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Sister size calculator

Find bras with the same cup volume on a different band, for when your size is sold out or a band feels off.

Your size

Your sister sizes

    How sister sizes work

    A cup letter only means something next to its band. Go up one band and down one cup and the cup volume stays the same, so 34C holds the same as 32D and 36B.

    Cup letters measure the difference between bust and band, and that difference covers a longer distance on a larger band. That is why a 38D holds noticeably more than a 30D, even though both say D.

    Sister size chart

    Read across any row: every size in it has the same cup volume. These are in US sizing. Change the system in the calculator above for other countries.

    3 bands down2 bands down1 band downBase size1 band up2 bands up
    28D30C32B34A36AA
    28DD30D32C34B36A38AA
    28DDD30DD32D34C36B38A
    28G30DDD32DD34D36C38B
    28H30G32DDD34DD36D38C
    28I30H32G34DDD36DD38D
    28J30I32H34G36DDD38DD
    28K30J32I34H36G38DDD

    Which way should you go?

    Sister size questions

    What is a bra sister size?

    Sister sizes share the same cup volume on a different band. To find one, go up a band and down a cup, or down a band and up a cup. 32D, 34C and 36B are all sisters.

    When should I wear a sister size?

    Use one when your size is out of stock, when a style runs tight or loose in the band, or between brands that size differently. It only works when your cups already fit well and only the band needs to change.

    How far can I go with sister sizes?

    One step usually feels fine. Two steps often changes the band enough to affect support or comfort, and the wire width and strap spacing shift too. Treat anything beyond that as a last resort.

    Is 34D the same as 36C?

    They hold the same cup volume, but they are not the same bra. The 36C has a band two inches longer, so it supports less and sits differently. Choose the band that feels firm and level on you first.

    Do sister sizes work the same in every country?

    Yes, the rule is the same, but the letters change. A UK 32E, a US 32DDD and an EU 70F are the same size, so look up the sisters in the system you are buying in. The calculator on this page does that for you.

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