Bra Size Calculator
Work out band and cup size from your underbust and bust measurements, with UK, US and EU equivalents and sister sizes.
How to use this calculator
- 1Measure your underbust snugly, directly under the bust, with the tape level.
- 2Measure your bust loosely across the fullest part, wearing an unpadded bra or none.
- 3Read the band and cup, then check the sister sizes if the band feels wrong.
How the calculation works
band = underbust rounded to the nearest even inch cup = bust − band, one letter per inch- underbust
- Measured snugly directly under the bust, tape level
- bust
- Measured loosely across the fullest part
- difference
- Bust minus band, in inches. The objective figure — letters are convention
- sister size
- Same cup volume at a different band: down a band means up a cup
The band is the underbust measurement itself, rounded to the nearest even inch. Adding four or five inches was standard advice decades ago when bras used non-stretch fabric, and it now produces bands several sizes too large.
A cup letter is not an absolute volume. A 30D and a 36D have very different cup volumes — the letter describes the difference from the band, so the same letter on a larger band means a larger cup.
Above D the letter systems diverge and are internally inconsistent. UK runs DD, E, F, FF, G; US brands variously run DD, DDD, DDDD or switch to E, F, G. The inch difference is the only figure that transfers reliably.
Sister sizing follows from cup volume being tied to the band. Dropping two inches of band and adding a cup letter keeps the volume roughly constant.
Worked example
A 32-inch underbust and 36-inch bust
- 1.Underbust is 32 inches, already even, so the band is 32.
- 2.Difference: 36 − 32 = 4 inches.
- 3.Four inches is a D cup, giving 32D in both UK and US systems.
- 4.In EU sizing the band is the underbust in centimetres rounded to the nearest 5: 32 in = 81.3 cm, so 80.
- 5.EU cups run single letters, so the same 4-inch difference is a D: EU 80D.
- 6.Sister sizes 30DD and 34C hold roughly the same cup volume.
Result: 32D (UK and US), EU 80D
Where UK and US diverge
- 1.Underbust 30 inches gives a band of 30.
- 2.Difference: 37 − 30 = 7 inches.
- 3.Seven inches is a UK F cup — the UK sequence runs D, DD, E, F.
- 4.Most US brands call the same volume a G, having gone D, DD, DDD first.
- 5.EU calls it a G too, since EU uses single letters throughout.
- 6.The 7-inch difference is what transfers between brands; the letters do not.
Result: 30F UK, 30G US — same volume
The "+4" rule is why so many bands are too big
For decades the standard advice was to add four or five inches to your underbust measurement to get the band size. That rule made sense when bras were constructed from non-stretch fabric that needed the slack. Modern bras use elastic, and the rule now produces a band several sizes too large.
The consequence is not merely cosmetic. The band is meant to provide the great majority of the support — around 80% of it — with the straps doing very little. A loose band rides up at the back, which shifts the load onto the straps, which is what produces shoulder marks and the general impression that bras are uncomfortable.
The modern method is simply to use the underbust measurement itself, rounded to the nearest even inch. It typically produces a smaller band and a larger cup letter than the old rule, and the larger letter is what tends to alarm people — even though the cup volume has not changed at all.
A cup letter is a difference, not a volume
The most persistent misconception in bra sizing is that a D cup means a fixed size. It does not. The letter describes the difference between bust and band, so a 30D and a 38D have substantially different cup volumes — the same four-inch difference wrapped around very different frames.
This is what makes sister sizing work. Drop two inches of band and go up one cup letter, and the cup volume stays roughly the same: 34C, 32D and 30DD are all approximately the same cup on progressively tighter bands. If the cup fits but the band rides up, a sister size is the fix, not a different letter.
It also explains why comparing letters between people is meaningless. Someone in a 28F may have a smaller cup volume than someone in a 38C, despite the letter being much further along the alphabet.
Why the letters stop agreeing above D
Through D the systems match. Above it they diverge, and not in a way that can be resolved by a simple lookup, because the US industry is not consistent with itself.
UK sizing continues DD, E, F, FF, G, GG, H — inserting doubled letters at regular intervals. Many US brands instead run DD, DDD, DDDD before switching to single letters, while others jump straight to E and F for the same volumes. Two American brands can label the identical cup differently.
This is why the inch difference is the figure worth carrying. It is objective and transfers between every system, while the letter depends on which chart the manufacturer happened to adopt. Published conversion tables disagree with each other for precisely this reason, and any chart claiming an exact universal mapping above D is overstating what the industry actually does.
What this assumes, and where it stops
Assumptions
- The band is the underbust measurement rounded to the nearest even inch, which is current fitting practice.
- One inch of difference between bust and band corresponds to one cup letter.
- EU band sizes are the underbust in centimetres rounded to the nearest 5.
- Measurements are taken correctly: underbust snug, bust loose, tape level.
Limitations
- A starting point, not a fitting. Brand variation of a full band and cup is routine, and no calculator can account for it.
- Cup letters above D are not consistent between systems or even between US brands. The inch difference is the reliable figure.
- Measures volume only. Breast shape — full-on-top, bottom-heavy, wide-set, close-set — affects which style fits as much as size does, and cannot be measured with a tape.
- Does not cover maternity, nursing, post-surgical or sports bras, which use their own sizing conventions.
- Measurements change with weight, menstrual cycle, pregnancy and age, so a size is not permanent.
Common questions
How do I measure my bra size?
Measure snugly directly under the bust for the band, and loosely across the fullest part for the bust, keeping the tape level both times. Round the underbust to the nearest even inch for the band, then subtract it from the bust — each inch of difference is one cup letter.
Should I add 4 inches to my underbust measurement?
No. That rule dates from when bras used non-stretch fabric and now gives a band several sizes too large. Use the underbust measurement itself, rounded to the nearest even inch. A loose band rides up and shifts support to the straps, which is the most common cause of discomfort.
What is a sister size?
A different band with the same cup volume. Going down two inches of band and up one cup letter keeps the cup roughly constant — 34C, 32D and 30DD are all about the same cup. Use one when the cup fits but the band is too loose or too tight.
Why do UK and US cup sizes differ?
They match through D, then diverge. UK runs DD, E, F, FF, G while many US brands run DD, DDD, DDDD before switching to single letters — and US brands are not even consistent with each other. The inch difference between bust and band is the only figure that transfers reliably.
Sources
Formula and content last reviewed on .
Results are estimates for information only, not professional advice.
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