What a bra size actually encodes
A bra size is two independent numbers wearing one label. The number is the band, a length: how far it is around your ribcage directly under the bust. The letter is the cup, and it is not a size at all — it is a difference. A D cup means the bust measures four inches more than the band, whatever the band happens to be. That is why a 30D and a 40D are nothing like the same garment: they hold the same four inches of projection on ribcages ten inches apart in circumference.
Reading the letter as an absolute volume is the single most common misunderstanding in bra buying, and it drives the second one: people who find a D cup too small on a loose band assume they need a bigger band, when they need a smaller band and a bigger letter. The band carries most of the support. The straps and the cups shape what the band is already holding up, which is why a band that rides up at the back leaves the whole bra doing nothing useful.
This calculator turns your two tape measurements into that pair, converts it to the five labelling systems you will meet online, and lists the sister sizes — the neighbouring band-and-letter pairs that keep the cup volume roughly fixed. If your tape reads in millimetres, the millimetre-to-inch converter and the height converter translate it first.
The arithmetic: rounding the band, counting the cup
Start with the band. Take the underbust measurement, divide by two, round to the nearest whole number, multiply by two. That is the snug-band method: your band is simply your underbust rounded to the nearest even inch. Bands are cut in even inches rather than in every inch because band elastic stretches: as a working rule a band accommodates about an inch either side of its nominal length, so a two-inch grid leaves no gaps between sizes.
Now the cup. Subtract the band from the bust measurement and round the result to the nearest whole inch. That whole number is k, the number of cup steps. Zero steps is AA, one step is A, two is B, three is C, four is D. Above D the systems diverge, which is the whole reason cross-market conversion is confusing: the United States doubles the letter and keeps going (DD, DDD, DDDD), the United Kingdom alternates single and doubled letters (DD, E, F, FF, G, GG) and skips I entirely, and continental Europe just walks the alphabet without doubling anything (D, E, F, G, H).
The subtraction uses the band, not the underbust. On the snug-band method the two are within an inch of each other, so it makes little difference. On the traditional +4 method it makes an enormous one, because the band you subtract is four to six inches larger — which is exactly why the older charts sent so many people home in a B cup.
Band numbers convert between markets by fixed arithmetic rather than by measurement. The continental band is 2.5 × band − 10, so a 34 becomes a 75. The French and Spanish band is the continental one plus fifteen, so 75 becomes 90. The Australian band is the inch band minus twenty-two, so 34 becomes 12. These are the conversions printed on brand charts, and they are exact, not approximate.
Worked example: 34 in underbust, 39 in bust
Take the calculator's default measurements and work them by hand.
- Underbust. The tape reads 34.0 in, snug but not compressing.
- Band. 34 ÷ 2 = 17.0, which rounds to 17, and 17 × 2 = 34. The band is 34 in.
- Bust. The tape reads 39.0 in at the fullest point, standing upright.
- Cup difference. 39 − 34 = 5.0 in.
- Cup steps. round(5.0) = 5. Counting from AA at zero: AA, A, B, C, D, DD. Five steps is a DD in the American and British sequences, and an E on the continent.
- Size. US 34DD, UK 34DD.
- Continental. 2.5 × 34 − 10 = 75, so EU 75E. French: 75 + 15 = 90, so FR 90E. Australian: 34 − 22 = 12, so AU 12DD.
- Sister sizes. One band down: band 32 with k = 6, which is 32DDD in the American sequence. One band up: band 36 with k = 4, which is 36D.
Now run the same tape through the traditional +4 method to see the size of the discrepancy. The rounded underbust is 34, an even number, so you add four: the band becomes 38. The cup difference is then 39 − 38 = 1.0 in, one step, an A cup. The same body is either a 34DD or a 38A depending only on which chart you picked up, and those are not two labels for one garment. Walk the 34DD out to a 38 band by sister sizes — 34DD, then 36D, then 38C — and the volume-equivalent size on a 38 band is a C. The +4 rule returns an A. So the older chart is wrong twice over: the band is four inches looser than your ribcage, and the cup is two whole letters smaller than the volume you actually need on that band. That is why the 38A rides up at the back and cuts in at the same time, and why going up one letter does not fix it.
How to check the number against the mirror
Treat the calculated size as your starting size, not your answer. A tape measure knows your circumference; it does not know your breast shape, your tissue density, or how a particular brand cuts its cups. Fitters call the calculated size the “first try-on” and expect to move one band or one letter from it.
Check the band first, because everything else depends on it. Fastened on the loosest hook, a correct band sits level all the way round and you can pull it about an inch from your back, no further. If it rides up between the shoulder blades, or you are already on the tightest hook in a new bra, go down a band and up a letter — that is the sister size down. A new bra should fit on the loosest hook so that you have somewhere to go as the elastic relaxes.
Then check the cup. The wire should sit flat against the ribcage all the way round and behind the breast tissue, not on it. Tissue escaping at the top or under the arm means the cup is too small: go up a letter at the same band. Wrinkling or gaping fabric at the top of the cup means it is too large: go down a letter. A wire that stands away from the sternum in the centre is usually a cup that is too small rather than a shape problem, and it is the fastest single diagnostic there is.
Half-inch differences sit exactly on a boundary. If your cup difference is 4.5 in, buy both the D and the DD in the same band and keep the one that lies flat. The same applies to an odd underbust: 33 in rounds to a 34 band by this calculator, but a 32 band with the cup one letter up is an equally defensible reading, and the fabric decides.
Cup letter for each inch of difference
| Difference (in) | United States | United Kingdom & Australia | Europe & France |
|---|---|---|---|
| 0 | AA | AA | AA |
| 1 | A | A | A |
| 2 | B | B | B |
| 3 | C | C | C |
| 4 | D | D | D |
| 5 | DD | DD | E |
| 6 | DDD | E | F |
| 7 | DDDD | F | G |
| 8 | H | FF | H |
| 9 | I | G | I |
| 10 | J | GG | J |
| 11 | K | H | K |
| 12 | L | HH | L |
The British sequence skips I to avoid confusion with the numeral one. American labelling above DDD is the least settled part of the whole system: brands sell the seventh step as DDDD, F or G, so read the brand's own chart at that end.
Band conversion between markets
| US / UK | Europe | France / Spain | Australia / NZ |
|---|---|---|---|
| 28 | 60 | 75 | 6 |
| 30 | 65 | 80 | 8 |
| 32 | 70 | 85 | 10 |
| 34 | 75 | 90 | 12 |
| 36 | 80 | 95 | 14 |
| 38 | 85 | 100 | 16 |
| 40 | 90 | 105 | 18 |
| 42 | 95 | 110 | 20 |
| 44 | 100 | 115 | 22 |
Continental band numbers are quoted on a centimetre scale, and European brands usually publish an underbust range in centimetres against each one. Where that published range is far from your own measurement, follow the brand's chart rather than this conversion.
Do not add four inches
If a chart tells you to add four inches to your underbust, it is reproducing a rule from the era when band elastic barely stretched and the extra inches were structural allowance. Modern band elastic does the stretching for you. Adding four inches now gives you a band that cannot grip and a cup letter at least four places below the snug-band answer. The two errors do not cancel each other. Convert the snug-band answer to the looser band by sister sizes and you still land above the +4 letter: a 34DD is a 38C on a 38 band, not the 38A the old rule hands you. The band is too loose and the cup is genuinely too small. Specialist fitters and most British brands now measure snug and add nothing. This calculator keeps the +4 option only so you can see what an old chart would have told you.
Measuring mistakes that change the answer
- Measuring the underbust over a padded bra. Padding adds nothing under the band but it does lift the tape off the ribs. Measure bare or in a thin, unlined bra.
- Holding your breath. Exhale normally and let the tape settle. Measuring on a full inhale can add an inch to the underbust and knock a full cup letter off the answer.
- Letting the tape ride up at the back. Check in a mirror that the tape is parallel to the floor all the way round. A back that sits an inch high reads short at the front.
- Pulling the bust measurement tight. The underbust tape is snug; the bust tape is loose enough to lie on the fabric without compressing. Compressing the bust is the most common cause of an under-sized cup.
- Rounding twice. Round once, at the end. Rounding the underbust to a whole inch and then rounding the band again can move you a full size.
- Using an old measurement. Weight change, pregnancy, breastfeeding, hormonal contraception and surgery all move both numbers, often in opposite directions. Re-measure rather than assume.
- Trusting the letter across brands. Two brands can cut the same nominal cup with different depth and different wire width. The size is a starting point for a fitting, not a specification.
Key terms
- Band
- The circumference of the bra around the ribcage, labelled in inches in the UK/US systems and in centimetres on the continent. It supplies most of the support.
- Cup difference
- Bust girth minus band, in inches. It is the quantity the cup letter encodes, so the letter is meaningless without the band beside it.
- Sister size
- A neighbouring band-and-letter pair with approximately the same cup volume: go down two inches of band and up one letter, or up two inches of band and down one letter.
- Snug-band method
- The modern approach used here: the band is the underbust rounded to the nearest even inch, with nothing added.
- Wire spring
- A wire that will not lie flat against the ribcage at the centre front. It nearly always signals a cup that is too small rather than a wrong band.
Sister sizes, brand drift, and when a formula stops helping
Sister sizes are the most useful thing a size chart gives you, because they are the vocabulary for fixing a near-miss. The cup of a 34D and the cup of a 32DD hold roughly the same volume; what differs is the length of elastic around the ribcage. So when a bra fits in the cup but the band rides up, you do not need a different cup — you need the sister size down, 32DD. When the band is right but tissue escapes, you stay at 34 and take the next letter. Keeping those two adjustments separate in your head is what turns online buying from a lottery into a process.
Brand drift is the limit of the method. Cup depth, wire width, gore height and the stretch of the band elastic are design choices, not standards, and two garments labelled 34DD from different makers can differ by a full letter in practice. There is no international standard that fixes bra cup volume; ISO 8559-1 standardises how you take the body measurements, and EN 13402-3 standardises how garment sizes are designated from body measurements, but neither dictates how a manufacturer cuts a cup. Treat the calculated size as the size to order first and expect to keep a second size in the same order.
The formula also stops being the right tool when breast shape rather than volume is the problem — very full-on-bottom or very shallow shapes, marked asymmetry, or fitting after breast surgery. Those are fitting-room problems, and a specialist fitter or a post-surgical fitter solves them by style selection, not by arithmetic. For post-operative fitting, follow your surgical team's advice on timing before you buy anything wired.
If you are tracking body measurements more broadly, the same tape feeds several other tools: body mass index from height and weight, the tape-based Navy body fat method, and ideal body weight formulas. Where a tape reads in fractions of an inch rather than decimals, the inch-to-fraction converter will translate 4.375 in into the sixteenth you can actually see on the tape.