Bra Size Calculator

Background.

A bra size calculator converts two tape-measure readings — a snug girth around the rib cage directly under the bust, and a loose girth around the fullest point of the bust — into a band number and a cup letter. The underlying idea is genuinely standardised even though the labels are not: ISO 8559-1, the international standard for anthropometric definitions used in clothing size designation, defines cup size as the difference between bust girth and under-bust girth. Everything else on a bra label is a national labelling convention layered on top of those two measurements, which is why the same body is a 30G in the United States, a 30F in the United Kingdom, an 80G in Germany, a 95G in France and an 8F in Australia. This calculator does the two measurements once and then prints whichever of those five series you are actually shopping in.

There is no sizing standard behind bra labels, and it is worth being blunt about that before you read a result. The United States withdrew its only apparel sizing standard — Voluntary Product Standard PS 42-70, "Body Measurements for the Sizing of Women's Patterns and Apparel" — decades ago, and it never covered brassieres in the first place. What remains is a set of conventions that most brands broadly follow and none are bound by. A calculated size is therefore a starting point for a fitting, not a specification you can order blind, and the honest expectation is that you will try two or three sizes in a given style before one is right.

The band is where this page deliberately departs from the rule you were probably taught. The traditional method says to measure your rib cage and add four or five inches to get the band. White and Scurr, publishing in Ergonomics in 2012, compared that rule against professional bra fitting criteria in 45 women and found it systematically overestimated band size and underestimated cup size — and, importantly, that it grew more inaccurate as band size increased. This calculator therefore bands you on the snug under-bust measurement itself, rounded to the nearest even inch, and shows the traditional result beside it purely so you can see the size of the discrepancy the research describes. For many people the two answers differ by two band sizes and three or four cup letters, which is exactly why so many people have spent years in a band that rides up and a cup that gapes at the top while the underwire sits on breast tissue.

Cup letters are the second place national conventions diverge, and they diverge in a way that no amount of care with a tape measure can fix. One inch of difference is one cup step in every system, but the letters used for those steps are not the same. The continental European series runs straight through the alphabet with no repeats: A, B, C, D, E, F, G. The UK series inserts DD between D and E and then starts doubling letters — DD, E, F, FF, G, GG, H, HH. The US series uses DD where Europe uses E and DDD where Europe uses F, and above DDD it stops being consistent even between American brands, with the same cup written as DDDD by one label and G or H by another. This is why a US 34DD and a UK 34DD are the same cup while a US 34DDD is a UK 34E, and it is why the inch difference is printed on this page as its own output: when the letters disagree, the number does not.

The sister sizes output exists because band and cup are not independent. Cup volume is defined relative to the band it sits on, so moving down one band and up one cup letter keeps roughly the same volume — a 34D and a 32DD hold a similar amount. If the cup is right but the band rides up or digs in, the sister sizes are the two sizes to reach for next, and they are the single most useful thing to know when a brand's band runs tight or loose. The page prints both, in the labelling system you selected.

Finally, a scope limit stated up front rather than buried: this page sizes an ordinary wired or soft-cup bra from two girth measurements on a standing body. It does not model breast shape, root width, projection, asymmetry between sides, the difference between a balconette and a full cup, sports-bra compression, or nursing and post-surgical fitting. Those are the things a trained fitter is for, and on the evidence in the literature they matter as much as the numbers.

What is bra size calculator?

A bra size is a two-part label: a band figure describing the girth of the rib cage the bra fastens around, and a cup letter describing how much further the bust projects beyond that rib cage. ISO 8559-1:2017, the international standard for anthropometric definitions in clothing size designation, is the authority for the two measurements themselves — bust girth and under-bust girth — and states the principle that cup size is the difference between them. It does not, and no standard does, tell a manufacturer which letter to print for a given difference.

That gap is filled by national convention. In the United States, the United Kingdom and Ireland the band figure is a number of inches; on the continent it is the under-bust girth in centimetres, which is why continental bands run 65, 70, 75, 80, 85 in five-centimetre steps and why EN 13402-3, the European size-labelling standard, works in centimetre girth intervals. France and Spain use the continental number plus fifteen. Australia and New Zealand use the US band minus twenty-two, giving the familiar 8, 10, 12, 14 series. The cup letter is one step per inch of difference in every system, but the alphabets differ, as the intro describes.

Because none of this is standardised, two bras marked with the same size from different brands can fit differently, and a size that fits perfectly in one cut can be wrong in another from the same maker. The label is a coordinate, not a guarantee.

How to use this calculator.

1. 01Choose your unit — centimetres or inches — and use the same one for both measurements.
2. 02Measure your under-bust: stand upright, put the tape directly under the bust, level all the way round, and pull it firm against the rib cage. Breathe out and read it. Snug is correct here; this is the measurement the band is built from.
3. 03Measure your full bust: keep the tape level, take it round the fullest point of the bust, and do not pull it tight. Wear an unpadded, non-minimising bra or nothing at all — a padded or push-up bra will inflate this reading and cost you a cup letter or two.
4. 04Pick the labelling system for the market you are buying from. The same body has a different label in the US, the UK, continental Europe, France and Australia.
5. 05Read the band and cup. Check the bust-minus-under-bust figure too — when brand letters disagree, that inch difference is the number to match against their own chart.
6. 06Note the sister sizes. If the cup is right but the band rides up or digs in, those are the next two sizes to try.
7. 07Compare the traditional "add four inches" result. If it differs sharply from your calculated size, that gap is the reason a previously worn size felt wrong, not a fault in the measurement.
8. 08Try the bra on and check the fit, not the label: band firm on the loosest hook, underwire behind breast tissue rather than on it, centre gore flat against the sternum, no gaping at the top edge and no spillage over it.

The formula.

Two independent calculations happen, and they use different measurements. The band comes from the under-bust girth alone: the girth is converted to inches and rounded to the nearest even inch, because bras are manufactured in even band sizes. The continental band takes the same girth in centimetres and rounds it to the nearest five, which is what the 65/70/75/80/85 series is — the under-bust girth itself, at five-centimetre steps, consistent with EN 13402-3's centimetre-girth labelling. The French and Spanish band is the continental number plus fifteen, and the Australian band is the US inch band minus twenty-two.

The cup comes from the difference between the two girths, converted to inches and rounded to the nearest whole inch, then used as an index into the cup series for the country you selected. Both roundings are half-up and both are applied to the unrounded measurement exactly once — nothing on this page classifies a number that has already been rounded for display. That distinction matters at the edges: an under-bust of 30.9 inches bands at 30 while 31.0 inches bands at 32, and a difference of 4.49 inches is a D while 4.50 inches is a DD. Those are real, tested boundaries, and they are the reason a careless tape reading can move you a whole size.

The traditional comparison figure is computed with the historical rule for contrast: four inches are added to the under-bust girth and the result is rounded to the nearest even band, then the cup is taken as the bust girth minus that inflated band. That is the method White and Scurr found wanting, and the two results are printed side by side so the difference is visible rather than asserted. Sister sizes are generated by stepping the band one size in each direction — two inches, or five centimetres on the continent — and moving the cup letter one step the opposite way, which preserves cup volume; at the very bottom of the cup range, where there is no smaller letter to step to, the cup is held rather than allowed to go off the end of the series.

Worked through with the page's own example: an under-bust of 32 inches gives a band of 2 × round(16) = 32. A full bust of 37 inches gives a difference of exactly 5.0 inches, which indexes to DD in the UK series, so the starting size is UK 32DD. The same body run through the traditional rule gets a band of 2 × round(18) = 36 and a cup of 37 − 36 = 1 inch, which is an A — UK 36A. Four cup letters and two band sizes apart, from the same tape readings.

A worked example.

Frequently asked questions.

References& sources.

1. [1]International Organization for Standardization, ISO 8559-1:2017 — Size designation of clothes, Part 1: Anthropometric definitions for body measurement. Defines bust girth and under-bust girth and the principle that cup size is the difference between them. Full text is sold by ISO; catalogue entry consulted, not the full document.
2. [2]White J, Scurr J. "Evaluation of professional bra fitting criteria for bra selection and fitting in the UK." Ergonomics 2012;55(6):704–711. doi:10.1080/00140139.2011.647096. Found that the traditional bra fitting method overestimated band size and underestimated cup size, and became more inaccurate as band size increased (p < 0.001).
3. [3]National Institute of Standards and Technology, "Withdrawn DOC Product Standards (PS)" — lists PS 42-70, Body Measurements for the Sizing of Women's Patterns and Apparel, among the withdrawn US Department of Commerce product standards. There is no current US government apparel sizing standard.
4. [4]European Committee for Standardization, EN 13402-3:2017 — Size designation of clothes, Part 3: Size labelling based on body measurements and intervals. Establishes centimetre body-dimension labelling with girth intervals of 4 cm, the basis of the continental 65/70/75/80/85 band series. Sold by CEN national members; abstract and scope consulted.
5. [5]ASTM International, ASTM D5585-21 — Standard Tables of Body Measurements for Adult Female Misses Figure Type, Size Range 00–20. The voluntary industry table that replaced the withdrawn PS 42-70 data set; it covers garment sizing rather than brassiere labelling, which is why no table in it maps a bust-minus-under-bust difference to a cup letter. Sold by ASTM; designation, title and scope consulted.
6. [6]Greenbaum AR, Heslop T, Morris J, Dunn KW. "An investigation of the suitability of bra fit in women referred for reduction mammaplasty." British Journal of Plastic Surgery 2003;56(3):230–236. All 102 women examined wore the wrong size; they underestimated their back measurement by a mean of 4 in and overestimated cup size by a mean of three sizes. A different population and a different direction of error from White and Scurr, but the same conclusion: a self-selected size is usually not the measured one.