Sister Sizes for Bras | Match Cup Volume, Adjust The Band

Sister sizes for bras keep the same cup volume while shifting the band size one step in the opposite direction of the cup letter.

One wrong band size makes even a perfect cup feel awful. When the band digs in or rides up but the cups fit fine, sister sizes for bras give you a nearby alternative that preserves the volume you already know works. The rule is simple: move up a band size, drop a cup letter — or drop a band size, move up a cup letter. That one adjustment keeps the cup volume constant while changing how the bra sits around your ribs.

How Sister Sizing Works

Sister sizing is a fitting shortcut based on a mathematical relationship between band and cup. As the band gets longer, the cup letter must get smaller to hold the same volume, and vice versa. Retailers describe the pattern as “up in the band, down in the cup” and “down in the band, up in the cup.”

For example, if you wear a 34C, your sister sizes include 32D and 36B. Each of these holds roughly the same cup volume as a 34C, but the band fits differently. Another common conversion: 32B and 30C are sister sizes, and so is 34A.

The main use case is when one element of the fit is wrong but the other is right. If your 34C band rides up your back, a 36B gives you a looser band without changing cup volume.

Reading A Sister-Size Chart

A sister-size chart organizes bras by cup volume in rows. Each row lists every band size that produces that same volume, so you can move across the row to find your alternative. To use one, find your current size, then look at the sizes immediately to its left and right on the same row.

Here is how a typical chart breaks down for a few starting sizes:

Notice the pattern: the cup letter moves exactly one step in the opposite direction of the band. If your starting size has a double letter or an uncommon letter, the system still works the same way, though the exact label depends on the brand’s cup progression. Bravissimo’s sister-size guide walks through this logic row by row for US shoppers.

The Limits Of Sister Sizing

Sister sizes preserve volume, not fit. A sister size is not an identical replacement — the band length genuinely changes, which shifts where the bra sits, how the straps fall, and how the cups hold the breast tissue. Treating a sister size as a perfect match is the most common mistake shoppers make.

The logic also does not transfer perfectly across every brand or style. Fit varies by manufacturer, fabric, and cut, so a sister size in one brand’s plunge bra may not fit the same in another brand’s balconette. Pour Moi’s fitting room notes that band styles and patterns alter how a size actually behaves on the body.

Sister sizing is a workaround for small adjustments, not a substitute for measuring properly or trying on multiple bras. If you are changing more than one band size, or if the cup fit is also questionable, a fresh measurement is the better move. Victoria’s Secret publishes its own bra sister-sizing guide and calculator for US shoppers who want to check their alternatives against the brand’s own sizing system. Cup-letter scales also vary by retailer and market, so the exact label for a sister size can differ from one store to the next — the row on the chart matters more than the letter.

When To Use A Sister Size

Sister sizing solves a specific problem: the band is wrong but the cup volume is right. If your usual 34C feels tight around the ribs but the cups fit perfectly, a 36B gives you breathing room without changing the cup.

Sister sizes are less useful when both band and cup are off, or when the bra’s construction changes the fit — molded cups, unlined styles, and different strap designs all behave differently. For those cases, start over with a tape measure and try the actual size.

For shoppers with a smaller band who need options at a specific fit, a curated roundup of the best 30AA bras available covers the narrowest band range with volume-matched picks.