Breast Size Calculator
Use your measurements to estimate a starting bra size. This tool provides a practical first guess you can refine with fit checks.
Measure around your ribcage, directly under your bust, with a snug tape.
Measure around the fullest part of your bust while standing naturally.
If entered, the calculator averages standing and leaning bust for a better estimate.
How this breast size calculator works
A bra size has two parts: the band size (number) and the cup size (letter). The band comes from your underbust/ribcage measurement. The cup comes from the difference between your bust measurement and band size.
- Band: rounded to the nearest even number (common fitting method).
- Cup: each inch of difference is roughly one cup step.
- Optional leaning bust: helps improve estimates for softer or projected tissue.
Step-by-step measuring guide
1) Measure your underbust
Wrap the tape around your ribcage right under the breast tissue. Keep it level and snug (not painful). Exhale gently and record the number.
2) Measure your full bust
Measure around the fullest part of your chest. Keep shoulders relaxed and tape parallel to the floor. Do not pull the tape too tight.
3) Optional: measure while leaning
Lean forward so your torso is roughly parallel to the floor and measure again. For many people, averaging standing and leaning bust gives a more realistic cup estimate.
Understanding the result
The calculator returns suggested sizes in common systems (US, UK, EU). Start with that size, then adjust based on fit:
- If the band rides up your back, try a smaller band.
- If the band feels painfully tight, try a larger band.
- If cups overflow at the top or sides, go up a cup.
- If cups wrinkle or gap significantly, go down a cup.
Quick cup difference chart (approximate)
| Bust - Band Difference | Typical Cup |
|---|---|
| 0 in | AA |
| 1 in | A |
| 2 in | B |
| 3 in | C |
| 4 in | D |
| 5 in | DD / E |
| 6 in | DDD / F |
| 7 in | G |
| 8 in | H |
Fit checklist after you calculate
Band fit
- Should be level all around, not creeping upward in the back.
- You should fit about two fingers under the band comfortably.
- Most support should come from the band, not the straps.
Cup fit
- No spilling over the top, center, or sides.
- No major wrinkling or empty space in the cups.
- Center gore (middle panel) should sit close to the sternum in most wired bras.
Strap fit
- Straps should be snug but not digging in.
- If straps constantly slip, the cup shape or band may be off.
Common mistakes to avoid
- Using an old stretched bra size as your only reference.
- Measuring over thick clothing or padded bras.
- Holding the tape at an angle instead of level.
- Ignoring comfort for the sake of a specific number/letter.
FAQ
Is this calculator 100% accurate?
No calculator is perfect. It gives a useful starting point, but final fit depends on breast shape, tissue distribution, and brand differences.
Why do sizes vary between brands?
Manufacturing standards differ. A 34D in one brand might fit like a 32DD or 36C in another. Always check fit, not just the label.
What are sister sizes?
Sister sizes keep cup volume similar while changing the band. Example: 34C has sister sizes 32D and 36B.
Final note
Your body can change over time due to cycle phase, weight shifts, pregnancy, training, and age. Re-measure every few months or whenever your bras start feeling uncomfortable.