How bra size is calculated
- Band size: your snug underbust measurement in inches, rounded to the nearest even number (31 → 32, 33 → 34).
- Cup size: bust minus band size, in inches. Each inch is one cup: 1 = A, 2 = B, 3 = C, 4 = D, 5 = DD and so on.
| Bust − band | US cup | UK cup |
|---|
| 0 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 |
Worked example
An underbust of 32 in and a bust of 35 in gives a band of 32 and a difference of 3 in, which is a C cup: 32C in the US and UK. Its sister sizes are 34B and 30D.
Signs your bra fits well
- The band sits level all the way around and you can fit about two fingers under it.
- The cups are smooth, with no gaps or spilling over the top or sides.
- The center panel lies flat against your breastbone.
- The straps stay put without digging in; most support comes from the band.
- New bras should fit on the loosest hook so you can tighten them as they stretch.
Curious about your overall proportions? Try the body shape calculator.
Last reviewed October 2026.
Frequently asked questions
How do I measure my bra size?
Wear an unpadded bra. For the band, measure snugly around your ribcage just under your bust, keeping the tape level. For the bust, measure loosely around the fullest part of your chest. Enter both and the calculator does the rest.
Should I add 4 inches to my band measurement?
Most modern fitters no longer do. The old "plus 4" method came from a time when bras were made of less stretchy material and often gives a band that is too loose. This calculator uses your actual underbust rounded to the nearest even number.
What are sister sizes?
Sister sizes hold the same cup volume on a different band. Going up one band size and down one cup (34C to 36B) keeps the same volume with a looser band; going down a band and up a cup (34C to 32D) does the opposite. They are useful when a band fits but the cup doesn't, or the reverse.
Why are US and UK cup sizes different?
Both start A, B, C, D, DD, but then they split. The US usually goes DDD (or F), G, H, while the UK goes E, F, FF, G, GG, H. European sizes use centimeter bands and single letters. The calculator shows all of them.
Will this size fit in every brand?
Not always. Sizing varies between brands and styles, so treat the result as a starting point and try a couple of sizes, including sister sizes. A well-fitting band sits level and does most of the support.