ReckonBoxTwo measurements, three regions
A bra size calculator turns a band and a bust measurement into a size, and this one reports the US, UK, and EU size together. A 34 inch band with a 37 inch bust is a 34C in the US and the UK, and a roughly 85C in the EU.
The two numbers do different jobs. The band, measured firmly directly under the bust, sets the number. The bust, measured loosely over the fullest point, sets the cup. Everything after that is arithmetic, and showing all three regions at once is the point, because the cup letters do not stay the same as you cross borders.
How the size is built
The band size is the underbust measurement in inches rounded to the nearest even number, and the cup is the difference between the bust and the band, one letter per inch. A one inch difference is an A, two is a B, three a C, four a D.
So a 34 inch underbust gives a 34 band, and a 37 inch bust is three inches larger, which is a C cup: a 34C. A five inch difference is a DD, and the letters keep climbing from there. Measure in a soft tape and a non-padded bra, keep the band tape level and firm, and let the bust tape rest without compressing, and the two numbers fall out cleanly.
Where US and UK letters part ways
US and UK cup letters agree up to D and DD, then split, and this is the mix-up that sends people home with the wrong size from an international brand. From a five inch difference upward the UK uses single letters, so six inches is a UK E and seven is an F, while the US doubles up into DDD and DDDD.
| Bust minus band | US cup | UK cup |
|---|---|---|
| 4 in | D | D |
| 5 in | DD | DD |
| 6 in | DDD | E |
| 7 in | DDDD | F |
| 8 in | H | FF |
A US 34DDD and a UK 34E describe the same body. Reading a label from a British brand as though it were American is how a well-fitting cup gets mistaken for the wrong size, so the tool prints both.
Sister sizes, for when the band is off
Sister sizes are different band-and-cup pairings that hold the same cup volume, reached by moving the band up or down while shifting the cup letter the opposite way. A 34C shares its cup with a 32D and a 36B. They earn their keep when the cup fits but the band rides up or digs in: drop to the sister size with the smaller band and a larger cup letter, and the cup stays the same while the band changes. This tool lists the sister size on each side.
What the number cannot know
A calculated size points you to the right shelf, and no further. Cut, fabric, and stretch differ by brand and by style within a brand, so two 34C bras can fit differently, and a circumference says nothing about breast shape, which drives real fit. Older calculators muddy this further by adding four or five inches to the band, a method that tends to oversize the band and undersize the cup; this tool skips that addition to match current fitting guidance. Use the size to narrow the search, then try the bra on, and treat a professional fitting as the last word.
Measure the band firmly and the bust gently, because the most common sizing mistake is a band read too loose, which quietly inflates the cup.
Frequently asked questions
How is bra size calculated? Bra size comes from two measurements: the band, taken firmly directly under the bust, and the bust, taken loosely over the fullest point, both in inches. The band size is the underbust measurement rounded to the nearest even number. The cup is the difference between bust and band, with one letter per inch, so a 34 inch band and a 37 inch bust is a 34C. Measure in a non-padded bra for the truest numbers.
Why do US and UK cup letters differ? US and UK cup letters agree up to D and DD, then split. From a five inch difference the UK adds single letters, so six inches is a UK E, seven is an F, while the US doubles up with DDD and DDDD. A US 34DDD is the same body as a UK 34E. This tool shows US, UK, and EU side by side so a size from an international brand is not a mystery.
What are sister sizes? Sister sizes are different band-and-cup combinations that hold the same cup volume, found by moving the band up or down while shifting the cup letter the other way. A 34C shares its cup with a 32D and a 36B. They matter when your cup fits but the band is loose or tight, since a sister size keeps the cup and changes only the band. This tool lists the two sister sizes on either side.
Is the band measurement plus four inches method right? That older method, adding four or five inches to the underbust, tends to produce a band too big and a cup too small, which is why modern fitting drops it. This tool uses the underbust measurement rounded to the nearest even number, with no addition, matching current fitting guidance. If a favourite brand still uses the older method, its own chart will say so.
How accurate is a calculated bra size? A calculated size is a strong starting point, not a guarantee, because cut, fabric, and stretch vary by brand and even by style within a brand. Two 34C bras can fit differently, and factors like breast shape are not captured by two circumferences. Use the number to narrow the search, then try the bra on or check the brand fit guide. A professional fitting is the final word.