Sister Size Chart

A sister size holds about the same cup volume on a different band. When the band feels loose, go down a band and up a cup letter. When it feels tight, go up a band and down a cup letter. Find your size in the chart below for both.

The rule

Cup letters scale with the band, so a 32D and a 36D hold different volumes. That is how bra sizing works: there is no fixed D cup. Move one band size and one cup letter in opposite directions and the cup volume stays about the same while the band changes. A 34D, a 32DD and a 36C are sisters.

Most-searched sister sizes

Your size	Smaller band (band feels loose)	Larger band (band feels tight)
30D	28DD	32C
32C	30D	34B
32D	30DD	34C
32DD	30DDD/F	34D
32DDD/F	30G	34DD
34C	32D	36B
34D	32DD	36C
36C	34D	38B
36D	34DD	38C
38B	36C	40A

Sister size chart by band

Every size from 28A to 46G, with the sister size one band down and one band up. Cup letters follow the US ladder on the cup chart: A, B, C, D, DD, DDD/F, G, H.

Your size	Smaller band	Larger band
28 band
28A	26B	30AA
28B	26C	30A
28C	26D	30B
28D	26DD	30C
28DD	26DDD/F	30D
28DDD/F	26G	30DD
28G	26H	30DDD/F
30 band
30A	28B	32AA
30B	28C	32A
30C	28D	32B
30D	28DD	32C
30DD	28DDD/F	32D
30DDD/F	28G	32DD
30G	28H	32DDD/F
32 band
32A	30B	34AA
32B	30C	34A
32C	30D	34B
32D	30DD	34C
32DD	30DDD/F	34D
32DDD/F	30G	34DD
32G	30H	34DDD/F
34 band
34A	32B	36AA
34B	32C	36A
34C	32D	36B
34D	32DD	36C
34DD	32DDD/F	36D
34DDD/F	32G	36DD
34G	32H	36DDD/F
36 band
36A	34B	38AA
36B	34C	38A
36C	34D	38B
36D	34DD	38C
36DD	34DDD/F	38D
36DDD/F	34G	38DD
36G	34H	38DDD/F
38 band
38A	36B	40AA
38B	36C	40A
38C	36D	40B
38D	36DD	40C
38DD	36DDD/F	40D
38DDD/F	36G	40DD
38G	36H	40DDD/F
40 band
40A	38B	42AA
40B	38C	42A
40C	38D	42B
40D	38DD	42C
40DD	38DDD/F	42D
40DDD/F	38G	42DD
40G	38H	42DDD/F
42 band
42A	40B	44AA
42B	40C	44A
42C	40D	44B
42D	40DD	44C
42DD	40DDD/F	44D
42DDD/F	40G	44DD
42G	40H	44DDD/F
44 band
44A	42B	46AA
44B	42C	46A
44C	42D	46B
44D	42DD	46C
44DD	42DDD/F	46D
44DDD/F	42G	46DD
44G	42H	46DDD/F
46 band
46A	44B	48AA
46B	44C	48A
46C	44D	48B
46D	44DD	48C
46DD	44DDD/F	48D
46DDD/F	44G	48DD
46G	44H	48DDD/F

Keep stepping in the same direction to walk the whole ladder. Through 32DD, it runs 30DDD/F, 32DD, 34D, 36C, 38B, 40A. Each step away from your size moves the band further from where it fit, so start one step out and judge the band before going further.

When to use a sister size

The band feels tight and the cups fit. Go up a band, down a cup: 34D to 36C.
The band rides up or feels loose and the cups fit. Go down a band, up a cup: 34D to 32DD. A band that rides up in back is too big, not too small.
Your measurements land between two bands. Try your calculated size and its sister on the other band. The band fit, not the letter, decides. See between sizes.

When to change something else

The band fits and only the cups are off. Keep the band and change the cup letter: 34D to 34DD when cups spill, 34D to 34C when they gap.
The band and the cups are both off. That takes a band change and a cup change together. The fit adjustment table maps each combination to its fix.
The size is right and one style still fits wrong. That points to shape. The same cup letter fits differently in bras cut for projected or shallow, top-full or bottom-full shapes. See breast shape and fit.

US and UK cup letters

Some US brands label the fifth cup E instead of DD and the sixth F instead of DDD. Same cup, different label. US and UK letters match through DD and diverge above it: a US DDD/F is roughly a UK E. Put both sizes in the same system before you sister size, and check which system a brand uses. The cup chart shows both columns.

Same volume, different band. Sister sizes hold about the same cup volume, and the band sits differently on each. Comfort decides: if a sister size of your calculated size feels better, wear it.

Quick answers

What is a sister size in bras?

A sister size holds about the same cup volume as your size on a different band. Go down one band and up one cup letter, or up one band and down one cup letter. 34D, 32DD and 36C are sister sizes.

What is the sister size for 34D?

32DD on a smaller band, or 36C on a larger band. All three hold about the same cup volume.

What is the sister size for 32DD?

30DDD/F on a smaller band, or 34D on a larger band.

What is the sister size for 32D?

30DD on a smaller band, or 34C on a larger band.

What is the sister size for 32DDD?

In US sizing, DDD and F are the same cup. The sister sizes of 32DDD/F are 30G on a smaller band and 34DD on a larger band.

What is the sister size for 36D?

34DD on a smaller band, or 38C on a larger band.

What is the sister size for 38B?

36C on a smaller band, or 40A on a larger band.

Should I sister size up or down?

Go by the band. If the band feels tight and the cups fit, go up a band and down a cup. If the band rides up or feels loose and the cups fit, go down a band and up a cup.

Do sister sizes fit the same?

They hold about the same cup volume, and the band sits differently: a larger band sits looser on your ribcage and a smaller one sits snugger. Try the sister size on and judge by the band.

Is a 32DD the same as a 34D?

In cup volume, about the same. They are sister sizes: 32DD on the smaller band, 34D on the larger one. The band fit decides which one is yours.

Start from your own measurements

Sister sizing starts from a size you already wear. Radical's scan measures you from two photos on your phone, then builds the band to your actual ribcage and each cup to its own side.

Start your fit