How bra sizes work
A bra size has two parts: a number for the band around your ribcage and a letter for the cup. The cup letter is not a fixed volume. It shows the difference between your bust and your band, so a 36D holds more volume than a 32D even though both are a D.
This is why the same body can wear several sizes in different brands, and why the letter alone says little.
How to calculate your size manually
- Measure your underbust in inches, snugly beneath the bust.
- Round it to the nearest even number. That is your band.
- Measure around the fullest part of your bust.
- Subtract the band from the bust measurement and round to the nearest whole inch.
- Match the difference to a cup letter.
Cup size chart
| Bust minus band | US cup | UK cup |
|---|---|---|
| Under 1 in | AA | AA |
| 1 in | A | A |
| 2 in | B | B |
| 3 in | C | C |
| 4 in | D | D |
| 5 in | DD (E) | DD |
| 6 in | DDD (F) | E |
| 7 in | G | F |
| 8 in | H | FF |
| 9 in | I | G |
| 10 in | J | GG |
Letters differ between countries. UK sizing adds double letters (DD, FF, GG) in a different order than US sizing.
US, UK, EU and AU sizes
Band numbers also differ by region. US and UK bands use inches (32, 34, 36). EU bands use centimeters in steps of 5 (70, 75, 80). French bands run 15 higher than EU (85, 90, 95). Australian and New Zealand bands use small numbers such as 10, 12 and 14.
The conversions in this tool are approximate. Always check the size chart of the brand you are buying from.
What are sister sizes?
Sister sizes have a different band but hold about the same cup volume. Going down a band size means going up a cup letter, and going up a band means going down a cup. For example, 32D is a sister size to 34C and 30DD. They are worth trying if your usual size does not fit well, for instance if the band rides up or feels too tight.
Signs your bra does not fit
- The band rides up your back. It may be too big.
- Straps dig in and carry most of the support. The band may be too loose.
- Breast tissue spills over the top or sides. The cup may be too small.
- Gaps or wrinkles in the cup. The cup may be too big.
- The center panel does not rest against your chest.
Tips for better measuring
Measure standing straight and breathing normally. Use a soft tape and keep it level. Measure more than once, since weight changes, hormones, pregnancy and time of month can change your size. If your results fall between sizes, try both and the sister sizes. A professional fitting at a lingerie shop can also help.