Skip to content
CupSizer

Methodology

CupSizer turns measurements into a bra size with eight written rules taken from five published sources, each read on the date shown below, and it says where the sources disagree.

What the calculator does

You give an underbust and a bust, in inches or centimetres. The calculator applies one written rule to find a band, takes the difference between the bust and that band, and looks the difference up in a table of cup letters. The result is a size label in six systems: US, UK, EU, FR/BE, AU/NZ and JP.

Nothing here is a fitting. A size from any calculator is a starting point to try on, because brands cut the same label differently.

Where the numbers come from

Every table on the site is typed from published sources: an encyclopedia article on bra sizes, a retailer's measuring guide, a retailer's size chart and calculator, a lingerie maker's comparison of Australian and European sizing, and a conversion chart from a sizing aggregator. All five were read on November 10, 2026. The list at the end of this page gives their titles, publishers and addresses.

Where only one source gives a figure, the page says so. Where two sources give different figures, the page shows the difference and does not pick one.

The site publishes no cup volumes in millilitres, no average sizes and no health or surgical guidance. The sources use incompatible models for volume.

Rule 1: units and accepted ranges

One inch is 2.54 centimetres. An underbust from 20 to 60 inches (50 to 150 cm) and a bust from 20 to 90 inches (50 to 230 cm) are accepted. A bust smaller than the underbust has no cup size, and a difference of more than 15 inches after rounding is refused instead of guessed.

Rule 2: the underbust method

This is the default. The band is the snug underbust rounded to the nearest even number of inches, and an exact odd whole number rounds up. The cup is the difference between the bust and that band, rounded to the nearest whole inch with a half going up, looked up in the cup table.

Example: an underbust of 30 inches and a bust of 36 inches give band 30 and a difference of 6 inches.

Rule 3: the add-4 method

An older method that still appears on some charts. The underbust is rounded to the nearest whole inch, then 4 is added if the result is even and 5 if it is odd. The cup is again the bust minus that band. Some sources criticise the method, so the calculator offers it as a comparison and does not use it by default.

Rule 4: the EU method from centimetres

The EU band is the underbust in centimetres rounded to the nearest multiple of 5. The cup is the difference between the bust in centimetres and that band number, in steps of 2 cm: AA from 10 to 12, A from 12 to 14, B from 14 to 16, C from 16 to 18, D from 18 to 20, E from 20 to 22, F from 22 to 24, G from 24 to 26 and H from 26 to 28. A value exactly on a boundary takes the higher letter. This method gives EU and FR/BE labels only.

Rule 5: cup letters by difference in inches

One table lists the cup letter each system prints for every whole-inch difference from 0 to 15. US and UK letters follow two published charts that agree. The US runs AA, A, B, C, D, DD or E, DDD or F, DDDD or G, then H and on. The UK runs AA, A, B, C, D, DD, E, F, FF, G, GG, H, HH, J, JJ, K.

EU and FR/BE letters run AA, A, B, C, D, E, F, G, H and on, without double letters. AU/NZ uses the UK letters through DD, then single letters. JP comes from one label chart only and is marked as varying by brand. A letter a system does not list is shown as not listed and is never extended.

Rule 6: band labels

A band on a tag is a label, not a measurement. The six systems label the same band differently: US and UK count in inches, EU in centimetres, FR/BE as the EU number plus 15, AU/NZ as the US and UK number minus 22, and JP follows the EU number. A band 34 is EU 75, FR/BE 90, AU/NZ 12 and JP 75.

Band 34 on a tag does not mean a 34 inch body. Under rule 2 it comes from an underbust between 33 and 35 inches.

Rule 7: sister sizes

A sister size moves the band one step (2 inches, 5 cm) and the cup one letter the other way: one band up with the cup one letter down, or one band down with the cup one letter up. The calculator shows two steps each way, only where the bands and the letters exist in the tables.

Rule 8: the body range of a size

Each size page lists the underbust and bust measurements that lead to that size under each method. They are found by running the calculator's own rules across every accepted measurement, so a size page and the calculator cannot disagree.

For band 34 the underbust method accepts an underbust from 33 up to, not including, 35 inches (83.8 to 88.9 cm). The add-4 method accepts 28.5 up to 30.5 inches, and the EU method for EU 75 accepts 72.5 up to 77.5 cm.

Where the sources disagree

The sources do not always agree. The site shows each difference and does not smooth it over. Each point below states the day it was last checked.

Band: one rule or three. An underbust of 28 inches (71 cm) gives band 28 under rule 2, band 32 under rule 3 and EU 70 under the EU method, and the label chart pairs band 32 with EU 70. One retailer's calculator gives band 30 for the same body, with a cup that depends on the band. The compare box on the calculator shows the three answers side by side. Checked November 10, 2026.

US cup letters E, F and G. Two published charts agree on DD or E, DDD or F and DDDD or G. One source adds that some US brands use other sequences, such as D1 to D5, E, EE and EEE, or the UK letters. Checked November 10, 2026.

UK double letters. FF, GG, HH and JJ appear on two charts, and one source says that one UK retailer leaves out some of them. Checked November 10, 2026.

AU/NZ letters above DD. Two sources give single letters, E, F, G and on, and a third lists FF and GG. The site shows the single letters and notes the other list. Checked November 10, 2026.

JP cup letters. The one chart that lists them puts each letter one step above the US letter from cup B, because JP measures against a different band. Brands vary, so every JP cup is marked as varying by brand, and the first two differences are not listed. Checked November 10, 2026.

Rounding. One source rounds the bust up, one rounds the band to the nearest even number and one works in half-inch ranges. The site uses the nearest even band and the nearest whole inch for the difference, as rule 2 says. Checked November 10, 2026.

EU cup letters. The EU rule counts in centimetres in steps of 2 cm, while the European column of a retailer's chart follows inches. The two letters coincide from E upward only on the label chart. Checked November 10, 2026.

Checks and corrections

Every table is validated on each build: its fields, its links to other tables and the check date of each source. Tests pin the calculator's rules against known examples, every table row and each rule at its boundary.

A report of an error is compared with its source. When it holds up, the data, the page and its date change together. The contact page says where to write.

What the dates mean

The date on a page is the day its figures or its words last changed. A source's check date is the day someone last compared the tables with it. Rebuilding the site changes neither.

Sources in use

Every source the tables cite, with the day it was last checked.