 − 7.5) ÷ 2.5 = 3 steps above AA, so AA → A → B → C.
Size: 75C (EU band 75).
US/UK band: 75 × 2 ÷ 5 + 4 = 34.

How to read the result

Read the main result, 75C, against a Korean, Japanese or European chart. For UK and US labels the band becomes 34: their band numbers are larger than your underbust in inches (73 cm is only about 28.7 in). Cup letters after D also diverge (UK uses DD, E, F, FF; US often DD, DDD), so convert the letter using the brand's own chart. Near a rounding boundary (such as 72.5 cm), try both neighboring bands.

Common mistakes

Measuring over a padded bra or loosely, which inflates the bust and the cup.
Pulling the tape too tight or letting it tilt at the back.
Converting the band by dividing centimetres by 2.54 instead of using the label table.
Ignoring sister sizes: going down a band usually needs one cup up.

Keep in mind: The result is a starting point based on two measurements. Breast shape, brand cut, fabric stretch and style change the fit, so always try on or check the maker's size chart.

FAQ

Frequently asked questions

What is the sister size of 75C?

70D and 80B hold a similar cup volume with a tighter or looser band.

When is the best time to measure?

Stand upright, breathe out normally and keep the tape level. Measuring at a similar point in your cycle gives more consistent results.

How often should I re-measure?

Whenever your weight changes noticeably, after pregnancy or breastfeeding, or if your current bras ride up at the back or gap at the cup.

Sources: JIS L 4006 Sizing systems for foundation garments (Japanese Industrial Standards)

Size charts differ between brands and countries — treat this as a starting point and check the maker’s own chart.

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