Bra Size Calculator

A bra size has two parts. The band number is the length around your ribcage. The cup letter shows how far your bust measures beyond your band, so the same letter holds a different volume on a different band⁸. This calculator reads both from two tape measurements and shows them in five sizing systems.

What Bra Size Measures

Your band comes from the snug measurement under your bust, and your cup from the gap between your bust and your band. It does not measure breast shape, firmness or width, or how a particular bra is cut.

The Formula

The European standard EN 13402-3 defines cup size as bust girth minus underbust girth¹⁰. We could read only a preview of it, so its letter tables are not used. Brastop rounds the underbust to an even band and subtracts that from the bust, and so does this tool⁶. A 1998 paper by Pechter recommended the raw underbust for the band, as a 2006 study describes².

Band (US and UK) = underbust in inches, rounded to the nearest even number

Cup step = bust in inches (rounded to a whole number) minus band

Each inch of difference moves up one cup letter. One inch is an A cup and two inches is a B cup².

How This Calculator Computes Your Result

Enter your underbust and bust in inches or centimeters. Centimeters are divided by 2.54, and nothing is rounded until the last step. An odd underbust goes up to the next even band⁶. The US and UK size is then converted: EU band = 2.5 x US band - 10, FR/BE band = EU band + 15, and AU/NZ band = US band - 22. These rules match retailer charts⁸. Bands below 28 are shown as 28. You can switch to the traditional add 4 or 5 method. The result card shows what the other method would give, plus sister sizes one step each way and a fit check.

What Your Result Means

Your size reads band first, then cup, for example 34D. For 1 to 4 inches of difference the letters are A, B, C, D in every system. After that they part ways. The UK adds double letters (DD, FF, GG). The US runs DD/E, DDD/F, then G and onward⁶ ⁸.

For example, a 34 band is EU 75, FR/BE 90 and AU/NZ 12. Sizing varies across brands, so treat your result as a starting size and try it on⁹.

How to Measure Correctly

• Use a soft tape and stand upright, with no bra or a thin unpadded one.

• Underbust: wrap the tape around your ribcage right under the bust, level at the back and front, and snug enough to slip two fingers under⁶.

• Bust: wrap the tape level across the fullest part, without pressing in. Round to the nearest whole inch⁶.

• Breathe out normally and measure at the end of a relaxed breath. In 16 large-breasted women, a deep breath in raised the calculated band size by 1.9 inches on average and lowered the cup size, and the authors said breathing should be standardized².

• If your breasts differ in size, use the larger one.

Direct Method Versus Add 4 or 5

The traditional rule adds 4 inches to an even underbust and 5 to an odd one. One study found that 26 bra-sizing websites added anything from 2 to 6 inches, and used 4 and 5 as its standard². In 45 UK women, the traditional method tended to overestimate band size and underestimate cup size, and it was less accurate at larger bands¹. Some retailers now say to add nothing⁶, while some health service leaflets still teach add 4 or 5⁷. We found no count of fitters by method, so we make no claim about "most". With a 30 inch underbust and 36 inch bust, the direct method gives 30E (UK) and the traditional method gives 34B.

Sister Sizes and the Fit Check

A sister size keeps the same cup volume on a neighboring band. Go up a band and down a cup, or down a band and up a cup⁸. Here is a 34C and its neighbors.

Check the fit⁹:

• The band sits level at the front and back, close to the skin, neither tight nor loose.

• The cups hold all of your breast, with no spilling over and no loose fabric.

• The center panel lies flat, and wires and straps do not dig in.

• The bra stays in place when you lift your arms.

How Common Is a Wrong Size?

In 104 women (average age 43.5), 85% wore ill-fitting bras by professional fitting criteria, and neither trying bras on nor measuring fixed this³. In 30 women aged 18 to 26 with thoracic (upper back) pain, 80% wore a wrongly sized bra, mostly too small. That study found only a negligible link between fit and pain, so this page makes no claim about pain or posture⁴. In 115 girls with an average age of 16, an education booklet raised the share passing a fit assessment by 39 percentage points (95% CI 19 to 54), but exercise breast discomfort did not differ⁵. These studies are small and define "wrong" differently.