Bra Size Calculator – Band, Cup, Sister Sizes and Conversions
This Bra Size Calculator helps you estimate your bra size based on your underbust and bust measurements, then refine the fit with sister sizes, international size conversions and simple fit suggestions. It is designed as a practical starting point for finding a more comfortable and supportive bra.
How the Bra Size From Measurements Works
The calculator uses a measurement-based approach commonly used in US and UK sizing:
- Band size: Based on your snug underbust measurement, rounded to the nearest even band size.
- Cup size: Based on the difference between your bust and band in inches. The larger the difference, the larger the cup letter.
This method provides an estimate and is intended as a guide. Trying on bras and checking the fit in a mirror is still important.
Understanding Sister Sizes
Sister sizes are bra sizes that have the same cup volume but different band sizes. Moving to a tighter band usually means going up a cup letter, and moving to a looser band usually means going down a cup letter. For example, 34C often has a similar cup volume to 32D and 36B.
Using the Size Converter
Bra sizing varies by region. The size converter helps you map a known size into several common systems:
- US/UK
- EU
- FR/BE
- AU/NZ
- JP
This is useful when shopping online from international brands or when moving between countries.
Fit Checker Suggestions
The fit checker uses your current size and a few simple fit questions to give guidance such as:
- When to try a tighter or looser band
- When to size up or down in the cup
- How sister sizes can help if only one part of the fit feels wrong
The tool cannot replace a professional fitting, body scan or expert consultation, but it offers a helpful, quick way to understand how band and cup changes affect overall fit.