Size Guide
Bra Size Guide: Band, Cup & Sister Sizes Explained
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A bra size is really two measurements wearing one label. The number is your band (how big around your ribcage is just under your bust), and the letter is your cup, which describes how much bigger your bust is than that ribcage measurement. Once you understand that the letter is a difference, not an absolute volume, the whole system clicks: it explains why a D cup on a 32 band is smaller than a D cup on a 38 band, and why "sister sizes" exist at all. This guide walks through how to measure yourself with an ordinary tape measure, works one example from start to finish, and untangles the US, UK, and EU labeling systems.
How band and cup sizing works
The band size comes from your underbust measurement: the circumference of your ribcage directly below your breasts. In US and UK sizing that number is expressed in inches, usually rounded to the nearest even number (30, 32, 34, 36…). The band does most of the supporting work, which is why a well-fitting band sits level and snug rather than riding up your back.
The cup size is the difference between your full bust measurement and your underbust measurement. As a rule of thumb, each inch of difference equals one cup letter: about 1 inch is an A cup, 2 inches a B, 3 inches a C, 4 inches a D, 5 inches a DD, and so on. That is why the same letter means different things on different bands: the cup is always relative to the ribcage it sits on.
How to measure yourself, step by step
You need a soft tape measure and, ideally, a mirror. Wear an unpadded or lightly lined bra, or none at all. For more detail on taking accurate body measurements generally, see our how to measure guide.
- Measure your underbust. Wrap the tape around your ribcage directly under your bust, keeping it level all the way around. It should be snug: parallel to the floor, no slack, but not digging in. Exhale normally and note the number.
- Find your band size. Round the underbust measurement to the nearest even number. Many fitters suggest that if you land on an odd number, you try both neighboring even bands and keep whichever stays put when you raise your arms.
- Measure your full bust. Wrap the tape around the fullest part of your bust, usually at nipple level. Keep the tape level across your back and let it rest gently — don't compress the tissue.
- Subtract to get your cup. Full bust minus underbust, rounded to the nearest whole inch. Count one cup letter per inch of difference.
A worked example
Say your underbust measures 31 inches and your full bust measures 36 inches. Rounding 31 to the nearest even number gives a 32 band. The difference between bust and band is 36 − 32 = 4 inches, and four inches of difference corresponds to a D cup. Put the two together and your starting size is 32D. Treat that as a starting point, not a verdict: cut, padding, and stretch vary between styles, so the tape gets you to the right shelf and trying on gets you to the right bra.
Sister sizes: same cup volume, different band
Because the cup letter is relative to the band, you can trade band for cup and keep roughly the same cup volume. Go down a band, up a cup or up a band, down a cup and you have a "sister size." This is the trick to reach for when the cups fit perfectly but the band is wrong, or when a brand simply doesn't stock your exact combination.
| Band down, cup up | Your measured size | Band up, cup down |
|---|---|---|
| 30C | 32B | 34A |
| 30D | 32C | 34B |
| 30DD | 32D | 34C |
| 32DD | 34D | 36C |
| 34DD | 36D | 38C |
One caveat: sister sizing changes the band, and the band is where the support comes from. A sister size is a good backup plan, not an upgrade — if your measured band fits, stick with it. Sizing also matters beyond bras: sports bras at activewear brands often use S–M–L instead of band-and-cup sizing, which our Lululemon size chart and fit guide covers in detail.
US vs UK vs EU: the cup letters diverge
Through a D cup, US and UK letters agree. Above that they split: the UK system runs DD, E, F, FF, G, GG, while the US commonly runs DD, DDD, G, H (with some brands using E and F instead of DD and DDD, which adds to the confusion). EU sizing labels bands in centimeters (a US/UK 32 band is roughly a 70 EU band), and its cups run in single letters. The tables below show the approximate equivalences; treat them as a guide, since brands apply these systems unevenly.
Band conversion (approximate)
| US / UK band (in) | 30 | 32 | 34 | 36 | 38 |
|---|---|---|---|---|---|
| EU band (cm) | 65 | 70 | 75 | 80 | 85 |
Cup letter conversion (approximate)
| Bust − band | US cup | UK cup | EU cup |
|---|---|---|---|
| 3 in | C | C | C |
| 4 in | D | D | D |
| 5 in | DD | DD | E |
| 6 in | DDD | E | F |
| 7 in | G | F | G |
| 8 in | H | FF | H |
Shopping across borders? Bra bands are only one piece of the puzzle. Our international size conversion charts cover clothing and shoe sizes across the US, UK, and EU as well.
Bra sizing FAQ
How do I calculate my bra size at home?
Measure snugly around your underbust for your band size, then measure loosely around the fullest part of your bust. Subtract the underbust number from the bust number: each inch of difference is roughly one cup letter (1 inch = A, 2 = B, 3 = C, 4 = D, and so on).
What is a sister size in bras?
A sister size is a different band-and-cup combination that holds roughly the same cup volume. Go down a band and up a cup (32D to 30DD) or up a band and down a cup (32D to 34C). Sister sizes are useful when a bra fits in the cup but the band feels too tight or too loose.
Is a UK DD cup the same as a US DD cup?
At DD the two systems still line up, but they diverge just above it. After DD, UK sizing runs E, F, FF, G, GG, while US sizing commonly runs DDD, G, H. So a UK E is roughly a US DDD, not a US E, which is why converting letter by letter goes wrong past DD.
What EU bra size is a US 32D?
Approximately a 70D. EU bands are labeled in centimeters, and a US or UK 32 band corresponds to about a 70 EU band (30 is about 65, 34 about 75, 36 about 80). Cup letters match closely through D, so 32D converts to roughly EU 70D.
Keep reading
- International size conversion charts
- Lululemon size chart & fit guide
- How to measure yourself at home
- Gap & Old Navy size chart & fit guide
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