Bra Size Calculator
Enter your underbust and full bust measurements to estimate your bra size. This is a starting point for fit, not a final rule.
How to use this determine bra size calculator
A bra that fits well can improve comfort, posture, and confidence. The challenge is that many people are wearing the wrong size band, cup, or both. This determine bra size calculator gives you a practical estimate based on two numbers:
- Underbust: measured snugly around your ribcage, directly under breast tissue.
- Full bust: measured around the fullest point of your bust while standing straight.
Use the result as your starting size. Bra fit can vary by brand, style, and fabric stretch, so trying nearby “sister sizes” is often the key to finding your best fit.
Step-by-step measuring guide
1) Measure your underbust correctly
Wrap a soft tape measure around your ribcage directly beneath your bust. Keep the tape level and snug (not painful). Exhale normally and record the number. If you are between two values, keep the decimal.
2) Measure your full bust
Measure around the fullest part of your bust, usually across the nipples. Keep the tape parallel to the floor and relaxed. Do not pull too tight; compressing tissue will skew cup calculations.
3) Keep posture and tape level consistent
The biggest measurement errors happen when the tape rides up at the back or angles downward at the front. Standing in front of a mirror can help you keep the tape level.
How the calculator estimates your size
The calculator follows standard sizing logic:
- Band size: estimated from underbust measurement (nearest even number in modern mode).
- Cup size: estimated from the difference between full bust and band size.
Every additional inch of bust-to-band difference generally corresponds to one cup step. This means cup volume is not absolute: a 32D and 34C are different labels but similar cup volume.
| Difference (inches) | Estimated Cup |
|---|---|
| 0 | AA |
| 1 | A |
| 2 | B |
| 3 | C |
| 4 | D |
| 5 | DD/E |
| 6 | DDD/F |
| 7+ | G and above |
What are sister sizes and why they matter?
Sister sizes are nearby sizes with similar cup volume but different band lengths. If your cups feel right but the band is too tight, go up one band and down one cup (for example, 34D → 36C). If the band is too loose, go down one band and up one cup (34D → 32DD).
This calculator displays sister-size suggestions to make fitting easier when your first try is close but not perfect.
Common fit issues and quick fixes
Band rides up in back
Usually means the band is too loose. Try a smaller band and one cup letter up to maintain cup volume.
Cups gape at the top
Try a smaller cup, a different cup shape, or both. Gaping is sometimes a shape mismatch, not just a size issue.
Spillage at top or sides
Try a larger cup or a different style with more coverage. If the band also feels tight, adjust both band and cup using sister sizing.
Straps digging into shoulders
The band should provide most support. If straps carry too much weight, your band may be too loose or worn out.
Tips for better bra shopping
- Try bras on the loosest hook first; tighten over time as elastic ages.
- Re-measure every 6–12 months or after major weight or hormonal changes.
- Know your size can differ across sports bras, balconette bras, plunge bras, and bralettes.
- Judge fit by comfort and support, not just by label.
Frequently asked questions
Is this calculator accurate for everyone?
It is accurate as a starting estimate for many people, but no calculator can perfectly account for breast shape, root width, firmness, and brand variation.
Should I use inches or centimeters?
Either works. The calculator converts units internally and gives equivalent results.
Why do I get a different size than my current bra?
Many people are fitted into convenient stock sizes rather than ideal sizes. Also, bra sizing standards vary across brands and countries.
Can this help with sports bras?
Yes. Start with the estimated size, then focus on bounce control, band security, and breathable fabric for your activity level.
Final note
This determine bra size calculator is designed to help you quickly estimate a bra size from your own measurements. Use it as your foundation, then fine-tune with sister sizes and real-world try-ons for the best comfort and support.