Bra Sister Sizes Explained

Sister sizes are bra sizes that hold roughly the same cup volume but use a different band length. To find one, go up a band size and down a cup size, or go down a band size and up a cup size. A 34B, for example, has sister sizes of 32C and 36A.

This guide explains the rule, shows worked examples and tells you when a sister size helps. For finding your starting size, a size recommendation for fashion stores can match measurements to a brand's own chart.

What is a sister size?

A bra size has two parts: the band number and the cup letter. The cup letter only has meaning together with the band. A C cup on a 32 band holds less volume than a C cup on a 38 band.

A sister size changes both parts together so the cup volume stays about the same. The band gets tighter or looser, and the cup letter moves in the opposite direction to compensate.

How do you find a sister size?

Use one of two moves.

1. Go up in band, down in cup. This gives a looser band with a smaller-lettered cup. 34B becomes 36A.
2. Go down in band, up in cup. This gives a tighter band with a larger-lettered cup. 34B becomes 32C.

Each move changes the band by one step (two units, such as 34 to 36) and the cup by one letter. You can repeat the move to find more sisters, though the further you go, the less reliable the match.

What do sister sizes look like in a table?

This chart is a worked example of the swap rule using UK-style band and cup letters. Cup letters vary by brand and country, so see the conversion guide linked below.

Your size	Tighter band sister	Looser band sister
32B	30C	34A
34B	32C	36A
34C	32D	36B
36C	34D	38B
36D	34DD	38C

Check the cup letters against the brand you're shopping with. Cup letters differ between countries. The guide to bra size conversion across UK, US, EU and AU covers those differences.

When is a sister size useful?

Sister sizes help in a few situations:

• The band feels too tight or too loose, but the cup fits. Try a sister size to change the band while keeping the cup volume.
• Your size is out of stock. A sister size may hold the same volume when yours is sold out.
• A brand runs small or large in the band. A sister size can balance that out.
• Your weight or body has changed a little. Sister sizes give a quick way to adjust.

They are not a way to fix a bra that is the wrong shape for you. If the cups gap, spill or wrinkle, the problem may be shape, not volume.

How do you know if a sister size is working?

Use the same fit check you'd use for any bra. The guide on how a bra should fit explains it in detail. In short:

• The band sits level around your back and doesn't ride up.
• The center gore (the part between the cups) rests flat against your breastbone.
• The cups hold your breast tissue without gaps or spilling.
• The straps stay in place without digging in.

If the band rides up after you go to a bigger band, the sister size is too loose. Go back and try the other direction.

Do sister sizes work for sports bras?

Sometimes. Sports bras are often sold in simple small, medium and large sizes, which use ranges instead of exact band and cup pairs. If a sports bra does use band and cup sizes, the same swap rule can apply, but a stretchy fabric and compression change how it fits. Read the brand's size chart first. The guide to how a sports bra should fit covers the differences.

What can't sister sizes tell you?

Sister sizes work from the arithmetic of band and cup. They don't account for:

• the shape or fullness of your breasts
• how much stretch the band has
• brand-specific cup shapes and depths

For stores, these gaps are one reason why virtual try-on can help alongside size charts. ZeDaq.ai offers size recommendations from a 30-second body scan, matched to the store's own size chart.

How should stores show sister sizes?

If you sell bras, list the band and cup clearly and link to your chart. Consider adding a note that explains sister sizes for shoppers whose size is sold out.

Keep your own chart as the source of truth. A shopper who finds a size elsewhere should still check yours.

FAQs

Is a 34B the same as a 32C?

They hold about the same cup volume, but the band is tighter on the 32C. They fit differently, so try both if you can.

Does a bigger band mean a bigger cup?

No. A bigger band number means a longer band. The cup letter only means something relative to the band.

Can I always wear a sister size?

Often, but not always. Brands cut cups differently, so test it with the brand you buy from.

Which direction should I go if I am between sizes?

If the band is tight, go up in band and down in cup. If the band is loose, go down in band and up in cup.