Band and cup from two measurements, in three conventions
A bra size is two measurements wearing a code: the band is your snug underbust rounded to its sizing step, and the cup letter is the difference between bust and band, which is why the same letter means different volumes on different bands. This calculator takes a snug underbust and a full bust measurement, applies the standard rounding for each system, and returns US, UK and EU sizes side by side, plus the sister sizes that hold the same cup volume on neighbouring bands, which are the practical answer when your exact size is out of stock or a brand runs small. The result is a reference size to start fitting from, stated as such, because brands and styles genuinely vary.
Disclaimer: This calculator gives estimates for general information only. It is not financial, medical or other professional advice. Sizes are a starting reference: brands, styles and fabrics vary, and a fitting, or trying the sister sizes either side, settles what a chart cannot.
What this calculator does
It converts two tape measurements into reference sizes in the three conventions a shopper actually meets, with each system’s own rounding rules applied openly: US and UK bands are the underbust in inches rounded to the nearest even number, EU bands are the underbust in centimetres rounded to the nearest 5. Cup letters come from the bust-minus-band difference, stepping one letter per inch in the US and UK (whose letter sequences agree up to D and then diverge, which the page handles by computing each from its own sequence) and per 2 cm in the EU. It also returns sister sizes, the band-up and band-down equivalents holding the same cup volume, and flags the between-sizes case when the underbust lands near a rounding boundary, because that flag is the difference between a size and a fitting plan.
How to use it
- Snug means snug. The underbust tape should be firm enough to stay level without digging: it is measuring the ribcage the band must grip, since a band does most of the supporting. A loose underbust reading inflates the band, and an oversized band is the root of most fit complaints later blamed on cups.
- The bust tape is level and uncompressed. Across the fullest point, around the back at the same height, without pulling tissue in. Wearing a thin unpadded bra that lifts without padding gives the most shop-useful figure; measuring over a padded bra adds the padding to your size.
- Read the three systems as translations, not options. Your body has one size per system; the letters differ because the sequences diverge after D (a UK F is not a US F). When a shop’s label names a system, use that system’s line from the results, and distrust any conversion chart that maps cup letters one-to-one across the Atlantic beyond D.
- Sister sizes are fitting tools. Same cup volume, different band: down a band and up a letter for a firmer band, up a band and down a letter for a looser one. They are the answer when a band feels wrong but the cups sit right, when a brand runs small, or when your size is simply out of stock in the style you want.
- Between sizes is a real result. When the underbust falls close to the boundary between two bands, the results say so, and the honest plan is to try both bands with their sister cups. Bodies also change across the month and across the year; a measurement is a dated sample, like any other.
How it is calculated
- Band, US/UK: underbust in inches rounded to the nearest even number (33 becomes 34; 32.4 becomes 32). Cup index = (full bust – band) in inches, rounded to the nearest whole step: 0 = AA, then 1 = A, 2 = B, 3 = C, 4 = D, and from there the sequences split: US continues 5 = DD, 6 = DDD, 7 = G, 8 = H, 9 = I, 10 = J, 11 = K; UK continues 5 = DD, 6 = E, 7 = F, 8 = FF, 9 = G, 10 = GG, 11 = H, 12 = HH, 13 = J.
- EU: band = underbust in cm rounded to the nearest 5 (75 cm stays 75). Cup from the bust-minus-underbust difference in cm, in 2 cm steps: under 10 = AA, 10-12 = A, 12-14 = B, 14-16 = C, 16-18 = D, 18-20 = E, 20-22 = F, 22-24 = G, 24-26 = H. Sister sizes: band – 2 with the next cup letter up, and band + 2 with the next cup letter down, in the same system, because cup volume tracks the difference, not the letter.
Worked examples
Underbust 33 in, bust 37 in. The band rounds to 34, the difference over the band is 3 inches, and the reference size is US 34C, UK 34C. In EU terms the underbust is 83.8 cm, band 85, difference 10.2 cm: EU 85A, a reminder that the letters do not travel between systems even when the body does not change. Sisters of the 34C: 32D for a firmer band, 36B for a looser one, same cup volume.
Underbust 75 cm, bust 93 cm. In inches that underbust is 29.5, rounding to a 30 band; the 18 cm difference is 7.1 inches, step 7: US 30G, UK 30F, the systems diverging after D exactly as the method describes. On its home system the EU reads band 75 and an 18 cm difference: EU 75E. Three labels, one body, and the page’s reason for existing is that none of them is the translation of another by letter.
The between case. An underbust of 33 in sits exactly between bands 32 and 34 in rounding terms close enough that the results flag it, and the fitting plan is both: the 34 with its cup, and the 32 with the sister cup up. Fitters start from exactly this flag; a single printed size hides it, which is why the note is in the results and not a footnote.
Reading your results
Treat the reference sizes as the centre of a small fitting range, not a verdict. Check fit by the band first: it should be firm and level, fastening comfortably on the loosest hook when new, because bands stretch with wear. Cups should contain without cutting in or gaping; if the band is right and the cups are wrong, move only the letter; if the cups are right and the band is wrong, move through the sisters, which is what they are printed for. A size that works in one brand’s balconette may not in another’s plunge, and that is the industry, not your measurements.
Limitations
- Brands vary, honestly and otherwise. Sizing is a convention, not a regulated unit: some brands run small or large by design, and cup progressions differ between makers at the larger letters. The sequences used here are the most common published ones; a specific brand’s own chart outranks this page for that brand.
- Shape is not in two measurements. Projection, spacing, fullness distribution and asymmetry all change which style and sometimes which size fits, and no tape captures them. The calculator sizes the band and the volume; style selection remains a fitting-room exercise.
- Measurements move. Across the day, the month, training, weight change and life stages, underbust and bust both shift by enough to cross a band boundary. A size from a year ago is a historical document; remeasure for important purchases, which is cheap advice that this page exists to make easy.
- Rounding is a convention too. Nearest-even band rounding is the modern standard; some older charts add inches to the underbust before sizing (the plus-four method), which produces bands two sizes larger from the same body. If a brand instructs that method on its own chart, follow its chart for its garments and this page for the rest.
Frequently asked questions
How do I measure myself for a bra size?
Two measurements with a soft tape, standing straight: snug underbust, firm and level under the bust, and full bust, level across the fullest point without compression. Enter them in inches or centimetres and the page applies each system’s rounding. The snugness of the first is the detail that decides the band, and the band decides most of the fit, so take it twice and use the consistent reading; a tape that slopes up the back is measuring a diagonal, not a ribcage.
What are sister sizes?
Sizes with the same cup volume on different bands: a 34C’s sisters are 32D and 36B. Cup volume is set by the difference between bust and band, so stepping the band down and the letter up (or the reverse) holds the volume while changing the fit of the band. They matter in fitting, when the cups are right but the band is not, and in shopping, when your size is sold out: the results print both sisters for the US size because those are the sizes to reach for in each case.
Why is my UK size different from my US size?
Because the letter sequences divorce after D. Up to D they agree (A, B, C, D); then the UK inserts DD, E, F, FF, G, GG with extra steps, while the US runs DD, DDD, G, H. At a difference of 7 inches the same body is a UK F and a US G, as the worked example shows. Neither is wrong; they are different codebooks, and a label’s system, printed or asked for, decides which line of the results to use.
What if I am between band sizes?
Then the fitting plan is genuinely two sizes: your rounded band and the neighbouring band with its sister cup. The calculator flags the case when the underbust sits near a rounding boundary, an underbust of 33 inches being the textbook example between 32 and 34. Try the smaller band with the larger cup for support, and the larger band with the smaller cup for ease, and judge by the fit checks in the results section rather than by which label sounds more familiar.
Related calculators
Method: snug underbust rounded to the nearest even band (US/UK) or nearest 5 cm (EU), cup from the bust-minus-band difference on each system’s published letter sequence, sister sizes by equal-difference steps, exactly as defined on this page. Last reviewed: 13 October 2026.