everyday ToolUpdated 2026-09-20

Bra Size Calculator & Sister Size Chart

Over 80% of women wear the wrong bra size. Enter your snug underbust and standing bust measurements in inches to determine your exact band size, cup letter, and sister sizes.

Formula & Mathematical Methodology

Band = Underbust + 4 (even) or + 5 (odd); Cup = Bust - Band Difference in inches

Band size provides the foundation of support. The cup size is determined by the numerical difference between the fullest bust measurement and the band size (1" = A, 2" = B, 3" = C, 4" = D, 5" = DD/E).

Step-by-Step Worked Example

Scenario: 30-inch underbust and 35-inch bust.

1. Even underbust 30 + 4 = 34 Band Size.

2. Bust measurement = 35 inches.

3. Difference: 35 - 34 = 1 inch difference = A Cup.

4. Direct measurement with snug fit: 30 underbust gives modern 30DD sister size or standard 34B.

Final Calculated Result: 34B (Sister sizes: 32C, 36A)

Frequently Asked Questions

What are sister sizes in bras?

Sister sizes are bras with identical cup volume but different band sizes. If your band feels too tight or loose, moving one band size up or down while adjusting the cup in the opposite direction preserves the cup fit.