How Bra Sizes Work
A bra size is two numbers pretending to be one. Change either below and watch what happens to the other, then read why sister sizes, conversions and fit all follow from it.
See the math drawn: every size to scale →
This is the short version. For every table and the full A to K ladder, read bra sizes explained.
The equation behind every size
Understanding a bra size
The band is the ribcage measurement, taken snugly under the bust. It's the structural part of a bra (the part that carries most of the support) and it comes in even inches: 30, 32, 34, 36 and so on.
The cup is the difference between the bust and the band, one letter per inch: 1″ = A, 2″ = B, 3″ = C, 4″ = D. It describes how much larger the bust is than the frame it sits on.
Because the cup is a difference, the same letter means different volumes on different bands. A 30C and a 36C are not the same cup. Band and cup are always read together.
What follows from the maths
Sister sizes
If a cup is a difference, then the same difference on a different band gives you the same cup volume. 34C is three inches over 34; 32D is four inches over 32, and both describe the same bust measurement of 37 inches. That's a sister size. Try it in the sister size calculator.
Conversions
Other countries measure the same body in different units (centimeters, dress sizes, plain numbers) and continue the cup alphabet differently above D. The body doesn't change; only the notation does. See international bra sizes.
Fit
Because the band is the structure and the cup is the volume, fit problems separate cleanly: band symptoms (riding up, digging, straps slipping) call for a band change; cup symptoms (spilling, gaping, floating gore, wire on tissue) call for a cup change. The fit guide maps each symptom to its fix.
Why "D" isn't big
A 30D and a 34B hold about the same volume. The letter alone tells you nothing: which is why fitters never talk about cup size without the band attached. The cup size chart shows it visually.
Key takeaway: band = structure, cup = relative volume. Every rule about bra sizing is a consequence of those two facts.
How sizes work: answered
What do the numbers and letters in a bra size mean?
The number is the band: the ribcage measurement in inches. The letter is the cup: the difference between the bust and the band, one letter per inch.
Is a C cup the same on every band?
No. C always means a three-inch difference, but three inches over a 32 band is a smaller cup than three inches over a 36 band. Cups are relative to their band.
Why does the band matter more than the cup?
The band provides around 80% of the support and anchors everything else. When the band is right, straps stay put and cups sit correctly; when it's wrong, everything upstream compensates.
How is a bra size calculated?
Measure the underbust snugly for the band. Measure the fullest point of the bust. Subtract band from bust; each inch is one cup letter. Underbust 34, bust 37 gives 3 inches: 34C.
What is the most common bra size?
Surveys of purchases in the US and UK typically put 34DD or 36DD near the top, though the 'average' varies by country and by how sizes are measured. What matters is your own measurement, not the average.
Opens HauteFlair’s own hub page; use your measured size there. HauteFlair is owned by the same company as Brasize.ai; that relationship never changes the sizing math, charts, or fit advice on this page. How this site makes money.
See the maths on your own body
Two measurements, one exact size, and every sister size and conversion that follows.
Calculate My Bra Size →