How to measure for a bra
You only need two measurements, taken with a soft tape measure and ideally while wearing a thin, non-padded bra:
- Band (underbust): wrap the tape snugly and level around your ribcage, directly underneath your bust. This should be firm — the band does most of the support.
- Bust (fullest): wrap the tape loosely and level around the fullest part of your chest, usually across the nipples, without compressing.
How band and cup size are figured
Your band size is your underbust measurement rounded to the nearest even number — so a 31½-inch underbust becomes a 32 band. Your cup size comes from the difference between your bust and your band: roughly one cup letter per inch.
Cup = Bust − Band, at about one letter per inch of difference
Cup size by measurement difference
| Bust − band difference | US cup |
|---|---|
| Less than 1 in | AA |
| 1 in | A |
| 2 in | B |
| 3 in | C |
| 4 in | D |
| 5 in | DD (E) |
| 6 in | DDD (F) |
| 7 in | G |
| 8 in | H |
Sister sizes: when you're between fits
Cup volume stays the same when you move a band up and a cup down, or a band down and a cup up. If your calculated band digs in, try the next band up with one cup smaller; if it rides up your back, try the next band down with one cup larger. The calculator shows both sister sizes above so you have ready alternatives to test.
A quick example
A 32-inch underbust and a 36-inch bust give a 32 band and a 4-inch difference, which is a D cup — a size 32D. Its sister sizes are 30DD and 34C, all sharing the same cup volume.
Frequently asked questions
How do I measure my bra size at home?
Take two measurements with a soft tape, ideally while wearing an unpadded bra. First, measure your band: wrap the tape snugly around your ribcage directly under your bust, keeping it level. Second, measure your bust: wrap the tape around the fullest part of your chest, loosely and level. Enter both numbers and the calculator does the rest.
How is cup size calculated?
Cup size comes from the difference between your bust measurement and your band size. Each inch of difference is roughly one cup: a 1-inch difference is an A, 2 inches a B, 3 inches a C, 4 inches a D, 5 inches a DD, and so on. So the cup letter is not an absolute size — it only makes sense paired with a band number.
What are sister sizes?
Sister sizes are bra sizes that share the same cup volume but have a different band. When you go down one band you go up one cup (34C → 32D), and when you go up one band you go down one cup (34C → 36B). They are useful when your calculated band feels too tight or too loose but the cups fit — try a sister size instead.
Why is the same size different between brands?
There is no enforced global standard for bra sizing, so each brand drafts its cups and bands a little differently, and styles vary too. Your calculated size is an excellent starting point, but expect to try a size up or down — and the relevant sister sizes — between labels to find the best fit.
Is a D cup always large?
No — a cup letter only describes volume relative to its band. A 30D and a 38D have very different cup volumes because the band is larger on the 38. That is why fitters always quote the full size, band and cup together, rather than the cup letter alone.
How do US, UK, and European sizes differ?
Band numbers in the US and UK are based on inches and are usually the same, but cup letters diverge above a DD: a US DDD is roughly a UK E, and the letters continue differently from there. European sizes use a centimeter-based band (such as 70, 75, 80) with letter cups. This tool gives the US/UK band number with US cup letters; convert from there for EU sizing.
Disclaimer: Bra sizing is not standardized, so this is a starting estimate using the common US/UK method. Fit varies by brand, style, and body shape — use your calculated size and its sister sizes as a guide, and try bras on where you can. This tool does not provide medical advice.