Tip: Measure with a soft measuring tape, level around your body, while wearing a non-padded bra.
How this bra cup size calculator (cm) works
This calculator uses two body measurements in centimeters: your underbust (around your ribcage under the breasts) and your full bust (around the fullest part of the bust). The difference between these two measurements estimates your cup volume, and underbust is used to estimate band size.
You will get an estimated bra size in EU and/or UK format. Keep in mind that bra sizing can vary by brand, style, and fabric stretch, so the result is a strong starting point rather than a final answer.
Step-by-step measuring guide (in cm)
1) Measure your underbust
- Stand upright and breathe out normally.
- Wrap the tape snugly around your ribcage directly under your bust.
- Keep the tape level and parallel to the floor.
- Record in centimeters (for example: 76.8 cm).
2) Measure your full bust
- Measure around the fullest part of your bust.
- Keep tape comfortably snug, not compressing tissue.
- Check that tape is level all around your body.
- Record in centimeters (for example: 91.5 cm).
3) Calculate your cup difference
Cup difference is simple: full bust (cm) − underbust (cm). A larger difference typically means a larger cup letter.
Approximate cup mapping by difference (cm)
| Difference (cm) | EU Cup | UK Cup (approx.) |
|---|---|---|
| < 10 | AA | AA |
| 10–11.9 | A | A |
| 12–13.9 | B | B |
| 14–15.9 | C | C |
| 16–17.9 | D | D |
| 18–19.9 | E | DD |
| 20–21.9 | F | E |
| 22–23.9 | G | F |
| 24–25.9 | H | FF |
| 26–27.9 | I | G |
| 28–29.9 | J | GG |
Understanding your result
If your result says EU 75C, that means:
- 75 is the estimated band size (ribcage support).
- C is the estimated cup volume relative to that band.
In UK sizing, the same person may see a different label format (for example, 34C). Cup letters are not identical between all systems after D cup, so cross-system conversion is always approximate.
Common fitting checks after calculating
Band fit
- The band should feel firm and supportive, not painful.
- It should stay level and not ride up your back.
Cup fit
- No major gaping at top of cups.
- No spillage over the cup edge.
- Underwire (if present) should surround tissue, not sit on it.
Straps and comfort
- Straps should not carry all the weight.
- You should be able to move and breathe comfortably.
Why online calculators can differ
Different calculators use different rounding rules, cup scales, and regional assumptions. Some follow modern fitting methods, others still use older “add inches” rules. That is why your output may differ slightly across websites.
The best approach is to use the calculated size as your baseline, then test nearby sister sizes. For example, if one size feels too tight in band but cup is good, try one band up and one cup down.
Quick FAQ
Is this calculator accurate?
It is accurate enough to provide a practical starting size for most people. Final fit still depends on bra model and brand.
Can I use this for sports bras?
Yes for a starting estimate, but sports bras often use S/M/L systems and compression styles, so fit testing is especially important.
Should I measure over clothing?
For best results, measure over a thin, non-padded bra or directly on skin. Thick clothing introduces errors.
Final note
Good bra fit is about comfort, support, and confidence. Use the calculator, then fine-tune by trying adjacent sizes if needed. A small adjustment in band or cup can make a big difference in all-day comfort.