Sister Size Calculator
Sister sizes are bra sizes that hold the exact same cup volume despite having different band measurements. Enter your current bra size below to calculate your tighter-band and looser-band sister sizes.
Sister Size Calculator Form
Sister Sizes for 34C
If band feels too loose: Try a smaller band size with a larger cup letter.
Starting baseline size entered above.
If band feels too tight: Try a larger band size with a smaller cup letter.
What Are Sister Sizes?
Sister sizes are bra size combinations that share the exact same cup volume capacity despite using different band sizes. Because bra cups are manufactured proportionally to the band size, cup letters are not fixed volumes.
Figure 1: Equal 590 cm³ cup volume capacity across 32D, 34C, and 36B sister sizesWhen you move up one band size (e.g., 34 to 36), going down one cup letter (e.g., C to B) maintains the exact same cup volume capacity.
Why Would I Use a Sister Size?
1. When Your Band Feels Too Tight
If your current band digs into your ribcage or leaves uncomfortable marks, try the sister size with a larger band and smaller cup letter (e.g., move from 34C to 36B).
2. When Your Band Feels Too Loose
If your bra band rides up your back or shifts throughout the day, try the sister size with a smaller band and larger cup letter (e.g., move from 34C to 32D).
3. Brand & Fabric Variations
Some lingerie brands use firmer elastic or stiffer fabrics. Using sister sizes allows you to adjust band firmness without losing your cup fit.
Explore More Bra Sizing Tools
- Calculate your baseline starting size with our Bra Size Calculator.
- Compare international band and cup labels on our Bra Size Chart.
- Convert known sizes between regions using our Bra Size Converter.
- Understand cup volume capacity ratios in our How Cup Sizes Work Guide.
- Fix sliding bands with our Band Riding Up Diagnostic.
- Diagnose common fit issues with our comprehensive Fit Guide.