OneTaskBy Tim King · Updated
Bra Size Calculator
Band and Cup in UK, US, EU, FR and AU Sizes
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How the Bra Size Calculator works
This bra size calculator turns two tape-measure readings into a band and cup size. Measure snugly around your ribcage just under the bust, then loosely around the fullest part, and enter both in centimetres or inches. You get the size in UK, US, EU, French and Australian sizing, plus the sister sizes to try if the fit is close.
Bra sizes look simple but each country counts differently. UK and US bands are in inches, EU bands are in centimetres, and French and Australian bands are offset from those. Cup letters also part ways after D: the UK uses DD, E, F and FF, while the US uses DD, DDD and G. Getting both numbers from one pair of measurements saves a lot of guessing between brands.
How the bra size calculator works
The method uses the European standard. The formula for the band is the underbust in centimetres rounded to the nearest 5, so 73 cm becomes an EU 75. The UK and US band is (EU band − 60) ÷ 5 × 2 + 28, which makes EU 75 a UK 34. French bands add 15 to the EU band and Australian bands subtract 22 from the UK band. The cup comes from the difference between bust and underbust: 12 to 14 cm is an A, and each further 2 cm is one cup larger.
First example. An underbust of 75 cm and a bust of 91 cm give an EU 75 band and a 16 cm difference, which is a C cup. The size is UK 34C, US 34C, EU 75C, FR 90C and AU 12C. The sister sizes are 32D and 36B.
Second example. In inches, an underbust of 30 in and a bust of 35 in convert to 76.2 cm and 88.9 cm. The band rounds to EU 75, the 12.7 cm difference is an A cup, and the size is 34A with sister sizes 32B and 36AA.
How many cup sizes per inch? Roughly one cup for every inch, or every 2 cm, of difference between bust and underbust. The table shows the full run of cup letters in each system.
Bust minus underbust difference with the cup size in EU, UK and US sizing
| Difference (cm) | Difference (in) | EU cup | UK cup | US cup |
|---|---|---|---|---|
| 10 cm | 3.9 in | AA | AA | AA |
| 11 cm | 4.3 in | AA | AA | AA |
| 12 cm | 4.7 in | A | A | A |
| 13 cm | 5.1 in | A | A | A |
| 14 cm | 5.5 in | B | B | B |
| 15 cm | 5.9 in | B | B | B |
| 16 cm | 6.3 in | C | C | C |
| 17 cm | 6.7 in | C | C | C |
| 18 cm | 7.1 in | D | D | D |
| 19 cm | 7.5 in | D | D | D |
| 20 cm | 7.9 in | E | DD | DD/E |
| 21 cm | 8.3 in | E | DD | DD/E |
| 22 cm | 8.7 in | F | E | DDD/F |
| 23 cm | 9.1 in | F | E | DDD/F |
| 24 cm | 9.4 in | G | F | G |
| 25 cm | 9.8 in | G | F | G |
| 26 cm | 10.2 in | H | FF | H |
| 27 cm | 10.6 in | H | FF | H |
| 28 cm | 11.0 in | I | G | I |
| 29 cm | 11.4 in | I | G | I |
| 30 cm | 11.8 in | J | GG | J |
| 31 cm | 12.2 in | J | GG | J |
| 32 cm | 12.6 in | K | H | K |
| 33 cm | 13.0 in | K | H | K |
Tips and pitfalls
- Measure without padding. Wear an unpadded bra or none, stand straight and keep the tape level all round.
- The band does the work. Most support comes from the band, so it should feel firm on the loosest hook when new.
- Sister sizes keep the same cup volume. Going down a band and up a cup, such as 34C to 32D, holds the same cup but fits more firmly.
- Brands vary. Treat the result as a starting point and try the neighbouring sizes, especially between UK and US labels.
FAQ
How do I measure my bra size?
Measure snugly around your ribcage directly under the bust for the underbust, then loosely around the fullest part of the bust for the bust. Keep the tape level and breathe out normally. Enter both numbers above.
What are sister sizes?
Sister sizes hold the same cup volume on a different band. Going down one band and up one cup letter, or up one band and down one cup, keeps the cup volume the same. The sister sizes of 34C are 32D and 36B.
Is a UK 34DD the same as a US 34DD?
Yes, up to DD the UK and US cups match. After that they split: UK E is US DDD, UK F is US G, and UK FF is US H. Bands are the same in both, measured in inches.
Why does my bra size differ between brands?
There is no single enforced standard, and fabrics, wire shapes and cup styles all change the fit. Two brands' 34C can feel a size apart, so try sister sizes and the next band when a fit is close.
How do EU and French bra sizes work?
EU bands are the underbust in centimetres rounded to 5, such as 70, 75 and 80. French bands are 15 higher, so EU 75 is French 90. Cup letters run the same in both, rising one letter every 2 cm.
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