How this calculator works
A bra size has two parts. The band is set by the underbust measurement, and the cup is set by the difference between the bust and the underbust. In the US and UK the band is usually the underbust in inches rounded to the nearest even number. Each inch of difference adds a cup letter: one inch is an A, two a B, and so on. That is why a 34C and a 36B can hold similar volume: the band is larger and the cup is one step smaller. European band sizes are in centimetres, usually in steps of five, so an 80 cm underbust is an 80 band.
Formula
difference = bust - underbust. cup = the letter for the difference in inches, starting at A for 1 inch. US and UK band = underbust in inches rounded to the nearest even number. European band = underbust in cm rounded to the nearest 5. sister sizes = one band smaller with one cup larger, and one band larger with one cup smaller.
Worked example
An underbust of 80 cm and a bust of 95 cm is a 15 cm difference, or 5.9 inches, which rounds to 6 inches and cup E. The underbust is 31.5 inches, so the band is 32 and the size is 32E. The European band is 80, and the sister sizes are 30F and 34DD.
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Frequently asked questions
Why does the difference use inches for letters?
The cup scale was built on inch differences and is still defined that way in US and UK charts. Measuring in centimetres is fine; the calculator converts the difference before choosing the letter.
Is the plus-four method still used?
Older charts added four inches to the underbust to get the band. Modern fitting advice usually uses the underbust rounded to the nearest even inch, which gives a firmer band. If a band feels too tight, go up a band and down a cup rather than adding four inches.
What if the cup size is beyond HH?
The table stops at HH, which is a twelve-inch difference. Larger sizes exist in specialist ranges and use their own letter sequences. Use the difference in inches and check the maker chart rather than forcing a letter.
Are US and UK cup letters the same?
They match up to DD and then diverge in some ranges. This page uses the US sequence A to HH. For a UK brand, check whether the chart uses DD, E, F or DD, E, F, FF, as the letters can shift by one at the top of the range.