A starting point for the right fit
A well fitting bra is comfortable all day, and most of its support comes from the band, not the straps. Getting there starts with two numbers from a soft tape measure. This bra size calculator turns them into a band and cup size in US and UK letters, and gives two sister sizes that hold about the same volume.
Treat the result as a place to start. Sizing varies between makers and styles, and the way a bra feels on is the final test.
How to measure
- Wear a light, unpadded bra or none, and stand straight in front of a mirror.
- Underbust: wrap the tape snugly around your ribcage, just under the bust, keeping it level.
- Bust: wrap the tape loosely around the fullest part of the bust, without squeezing.
- Enter both numbers in inches or centimetres, and read your starting size.
How the size is worked out
band = underbust in inches, rounded to the nearest inch; an odd number goes up to the next even one
cup = bust (rounded to the nearest inch) − band
1 inch = A, 2 = B, 3 = C, 4 = D, 5 = DD, and so onUS and UK letters agree up to DD, then part ways: the next cups are DDD and G in the US, but E and F in the UK. Some charts add 4 or 5 inches to the underbust to get the band. That older method often gives a band that is too loose, so this calculator does not use it.
Worked example: underbust 31.5 in, bust 36 in
- Band: 31.5 rounds to 32, which is even, so the band is 32
- Cup: 36 − 32 = 4 inches, which is a D
- Size: 32D in both US and UK letters
- Sister sizes: 30DD (E) with a smaller band, 34C with a larger band
The starting size is 32D. If the band feels tight but the cups fit, try the sister size 34C; if the band rides up but the cups fit, try 30DD (E).
Cup letters by difference
| Difference | US | UK |
|---|---|---|
| 1 | A | A |
| 2 | B | B |
| 3 | C | C |
| 4 | D | D |
| 5 | DD (E) | DD |
| 6 | DDD (F) | E |
| 7 | G | F |
| 8 | H | FF |
| 9 | I | G |
A cup letter on its own means little; it only describes a size together with the band. A 30D and a 36D hold very different volumes.
Signs of a good fit
- The band sits level all the way round and stays put when you lift your arms.
- The centre front lies flat against the breastbone.
- The cups are smooth, with no gaps and no spilling over the top or sides.
- Straps stay up without digging in. A new bra should fit on the loosest hook, since bands stretch with wear.
Frequently asked questions
How do I measure my bra size at home?
You need two measurements with a soft tape. Measure snugly around your ribcage just under the bust for the band, then loosely around the fullest part of the bust. Enter both numbers above in inches or centimetres, and the calculator gives a starting size in US and UK letters.
What is a sister size?
Sister sizes hold about the same cup volume on a different band. Going down one band size and up one cup letter gives a sister size, as does going up a band and down a cup. If 32D fits in the cups but the band is wrong, 30DD (E) and 34C are the ones to try.
Why are US and UK bra sizes different?
Both systems start with the same letters, but they part ways after DD. The US continues with DDD and then G, while the UK goes to E, F and FF. Band numbers are usually the same in both. The calculator shows US and UK letters, so you can shop with either.
Why does my size vary between stores?
Makers cut their bras differently, and styles such as plunge, balconette and full cup fit the same body in different ways. Fabric stretch also varies. Use the calculated size as a starting point, try the sister sizes, and trust how the bra feels over the number on the label.
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