How this bra size calculator works
The calculator uses two measurements: your underbust and your bust. Your underbust helps estimate band size, and the difference between bust and band helps estimate cup size. This mirrors the method used in many online bra fit tools and in-store fitting guides.
- Band size: estimated by rounding your underbust to the nearest even number.
- Cup size: estimated from the inch difference between bust and calculated band.
- Result: a suggested size plus two sister-size options.
How to measure for best accuracy
1) Underbust measurement
Wrap a soft tape measure directly under your bust, parallel to the floor. Keep it snug but not painfully tight. Exhale normally and record the number.
2) Bust measurement
Measure around the fullest part of your bust, again keeping the tape level all the way around. The tape should rest gently without compressing breast tissue.
3) Enter your values and calculate
Choose inches or centimeters, enter both numbers, and click the button. If your result feels close but not perfect, try your sister sizes.
Understanding bra size: band + cup
A bra size has two parts, like 34C:
- 34 = band size (ribcage fit)
- C = cup volume relative to that band
Cup letters are not absolute by themselves. A 34C and a 36C are not the same cup volume because cup volume changes with band size. That is why sister sizing can be so useful.
Common fit problems and easy fixes
Band rides up in the back
Usually means the band is too loose. Try one band size smaller and one cup size larger (a sister size adjustment).
Spillage at top or sides of cups
Cups may be too small. Increase cup size while keeping band the same, or move to a sister size if band tension is also off.
Gapping cups
Cups may be too large, or the shape may not match your breast shape. Try a smaller cup or a different bra style (balconette, plunge, full coverage).
Why your calculated size is a starting point, not a final answer
Fabric stretch, underwire shape, cup height, and strap placement vary from brand to brand. Even within one brand, a T-shirt bra and lace balconette may fit differently in the same labeled size. Use this calculator to narrow your options quickly, then fine-tune by trying on nearby sizes.
Quick bra fitting checklist
- Band sits level all around and feels firm on the loosest hook when new.
- Center gore (middle panel) lies flat against your sternum.
- Cups fully contain tissue with no major spillage or collapse.
- Straps stay put without digging deeply into shoulders.
- You can breathe comfortably and move freely.
Frequently asked questions
Can I use centimeters?
Yes. The calculator converts cm to inches automatically before estimating your size.
What are sister sizes?
Sister sizes keep similar cup volume while changing band tension. Example: 34C has close cup volume to 32D and 36B.
How often should I re-measure?
Recheck every 6-12 months or after body changes such as weight changes, pregnancy, hormonal shifts, or new fitness routines.
Final thought
A good bra fit can improve comfort, posture, and confidence. Use the “get my bra size calculator” above to find your baseline size, then adjust with sister sizes and style preferences until it feels right for your body.