About

Bra sizing lacks a universal standard. A 34D in the US maps to 34D in the UK but 75D in EU/FR and 3D in Italy. The band number represents underbust circumference, but systems disagree on the unit: US/UK use inches, EU/FR use centimeters rounded to the nearest 5, and Italy uses an arbitrary ordinal index. Cup letters compound the confusion because the alphabetical progression differs. The UK system inserts DD before E, while the US system varies by manufacturer with some using DDD and others F at the same volume step. Ordering the wrong size from an international retailer wastes money on returns and shipping. This tool cross-references underbust measurements and cup volume indices across 8 sizing systems to produce an accurate conversion. It also computes sister sizes - adjacent band/cup combinations that preserve the same breast volume. Note: manufacturer tolerances vary by ±1 cup, so treat results as a fitting starting point, not an absolute guarantee.

Formulas

Band conversion maps underbust circumference between unit systems. US/UK bands express inches directly. EU/FR bands round centimeters to the nearest 5:

Band_EU = round5(Band_US × 2.54)

where round5 rounds to the nearest multiple of 5. Italian sizing uses a zero-based ordinal index:

Band_IT = Band_US − 282

Cup volume is indexed ordinally from the cup letter sequence of each system. The volume index i is the zero-based position of the cup letter in that system's progression. Conversion finds the letter at position i in the target system's sequence. UK cup progression: A, B, C, D, DD, E, F, FF, G, GG, H, HH, J, JJ, K, KK, L. US cup progression: A, B, C, D, DD(E), DDD(F), G, H, I, J, K, L, M, N. Sister sizes preserve cup volume V by shifting band and cup inversely:

Sister_up = (Band + 1 step, Cup − 1 step)

Sister_down = (Band − 1 step, Cup + 1 step)

where Band is the band value in the source system and Cup is the cup letter index. Each band step is 2 inches (US/UK), 5 cm (EU/FR), or 1 ordinal (IT/AU).

Reference Data

US Band	UK Band	EU/FR Band	IT Band	AU/NZ Band	JP Band	Underbust (cm)	Underbust (in)
28	28	60	0	6	60	58-62	23-24
30	30	65	1	8	65	63-67	25-26
32	32	70	2	10	70	68-72	27-28
34	34	75	3	12	75	73-77	29-30
36	36	80	4	14	80	78-82	31-32
38	38	85	5	16	85	83-87	33-34
40	40	90	6	18	90	88-92	35-36
42	42	95	7	20	95	93-97	37-38
44	44	100	8	22	100	98-102	39-40
46	46	105	9	24	105	103-107	41-42
48	48	110	10	26	110	108-112	43-44
50	50	115	11	28	115	113-117	45-46
52	52	120	12	30	120	118-122	47-48

Frequently Asked Questions

The UK system uses double-letter increments: DD, E, F, FF, G, GG, H, HH, J, JJ. The US system historically used DDD (or equivalently F) after DD and then continues G, H, I, J. This means a UK F cup is not the same volume as a US F cup. UK F sits at volume index 6, while US F (DDD) sits at volume index 5. Always specify which system you are referencing to avoid a one-cup error.

Sister sizes maintain the same cup volume while changing the band. Moving up one band step and down one cup step gives a looser band with the same breast coverage. For example, US 34D has sister sizes 32DD (tighter band, next cup up) and 36C (looser band, one cup down). Use sister sizes when your correct band size is unavailable or when a brand runs tight or loose.

Band sizes are identical between EU and FR - both use centimeters rounded to multiples of 5. Cup letter progressions are also the same in modern usage: A, B, C, D, E, F, G, H, and so on without double letters. Some older French manufacturers historically started cup letters at an offset, but contemporary brands align with the EU standard.

Italian sizing uses an ordinal system for both band and cup. Band 1 corresponds to a 30 US / 65 EU underbust, and cup 1 corresponds to an A cup. Each increment of 1 equals one step in the standard progression. So an Italian size 3/5 means band 3 (US 34 / EU 75) with cup 5 (approximately US/UK DD). This compact notation is common in Italian lingerie but can confuse buyers unfamiliar with the mapping.

Conversions are mathematically exact at the standards level. In practice, manufacturer tolerances introduce variation of approximately ±1 cup or ±1 band size. Elastic stretch, fabric cut, and cup shape (full-cup vs balconette) also affect fit. Treat conversion results as a starting point and try the adjacent sizes if the initial fit is imperfect. Bratabase crowd-sourced measurements can help calibrate for specific brands.

Australian and New Zealand bra band sizes align with AU dress sizes. A band size 10 corresponds to a US/UK 32 band or EU 70. The progression increments by 2: AU 8 = US 30, AU 10 = US 32, AU 12 = US 34, AU 14 = US 36, and so on. Cup letters follow the UK sequence (including DD, E, F, FF, G, GG), so a AU 12DD is equivalent to a UK 34DD.