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Bra Size Explained: Band, Cup, and Sister Sizes
The band is the support. The cup is the volume. Most people wear a band too big and a cup too small. Measure over an unlined bra, not a wish.
Updated 9 Aug 2026 · 8 min read · bra size explained
Band versus cup
The band, not the straps, carries most of the weight. It should sit level, on the loosest hook when the bra is new, snug enough that two fingers fit but the whole bra does not lift when you raise your arms. A band that rides up the back is too large — the cups then dump forward and the straps start doing a job they were not built for. Tightening the straps on a big band is how you get grooves in the shoulders and no lift.
The cup is a volume. It is not a moral category. UK letters run A, B, C, D, DD, E, F… US brands often use DD/DDD and then jump; EU uses centimetres (75B). A 34C is not “smaller” than a 32D. They are sistered. If you have been in a 36B for ten years and the band floats, you are probably a 34C or 32D, not “a B person.” Most people in a fitting room are wearing the band too big and the cup too small because a department-store “plus four” measure taught them to.
Sister sizes are the useful trick
Go down a band, go up a cup; go up a band, go down a cup. 32D ≈ 34C ≈ 36B in cup volume. The shape will not be identical — a 32 band on a 34 ribcage will be a compression garment — but this is how you shop when a brand runs long in the band. Try the sister, not a random leap to a bigger letter on the same band.
| If 34C is close but… | Try | Why |
|---|---|---|
| Band rides up, cups OK | 32D | Tighter band, same volume |
| Band digs, cups OK | 36B | Looser band, same volume |
| Band OK, spillage at the cup | 34D | Same band, more volume |
| Band OK, empty wrinkled cup | 34B | Same band, less volume |
| Straps slip and band is loose | 32D, not tighter straps | Straps are not the support |
Measure over an unlined bra
Wear a thin, unlined, non-padded bra you already own. Measure the band snug, level, at the bottom of the band, in inches, and round to the nearest even number for US/UK. Measure the fullest part of the bust, standing, arms down, tape not tight. Subtract band from bust. Roughly: 1 inch = A, 2 = B, 3 = C, 4 = D, and so on. This is a starting letter, not a sentence.
Do not measure over a padded push-up (you will add foam to the bust number). Do not measure over nothing if you have a lot of tissue to contain — the unlined bra gives a closer wearing number. The same tape logic as measuring yourself for clothing: level tape, exhale for the band, no sucking in for a wish size.
What to buy, and what it changes
A well-fitted T-shirt bra in the $35–$70 range from a shop that will actually alter or exchange, plus one unlined seamed bra so you can see shape, is a better start than four $18 moulded bras in the old size. Underwire should sit on the ribcage, not on breast tissue; the centre gore should tack on most shapes (it will not on a very close-set or a very full-on-bottom shape — that is a style problem, not always a size problem).
Fit changes how clothes hang. A capsule of white shirts and knitwear only works if the under-layer is quiet. For an interview, a smooth T-shirt bra in the correct band beats a fashion bra with visible seams. Makeup can be five minutes — the clean-girl version — but a riding band will still show in a blouse.
Questions, answered
- Why do most people wear the band too big and the cup too small?
- Department-store “plus four” measuring added inches to the band and then under-cupped to compensate. A snug 34D feels tighter than a loose 38B on day one and supports better on day thirty.
- Should I size up in the band for comfort?
- If the band leaves marks and you cannot fit two fingers, sister up a band and down a cup. If it is merely unfamiliar, wear it on the loosest hook for a week. A band that feels like a hug is doing the job.
- Do sister sizes fit the same?
- Same cup volume, different ribcage. A 32D on a true 34 rib will feel tight. A 36B on a true 34 rib will ride up. Use sisters to correct a brand that runs long, not to ignore the rib measurement.
