The cup letter describes how much fuller your bust is than your ribcage. It is not a fixed volume: a C cup on a 65 band is smaller than a C cup on a 90 band, because the cup grows with the band. This calculator takes two measurements, the bust and the underbust, finds the difference, and turns it into the European (EN 13402) cup letter and full bra size.
How to use the cup size calculator
- Measure your underbust directly under the bust with the tape level and snug.
- Measure your bust around the fullest part of your chest, with the tape level across the back and not squeezing the tissue. Stand upright and breathe normally.
- Enter both numbers in the same unit. You can pick centimetres or inches for each field.
- Read the result: the difference between the two measurements, your cup letter, your EU band and the combined size such as 80C.
Cup size formula
Cup letter = the letter whose range contains the difference
EU band = underbust rounded to the nearest 5 cm
EN 13402 steps the cup letter every 2 cm of difference. Following the published description, each range runs from just above the lower value up to and including the upper value, so a difference of exactly 18 cm is C and 18.1 cm is D.
| Cup | Difference (cm) | Difference (inches) |
|---|---|---|
| AA | 10–12 | 3.94–4.72 |
| A | 12–14 | 4.72–5.51 |
| B | 14–16 | 5.51–6.30 |
| C | 16–18 | 6.30–7.09 |
| D | 18–20 | 7.09–7.87 |
| E | 20–22 | 7.87–8.66 |
| F | 22–24 | 8.66–9.45 |
| G | 24–26 | 9.45–10.24 |
| H | 26–28 | 10.24–11.02 |
Differences of 10 cm or less are smaller than AA, and the range here stops at H (26–28 cm). The calculator tells you when your difference is outside the table instead of guessing a letter.
Worked example
Your underbust is 78 cm and your bust is 96 cm.
- Difference: 96 − 78 = 18 cm, which falls in the 16–18 cm range, so the cup is C.
- Band: 78 ÷ 5 = 15.6, rounded to 16, times 5 = 80.
- Your EU size is 80C.
More examples from the same rule:
| Underbust | Bust | Difference | EU size |
|---|---|---|---|
| 70 cm | 86 cm | 16 cm | 70B |
| 75 cm | 93 cm | 18 cm | 75C |
| 80 cm | 100 cm | 20 cm | 80D |
| 85 cm | 109 cm | 24 cm | 85F |
| 90 cm | 118 cm | 28 cm | 90H |
Things to keep in mind
- Edges differ between calculators. Some calculators use whole-centimetre ranges (for example 10–11 for AA, 12–13 for A), so a measurement close to a boundary can give a neighbouring letter there. Try both letters in the shop if you are within a centimetre of an edge.
- Cup letters are not universal. US, UK and Australian sizes use different letter sequences after D, such as DD or E, and brands differ again. This page gives the European letters.
- The band comes first. If the band feels too loose or tight, change the band and then re-check the cup with the Sister Size Calculator, which keeps the same cup volume.
- Shape matters. The tape cannot see shape, firmness or breast position. Gaping, spilling or wrinkling in the cup means try another cup letter.
To get only the band use the Band Size Calculator, and for the full result in one step try the European Bra Size Calculator or the bra-size-calculator.
Sources
The 2 cm cup steps and the "bust minus underbust" rule come from public descriptions of EN 13402-3. The paid standard was not available, so two public explanations were compared; they agree on the 2 cm step and differ only on how boundary values are assigned.
- MobileFish — EN 13402 pictogram help: cup and band — checked 2026-10-15
- Wikipedia — Brassiere measurement (European sizes) — checked 2026-10-15
- Calculator.net — Bra size chart (whole-cm ranges) — checked 2026-10-15
Frequently asked questions
How do I calculate my cup size?
Subtract your underbust measurement from your bust measurement. In the European system each 2 cm of difference is one cup letter: 14–16 cm is B, 16–18 cm is C, 18–20 cm is D and so on.
Is a C cup always the same size?
No. The cup letter is a difference, so it scales with the band. A 90C holds more volume than a 65C. If you change the band, use the Sister Size Calculator to keep the same volume.
Why does another calculator give a different cup letter?
Boundaries are handled differently. This page follows the EN 13402 description (lower value excluded, upper value included); some calculators use whole-centimetre ranges. Within a centimetre of an edge, try both letters.
Should I measure the bust with a bra on?
Measure the underbust and bust over an unpadded bra or without one, standing up with the tape level. Padding and wires add to the number and push the result up.
What if my difference is below AA or above H?
The calculator says so rather than guessing. Very small and very large differences are covered by specialist ranges and brand charts, so check a brand size guide or ask a fitter. The Band Size Calculator still gives a reliable band.