Sister Size Finder
Bra Sister Sizes
Updated
Sister sizes are bra alternatives that share the same cup volume on a different band. Enter your current size to find yours instantly.
Your Sister Sizes
What Are Sister Sizes?
Sister sizes are bra size equivalents that share the same cup volume but use a different band and cup letter combination. The concept works because bra cup molds are proportionally scaled to the band circumference -- when the band increases by one size, the cup letter decreases by one letter to maintain identical volume. A 34C, 32D, and 36B are all sister sizes holding approximately 275 ml of cup capacity. The difference is purely in band tension: the 32D band fits tighter around the ribcage while the 36B sits looser. According to industry fitting data from Bratabase, roughly 25% of women have difficulty finding a properly fitted bra in their primary size, making sister sizing a practical solution for cross-brand shopping, sold-out situations, and brands that run tight or loose. Sister sizing works best within one size up or down from the calculated fit. Going two sizes in either direction starts to change the wire width and cup shape enough to affect overall support. For a deeper look at how each cup differs in actual volume, see our cup size comparison.
Example
32D ↔ 34C ↔ 36B
All three hold the same cup volume. The band gets progressively looser.
When to Use Sister Sizes
These alternatives are useful when your calculated size isn't available in a specific brand or style, or when a bra fits in the cup but feels slightly too tight or too loose in the band. Going to an equivalent size lets you adjust the band without losing cup fit.
The Trade-Off
Sister sizing preserves cup volume but changes the band fit. If you sister-size up (looser band, smaller cup letter), you get more room around your ribcage but the same breast coverage. If you sister-size down (tighter band, larger cup letter), you get more support but less breathing room.
Sister sizing works best within one size up or down. Going two sizes away starts to affect the cup shape and wire width enough that the fit may feel noticeably different. Make sure you're starting from an accurate measurement -- see how to measure your bra size before exploring sister sizes.
When to Use Sister Sizes
Sister sizing is a practical fitting technique used when a bra fits correctly in the cup but feels too tight or too loose in the band, or when a specific size is out of stock. The approach is especially useful for cross-brand shopping, since brand fit surveys show that sizing varies by up to one full band size between manufacturers like Victoria's Secret, ThirdLove, and Wacoal. Rather than changing the cup volume, sister sizing adjusts the band while keeping breast coverage identical -- a 34C buyer who finds a brand runs tight can try 36B for the same cup capacity on a roomier band. According to fitting professionals, the most reliable results come from staying within one sister size in either direction. Moving two band sizes away changes the wire width and strap placement enough to alter the overall bra structure, which can reduce support and cause new fit issues. The three most common scenarios for sister sizing are detailed below.
Your size is sold out
If your 34C is unavailable, a 32D or 36B shares the same cup volume. The fit will differ slightly in the band, but the cup capacity is identical.
A brand runs tight or loose
Some bra brands run a full band size smaller. If your usual band feels too tight, go up one band and down one cup -- you keep the same cup volume with a more comfortable band. Shopping internationally? Use our bra size converter to translate your sister size into UK, EU, or AU sizing.
Between two band sizes
If your underbust measures an odd number (like 33 inches), try both 32D and 34C. They are sister sizes with the same cup volume -- let the band feel decide.
Important:
Sister sizing is a shopping workaround, not a permanent fit solution. Going more than one size away from your measured size will noticeably change support and comfort. For the most accurate starting point, use our bra size calculator.
Limitations of Sister Sizing
Sister sizing is a practical workaround that preserves cup volume, but it does change other structural elements of the bra that affect comfort and support over extended wear. Moving one band size up shifts the underwire width wider by approximately 5 to 8 millimeters per side, which means the wire may sit on breast tissue rather than the ribcage for women with narrow roots. Conversely, moving one band size down narrows the wire and may compress wider-set breast tissue at the sides. Strap placement also shifts proportionally with band size -- a larger band spaces the straps further apart, which can cause slipping on narrow shoulders, while a smaller band pulls them closer together. The elastic tension profile changes as well, since a band designed for a 32 frame uses different elastic density than a 36 band. For these reasons, sister sizing works best as a one-size adjustment in either direction. Going two or more sizes away introduces enough structural change that professional fitters recommend a full remeasurement rather than continued sister sizing. If you consistently need a sister size because your primary size is unavailable, that pattern may indicate a brand compatibility issue -- our brand sizing guide covers how different manufacturers fit across the size spectrum.
Not sure about your starting size? Use our bra size calculator to find your starting size, then come back here to find your sister sizes. For a full breakdown of what each cup letter means, see our cup size comparison. Sister sizing is especially useful for sports bra sizing, where encapsulation bras often run one band tighter than everyday styles. For teens going through growth spurts, our teen bra guide covers how to size starter bras that accommodate changing measurements.