Sister Sizes, Explained Properly

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A sister size is a different band and cup label that holds the same cup volume. The rule is one-for-one: each band size you go down, go one cup letter up to keep the volume, and each band up, one letter down. 32DD, 34D and 36C are one family wearing three name tags.

Why the math works

A cup letter is not a volume; it is a difference. The letter counts how many inches your bust exceeds your band: one inch is an A, four is a D, five a DD. So a DD on a 32 band means 37 inches of bust, and a C on a 36 band means 39, and when you compute the actual tissue those cups hold, geometry brings them to nearly the same capacity: the same volume projects further from a smaller frame, earning a bigger letter for the same breast.

This is also the punchline of every internet cup-size argument: a D cup is not a size, it is a ratio. A 30D is modest; a 40D is not. Band and letter only mean something together, which our comparison tool demonstrates with any two sizes you pick.

Using the ladder in real shopping

The classic case: cups fit beautifully but the band rides up by afternoon. Do not size the whole bra down and lose your cup room; step to the sister, band down and letter up, 36C to 34D. The cups hold the same tissue while the band finally anchors. Brands whose bands run famously firm (UK full-bust houses) or stretchy (US mall brands) are the other daily use: walk in knowing your sister family and you can adjust for the house cut on the spot.

The limits, honestly

Sister sizes match volume, not geometry. Wire width, cup height and strap spacing all scale with the band, so each step subtly reshapes the garment: two steps from your measured size usually fits like a different bra that happens to hold the same amount. Treat one step as routine, two as a last resort, and cup-shape problems (gaping tops, wrinkled bottoms) as shape problems that no amount of sister stepping fixes.

Find your own family instantly with the sister size calculator, or see every family as a highlighted diagonal on the interactive chart.

Frequently asked questions

What are sister sizes?

Sizes with different bands that hold the same cup volume: going one band down pairs with one cup letter up, and vice versa. 32DD, 34D and 36C form one sister family.

Is a sister size the same as my size?

Same cup capacity, different frame: the band length changes and wire width shifts slightly with it. One step usually wears interchangeably; two steps changes the garment's geometry noticeably.

Which direction should I sister size?

Band riding up or loose with fitting cups: band down, letter up. Band digging painfully on the loosest hook from new: band up, letter down. Cup problems alone are cup problems, not sister-size problems.

Written by the BraSizeConversion fit team. Last updated August 2026.