Size tool

Sister size calculator

Sister sizes have a similar cup volume on a tighter or looser band. They help when a band feels wrong but the cups fit.

Sources checked 17 October 2026

Rows of nude and blush bras hanging on hangers — Photo: Joost Crop (opens in a new tab) on Unsplash

The sister-size grid

Each diagonal is one family of sister sizes. Choose a size to highlight its family.

UK bands down the side, UK cups across the top. Sister sizes share a diagonal.
Band	A	B	C	D	DD	E	F	FF	G	GG	H	HH	J	JJ	K
28	28A	28B	28C	28D	28DD	28E	28F	28FF	28G	28GG	28H	28HH	28J	28JJ	28K
30	30A	30B	30C	30D	30DD	30E	30F	30FF	30G	30GG	30H	30HH	30J	30JJ	30K
32	32A	32B	32C	32D	32DD	32E	32F	32FF	32G	32GG	32H	32HH	32J	32JJ	32K
34	34A	34B	34C	34D	34DD	34E	34F	34FF	34G	34GG	34H	34HH	34J	34JJ	34K
36	36A	36B	36C	36D	36DD	36E	36F	36FF	36G	36GG	36H	36HH	36J	36JJ	36K
38	38A	38B	38C	38D	38DD	38E	38F	38FF	38G	38GG	38H	38HH	38J	38JJ	38K
40	40A	40B	40C	40D	40DD	40E	40F	40FF	40G	40GG	40H	40HH	40J	40JJ	40K
42	42A	42B	42C	42D	42DD	42E	42F	42FF	42G	42GG	42H	42HH	42J	42JJ	42K
44	44A	44B	44C	44D	44DD	44E	44F	44FF	44G	44GG	44H	44HH	44J	44JJ	44K

Grid shows bands 28 to 44 and cups A to K. The calculator supports bands 28 to 48 and cups AA to L.

How sister sizes work

Your cup letter only means something next to a band. A 34D cup holds more than a 32D cup, because the cup is sized for a bigger band. So when you change the band, you change the cup the other way to keep a similar volume:

One band smaller, one cup larger: a firmer band with a similar cup.
One band larger, one cup smaller: a looser band with a similar cup.

UK brands step through cups one position at a time, double letters included. Boux Avenue describes sister sizes as “equivalent sizes with the same cup volume, but different underband sizes”.

When to try one

The cups fit but the band feels too loose or too tight.
A brand or style is out of stock in your usual size.
A particular style runs firm or loose in the band.

Examples

Your size	Smaller band	Larger band
32DD	30E	34D
34E	32F	36DD
36F	34FF	38E
36H	34HH	38GG

Brastop’s own examples follow the same rule, for example 28G with sister sizes 30FF, 32F and 34E.

Is a sister size exactly the same cup?

Not exactly. Sister sizes have a similar cup volume, but the band, wire width and cup shape change with the band size, so they can feel different. Try them on.

Should I wear a looser band and go down a cup?

Only if the band is genuinely uncomfortable. Brastop advises not to make a habit of it, because a band that is too loose “won’t offer the correct support”.

Why does 32DD become 30E and not 30F?

UK cups use double letters, so the step after DD is E. Going down one band moves you up exactly one step in the UK order: D, DD, E, F, FF and so on.

Sources

Checked 17 October 2026. Brands change their charts, so check the retailer’s own size guide before you buy.

S3 Brastop: Sister sizes explained (opens in a new tab) Sister sizes and their limits
S4 Boux Avenue: How to measure bra size (opens in a new tab) Tape method; worked example 39 − 34 = 5 (DD); sister sizes; limits of measuring
S22 Pour Moi: Sister sizes (opens in a new tab) Sister sizes worked example (32F and 34E)
S17 Bravissimo: Bra fitting guide (opens in a new tab) Fitting without a tape measure; sister-size logic