Bra sister sizes are bra sizes that have the same cup volume but different band sizes. When you decrease the band size by one, you increase the cup size by one to maintain the same cup volume (and vice versa). For example, 34C, 32D, and 36B are all sister sizes—they have identical cup volumes despite different band and cup letter combinations.

Sister sizes work because cup size is relative to band size, not absolute. A "D cup" doesn't have a fixed volume—it represents the difference between your bust and underbust measurements. When you change band size, the cup letter must change to represent the same physical volume. Think of it as: Band down + Cup up = Same volume and Band up + Cup down = Same volume.

Yes, sister sizes are mathematically accurate for cup volume. However, the fit experience will differ because the band tightness changes. A 32D will feel tighter around the ribcage than a 34C, even though the cups hold the same volume. The wire width and cup shape may also vary slightly between sizes.

Yes! Sister sizes work well for many purposes: when a brand runs large or small, when your exact size is unavailable, when you're between sizes, or when you prefer different support levels. However, your "true size" will always provide the best overall fit. Sister sizes are a valuable tool, not a replacement for proper sizing.

36C sister sizes:

• 34D - One band smaller, one cup larger
• 32DD - Two bands smaller, two cups larger
• 38B - One band larger, one cup smaller
• 40A - Two bands larger, two cups smaller

All have the same cup volume as 36C.

30C sister sizes:

• 28D - One band smaller, one cup larger
• 32B - One band larger, one cup smaller
• 34A - Two bands larger, two cups smaller

Note: 28 bands are less common in stores but available from specialty retailers.

Victoria's Secret uses US sizing, so standard US sister sizes apply. However, VS is known to run small in cups. If your VS bra cups are too small, try a sister size with a smaller band and larger cup (e.g., if 34C is small, try 32D). Many VS customers find they need to size up one cup compared to UK brands.

US and UK band sizes are identical (32, 34, 36, etc.). The difference is in cup progression after D cup:

• US: D → DD → DDD → G → H
• UK: D → DD → E → F → FF → G

Sister size logic works the same in both systems. A US 34DD = UK 34DD, but US 34DDD = UK 34E.

EU uses centimeter bands (65, 70, 75, 80...) where each size is 5cm apart. The conversion: 70≈32, 75≈34, 80≈36. Sister sizes work identically—70D = 65E = 75C. EU cups progress linearly (A, B, C, D, E, F, G...) without the DD/DDD naming convention.

Australia and New Zealand use unique band numbers (8, 10, 12, 14...) but UK cup letters. Conversions: AU 8=US 30, AU 10=US 32, AU 12=US 34, AU 14=US 36. Sister sizes follow the same pattern: AU 12D = AU 10DD = AU 14C.

Sister sizes mean you have options! While cup volume stays constant:

• Smaller band sister: Tighter fit, more support, may feel more secure
• Larger band sister: Looser fit, more comfort, may feel less restrictive

Choose based on your preference and how the specific brand fits.

The mathematical relationship is the same, but brand sizing varies. A 34D in Brand A might fit like a 32DD in Brand B. This is exactly why sister sizes are useful—they help you adapt to different brands' interpretations of sizing. Always try on when possible.