The formula
How it works
Estimate your bra size from two measurements: around your ribcage just under the bust, and around the fullest part of the bust. The calculator gives a band size and a cup letter from the difference between them.
FAQ
How should I measure?
Use a soft tape. For the band, measure snugly around your ribcage directly under the bust. For the bust, measure loosely around the fullest part, keeping the tape level. Wear an unpadded bra or none. Both measurements drive the result, so take them carefully.
Why do sizes feel different between brands?
Bra sizing is not standardised, and cut, style and country conventions all vary. A 34C in one label can fit like a 34D in another, and sister sizes (going down a band and up a cup, or vice versa) often fit similarly. Treat the result as a starting point and try styles on.
How do I know if a size actually fits?
The band should sit level and snug on the loosest hook without riding up your back, and the cups should fully contain your bust with no gaping or spilling over the edges. If either sign shows up, try the next band or cup size before assuming the number itself is wrong.
What is a sister size and when would I use one?
A sister size has the same cup volume as your calculated size but in a different band — sizing the band down one step and the cup up one (or vice versa) keeps the fit similar. It’s useful when your exact size isn’t in stock or when a style runs slightly small or large.
How often should I remeasure?
It’s worth remeasuring roughly every six to twelve months, and any time your weight changes, after pregnancy or breastfeeding, or if a bra you’ve worn for a while suddenly feels off. Bust and ribcage size both shift gradually over time.
What happens if my underbust measurement is an odd number?
The calculator rounds to the nearest even band size, which is how most bra manufacturers size their bands. An underbust of 31 inches, for example, rounds to a 32 band, and the cup is then measured against that rounded band, not the raw number.
Are cup sizes past DDD or beyond J available?
Yes, in principle — the same one-inch-per-letter logic continues past DDD (also labelled E, F or G depending on the brand) into extended ranges, though not every retailer stocks every band-and-cup combination. The letters here follow common US sizing; other regions label the same volumes differently.
About the bra size calculator
This calculator estimates a bra size from two body measurements: the underbust, taken snugly around the ribcage, and the bust, taken around the fullest part. The underbust sets the band size, and the difference between bust and band sets the cup letter. Because getting either measurement wrong throws off the fit, having the calculator turn the numbers into a size takes the guesswork out of an often confusing system.
How to use it
Measure your underbust and your bust in inches or centimetres and enter both. The calculator rounds the underbust to a band size and works out the cup from the difference. For example, a 32-inch underbust with a 36-inch bust is a 4-inch difference, giving a 32D. Remeasure if the result seems off — a tape that is too loose or too tight is the usual culprit.
The formula
The band is the underbust rounded to the nearest even number, . The cup comes from the difference : each inch of difference steps up the cup letter, so 1 inch is an A, 2 a B, 3 a C, 4 a D, and so on through DD and beyond. Centimetre measurements are converted to inches before the difference is taken.
Where it is used
Shoppers use it to find a starting size before buying online, where trying on is not possible, and to check whether a long-worn size still fits after weight or body changes. Because band and cup interact — the same cup volume pairs with different letters at different bands — the calculator helps explain sister sizing too. It is a practical first step toward a comfortable, well-fitting bra.