What actually decides your bra size
Every bra size is really just two numbers stitched together: a band size and a cup letter. The band is how snug the elastic sits around your ribcage, just under the bust — it does most of the support work. The cup letter describes the gap between that band measurement and your bust at its fullest point, which stands in for how much volume needs covering.
Get the band right and the cup mostly falls into place. Get the band wrong — even by one size — and the cup letter that matches your actual shape changes too, which is the whole reason sister sizing (covered further down) exists.
How to measure yourself accurately
A soft measuring tape and two minutes in front of a mirror is all this takes. A little care with the tape makes a bigger difference to the result than anything the calculator does afterward.
- Wear a lightly padded or unlined bra, or go without — a heavily padded or push-up bra will throw off the bust reading.
- Underbust: wrap the tape directly under your bust, around your ribcage. Keep it level front and back, and snug enough that it doesn't slide, not tight enough to dig in.
- Bust: wrap the tape around the fullest part of your chest, usually right across the nipple line, with the tape level and relaxed rather than pulled tight.
- Take both measurements twice and use the average — it's easy to read a tape a half-inch off on the first try.
- Round each measurement to the nearest half inch or half centimetre before entering it below.
The band and cup formula, worked through
Once you have both numbers, the size calculation is genuinely just two short steps:
- Band size: round your underbust measurement to the nearest whole number, then up to the nearest even number. A 32.5-inch underbust rounds to 33, then up to 34.
- Cup size: subtract the band number from the bust measurement. Every inch of difference moves you up one cup letter — a 1-inch gap is A, 2 inches is B, 3 is C, 4 is D, and so on.
Worked example: underbust measures 32 inches (already even, so band = 32). Bust measures 36 inches. The difference is 36 − 32 = 4 inches, which lines up with a D cup. Final size: 32D.
This is the same shortcut most fitting guides teach, and it's a solid starting point — but bodies aren't perfectly standardised, so treat the result as your best first fitting, not a guarantee. If a size feels off, the next section is the fix.
Why the same cup letter means different things in different countries
This is the part that trips up online shoppers the most: a D cup in one country is not necessarily a D cup in another, because the letter progression itself is different. This calculator's 17 sizing systems fall into three families:
| System family | Countries | Cup progression |
|---|---|---|
| US-style | US, CA, MX | AA, A, B, C, D, DD, DDD, G, H, I… |
| UK-style | UK, AU, PK, IN, NZ, ZA | AA, A, B, C, D, DD, E, F, FF, G, GG… |
| Continental | EU, FR, IT, ES, DE, JP, CN, BR | AA, A, B, C, D, E, F, G, H, I… |
Notice the US-style chart skips E and F entirely and jumps from DDD straight to G, while the UK-style chart alternates single and doubled letters, and continental sizing just runs straight through the alphabet. Convert between them by cup position, not by matching letters — which is exactly what the conversion mode above does.
Sister sizing: what to try when the fit is close but not quite right
Sister sizes are different band-and-cup combinations that hold roughly the same cup volume. The rule: move one band size up, and the cup needs to drop one letter to match; move one band size down, and the cup needs to go up a letter.
So if 34C fits in the cup but the band rides up your back, 36B carries a similar cup volume with a looser band. If the band feels loose but the cups fit, 32D tightens the band while keeping roughly the same volume. This calculator lists sister sizes one and two steps in each direction, since real-world fit sometimes needs more than a single step to land right.
Sister sizing is a fitting tool, not a substitute for remeasuring — if you've had a significant weight change, gone through pregnancy, or just haven't checked your size in a couple of years, measuring again from scratch is worth the two minutes.
Signs your current bra doesn't fit
- The band rides up in the back instead of sitting level with the front — usually means the band is too big.
- Cups gape, wrinkle, or leave empty space — the cup is too large for the band it's paired with; try a sister size down in the cup.
- Breast tissue spills over the top or sides — the cup is too small; size up in the cup.
- The center gore (the piece between the cups) doesn't lie flat against your sternum — again, often a cup issue.
- Straps dig in and leave marks, or the band leaves a deep red line — the band is doing too much of the work; a smaller band with wider straps often helps.
- Underwires poke at the sides or dig into breast tissue rather than sitting flat against the ribcage.
If several of these sound familiar even after trying your calculated size and its sister sizes, that's the point where an in-person fitting is worth more than any calculator — a fitter can see how the fabric sits on your specific shape in a way no formula can.
A note on accuracy
Bra sizing has no single international standard, and brands frequently run their cut a size or more away from these generic charts. Treat every result here — and on any bra size calculator — as a well-informed starting point for trying on, not a guarantee of fit. Always check the specific brand's own size chart, and don't hesitate to size up or down between two options if one feels closer than the other.