Understanding Sister Sizes
What Are Sister Sizes?
Sister sizes are bra sizes that share the same cup volume but have different band sizes. The concept is simple: when you change the band size, you must also change the cup letter to maintain the same cup volume. Going down one band size means going up one cup letter, and vice versa.
How Sister Sizing Works
A bra size like 34C has sister sizes at 32D (one band tighter, one cup larger) and 36B (one band looser, one cup smaller). All three sizes have approximately the same cup volume. The difference lies in the band fit — a tighter band provides more support, while a looser band offers more comfort.
Why Sister Sizes Matter
Not all brands manufacture every size, and fit preferences vary. Sister sizes give you alternative options that maintain your cup volume. If your usual size is unavailable, or if you want a different band tension without changing how the cups fit, sister sizes are the answer.
International Sizing Systems
Sister sizing works across all major systems — US, UK, EU, French, and Australian. While the cup letters may differ between systems, the principle remains the same: decreasing the band by one size and increasing the cup by one letter preserves volume.
What Are Sister Sizes and Why Do They Matter?
Sister sizes are bra sizes that share the exact same cup volume but have different band measurements. This concept is fundamental to understanding bra fitting and is one of the most powerful tools for finding the right bra. The principle is simple: when you go down one band size, you go up one cup size to maintain the same cup volume. Conversely, going up a band size means going down a cup size.
For example, if you wear a 34C, your sister sizes are 32D, 36B, 30DD, 38A, and so on. All of these sizes have the same cup volume — the difference is in the band tightness. The cup of a 34C is the same volume as a 32D, just attached to a different band width.
The Mathematics Behind Sister Sizes
The mathematical relationship is straightforward. Cup volume is determined by the difference between bust circumference and band size. A 34C means: bust = 34 + 3 = 37 inches. Now take 32D: bust = 32 + 4 = 36 inches. Wait — that's not the same! Actually, cup volume isn't just about the absolute bust measurement. The cup volume depends on the proportional relationship between bust and band. The cup of a 32D is designed for a 4-inch difference from a 32-inch band, and when sewn into the smaller band, produces the same cup volume as a 3-inch difference on a 34-inch band. This is because cup patterns scale with band size in the manufacturing process.
When to Use Sister Sizes
Sister sizes are invaluable in several real-world scenarios. First, when a bra fits well in the cups but the band is too loose — try going down a band size and up a cup size (e.g., 34C → 32D). Second, when the cups fit but the band is too tight — go up a band and down a cup (34C → 36B). Third, when your preferred brand or style doesn't come in your exact size — try the sister size that's closest. Fourth, during weight fluctuations when your band size changes but your breast volume stays relatively constant.
Common Sister Size Mistakes
The most common mistake is assuming sister sizes will fit identically. While the cup volume is the same, the overall bra will fit differently. A smaller band provides more support and a firmer feel, while a larger band is more relaxed. The cup shape may also differ slightly because cup patterns change with band size. Additionally, very extreme sister sizes (more than 2 steps away) may not work well because the proportions become too distorted.
Another mistake is using sister sizes to avoid buying the correct size. If you're consistently buying a 36B because it's more available when you actually need a 34C, you're compromising on fit. Sister sizes are best used as a temporary solution or for fine-tuning fit between brands.
Sister Sizes Across International Systems
Sister sizes work the same way in all sizing systems, but the conversion can be confusing. In EU sizing, going down one band (e.g., 75→70) and up one cup (C→D) gives you the sister size. In French sizing, the same relationship holds (90C → 85D). The key is that each cup increment always represents approximately one additional inch of bust-band difference, regardless of the naming system used.
Sister Size Chart Quick Reference
Here's a quick reference for the most common sister size chains. Starting from 34C: 30DD → 32D → 34C → 36B → 38A. Starting from 34D: 30DD/E → 32DD → 34D → 36C → 38B. Starting from 36C: 32DD → 34D → 36C → 38B → 40A. These chains work identically in every sizing system — the band-cup inverse relationship is universal. Use our interactive tool above to see your complete sister size matrix for any starting size and any international system.
Professional bra fitters recommend keeping at least one sister size in mind when shopping. If your perfect fit is 32DD but it's not available, 34D (one sister size up in band) will often work with the band on the tightest hook setting. This is especially useful when shopping from brands that don't carry your exact size or when traveling abroad where certain sizes may be more or less commonly stocked.