Bra Sister Sizes: What They Are & How to Use Them

Bra sister sizes are alternate bra measurements that offer a better fit when your usual size isn't available. Learn how to find them and why they matter for comfortable, supportive underwear.

Understanding Bra Sister Sizes: The Basics

Sister sizes (or sister fits) refer to bra sizes with the same cup volume but different band and cup letter combinations. For example, if your usual size is 34C, its sister sizes are 32D (smaller band, larger cup) and 36B (larger band, smaller cup). This relationship exists because adjusting the band size by ±2 inches requires inversely adjusting the cup letter by ±1 to maintain the same breast volume.

How to Calculate Your Sister Sizes

Calculating sister sizes follows a simple rule: for every 2-inch increase in band size, increase the cup letter by 1; for every 2-inch decrease in band size, decrease the cup letter by 1. Let's take a 34B as an example:
- 34B → 32C (band -2" = cup +1)
- 34B → 36A (band +2" = cup -1)
Always start from your actual measured band and cup size for accurate calculations. Use online sister size charts if you need quick references, but verify with physical try-ons since brand sizing can vary.

Why Sister Sizes Are Critical for Bra Fitting

1. Size Availability: Many brands don't carry all size combinations. Sister sizes let you find a comparable fit when your exact size is out of stock.
2. Weight Fluctuations: Minor weight changes often affect band tightness more than cup volume. A sister size can bridge the gap between your pre- and post-weight change sizes.
3. Brand Variations: No two brands size identically. A 34D in Brand X might fit like a 36C in Brand Y—using sister sizes helps normalize these differences.
4. Comfort Adjustments: If your band feels too tight but the cups fit, try a sister size with a larger band and smaller cup (e.g., 34C → 36B) for better daily comfort.

Common Sister Size Myths to Avoid

Myth 1: "All sister sizes fit exactly the same."
Fact: While cup volume remains similar, wire width and cup shape can vary between sizes. Always check underwire placement and cup coverage.
Myth 2: "Sister sizes work for all bra types."
Fact: They're most reliable for full-coverage bras. Push-up or molded cups may have different fit dynamics due to padding and construction.
Myth 3: "You only need one sister size."
Fact: Maintain at least two sister sizes (one up, one down) in your size range to handle band stretch, laundry shrinkage, or temporary body changes like bloating.