Bra Cup Calculator
Enter your bust and underbust measurements to estimate a starting bra size. This gives you a practical baseline for shopping online or narrowing options before an in-store fitting.
Tip: Measure while braless or in a thin, unpadded bra for the most consistent result.
How this bra cup calculator works
A bra size has two parts: band size and cup size. The band is based on your ribcage (underbust), while the cup is based on the difference between your bust measurement and your band size.
This calculator follows a common fitting approach:
- Band size is estimated by rounding your underbust to the nearest even number.
- Cup size is estimated from bust-to-band difference (in inches).
- You get a suggested starting size and nearby sister sizes.
How to measure correctly
1) Underbust measurement
Wrap the tape measure directly under your bust, parallel to the floor, and pull snugly (not painfully tight). Exhale naturally and record the number.
2) Bust measurement
Measure around the fullest point of your chest. Keep the tape level and comfortably loose so it does not compress tissue.
3) Take each measurement 2–3 times
If your readings vary, use the average. Small errors can shift your cup estimate by a full letter.
Cup difference chart (quick reference)
| Bust - Band Difference | Approx Cup (US) |
|---|---|
| 0" | AA |
| 1" | A |
| 2" | B |
| 3" | C |
| 4" | D |
| 5" | DD (E) |
| 6" | DDD (F) |
| 7" | G |
| 8" | H |
Understanding sister sizes
If one size feels close but not perfect, sister sizing can help:
- Tighter band, bigger cup letter: e.g., 34C → 32D
- Looser band, smaller cup letter: e.g., 34C → 36B
Remember: cup volume is tied to band size. A 34C and 36C do not have the same cup volume.
Common fit checks after calculating
Band fit
The band should sit level around your torso and do most of the support work. If it rides up your back, it is usually too loose.
Cup fit
No spilling over the top, no gaping, and no major wrinkling. If tissue spills out, go up a cup. If cups wrinkle, try a smaller cup or a different cup shape.
Straps and center gore
Straps should not dig deeply into shoulders. The center gore (middle panel) should lie flat against your sternum in most underwire bras.
Final note
This bra cup calculator gives an estimate, not an absolute rule. Brand sizing, bra style (balconette, plunge, full cup, sports), and breast shape can all change your best fit. Use this result as your starting point, then refine by trying adjacent sizes.