The Bra Band-to-Cup Ratio Calculator helps you understand the relationship between your band size and cup volume, which is key to understanding 'sister sizing.' You'll input your current bra size (e.g., 34C). The calculator then shows you a range of equivalent 'sister sizes'—sizes that have the same cup volume but different band and cup letter combinations (e.g., 32D, 36B).

This calculator is a game-changer for bra shopping. Brands can have inconsistent sizing, and understanding your sister sizes gives you more options. If a bra's band is too tight but the cups fit perfectly, you can confidently try the next sister size up (e.g., from 34C to 36B). This knowledge empowers you to find the best possible fit, regardless of brand variations.

Your results will display a chart of your sister sizes. The key takeaway is the principle: if you go up in the band, you must go down in the cup letter to maintain the same cup volume, and vice versa. Use this chart as a powerful tool when you're in the fitting room. It unlocks a wider range of potential sizes, dramatically increasing your chances of finding a bra that fits you perfectly.

• If you sister size down in the band (e.g., 34C to 32D), you may need a band extender for the first few wears as it will feel much snugger.
• Write down your primary sister sizes and keep them in your wallet or on your phone for easy reference when shopping.
• Don't be afraid to ask a professional fitter to check your sister size fit. They can confirm if you've made the right choice.
• Sister sizing is especially helpful with European brands, which can have different sizing standards.
• Remember the rule: Band up, cup down. Band down, cup up.